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Solar
Detoxification
by Julian Blanco Galvez, Head of Solar Chemistry and
Sixto Malato Rodriguez, Researcher in the Solar Chemistry Area,
Plataforma Solar de Almeria, Spain
United Nations Educational, Scientific and Cultural Organization
2003
TABLE OF CONTENTS
PART A. SOLAR DETOXIFICATION THEORY
1. Introduction
Aims
Objectives
Notation and units
1.1 Solar Chemistry
1.2 Water contaminants
1.3 Photodegradation principles
1.3.1 Definitions
1.3.2 Heterogeneous photocatalysis
1.3.3 Homogeneous photodegradation
1.4 Application to water treatment
1.5 Gas-phase detoxification
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
2. Solar irradiation
Aims
Objectives
Notation and units
2.1 The power of light
2.1.1 Ultraviolet light
2.1.2 Visible light
2.1.3 Infrared light
2.2 The solar spectrum
2.3 Solar ultraviolet irradiation
2.4 Atmospheric attenuation of solar radiation
2.4.1 Annual available ultraviolet radiation
2.5 Solar radiation measurement
2.5.1 Detectors
2.5.2 Filters
2.5.3 Input Optics
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
3. Experimental systems
Aims
Objectives
Notation and units
3.1 Laboratory systems
3.2 Solar detoxification pilot plants
3.3 Operation of pilot plant
3.3.1 Once-through operation
3.3.2 Batch operation
3.3.3 Modelling once-through and batch operation
3.4 Evaluation of solar UV radiation inside
photoreactors
3.4.1 Radiometers calibration
3.4.2 Correlation between radiometric and
spectroradiometric data
3.4.3 Collector efficiency
3.4.4 Actinometric experiments
3.5 Simplified method for the evaluation of solar UV
radiation inside photoreactors
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
4. Fundamental parameters in photocatalysis
Aims
Objectives
Notation and units
4.1 Direct photolysis
4.2 Oxygen influence
4.3 pH influence
4.4 Catalyst concentration influence
4.5 Initial contaminant concentration influence
4.6 Radiant flux influence
4.7 Temperature influence
4.8 Quantum yield
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
5. Water decontamination by Solar Detoxification
Aims
Objectives
Notation and units
5.1 Detoxification of pollutants
5.1.1 Total mineralization
5.1.2 Degradation pathways
5.1.3 Toxicity reduction
5.1.4 Detoxification of inorganic pollutants
5.2 Quantum yield improvement by additional
oxidants
5.2.1 Hydrogen peroxide
5.2.2 Persulphate
5.2.3 Other oxidants
5.3 Catalyst modification
5.3.1 Metal semiconductor modification
5.3.2 Composite semiconductors
5.3.3 Surface sensitisation
5.4 Recommended analytical methods
5.4.1 Original contaminants
5.4.2 Mineralization measurements (TOC)
5.4.3 Intermediate analysis (GC-MS/HPLC-MS)
5.4.4 Extraction methods
5.4.5 Toxicity analysis
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
PART B. SOLAR DETOXIFICATION ENGINEERING
6. Solar Detoxification Technology
Aims
Objectives
Notation and units
6.1 Solar collector technology generalities
6.2 Collectors for solar water detoxification.
Peculiarities
6.2.1 Peculiarities of solar UV light utilization
6.2.2 Parabolic trough collectors
6.2.3 Non-concentrating collectors
6.2.4 Compound parabolic concentrator (CPC)
6.2.5 Holographic collectors
6.3 Concentrated versus non-concentrated sunlight
6.4 Technology issues
6.4.1 Reflective surfaces
6.4.2 Photocatalytic reactor
6.5 Catalyst issues
6.5.1 Slurry versus supported catalyst
6.5.2 Catalyst recuperation and re-use
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
7. Solar Detoxification Applications
Aims
Objectives
Notation and units
7.1 Introduction
7.2 Industrial waste water treatment
7.2.1 Phenols
7.2.2 Agrochemical compounds
7.2.3 Halogenated hydrocarbons
7.2.4 Antibiotics, antineoplastics and other
pharmaceutical biocide compounds
7.2.5 Wood preserving waste
7.2.6 Removal of hazardous metals ions from water
7.2.7 Other applications
7.3 Maritime tank terminals
7.4 Groundwater decontamination
7.5 Contaminated landfill cleaning
7.6 Water disinfection
7.7 Gas-phase treatments
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
8. Economic Assessment
Aims
Objectives
Notation and units
8.1 Photochemical and biological reactors coupling
8.2 Cost calculations
8.2.1 Example A: TiO2-based detoxification plant
8.2.2 Example B: Photo-Fenton based detoxification
plant
8.3 Solar or electric photons?
8.4 Solar resources assessment
8.5 Comparison with other technologies
8.5.1 Thermal oxidation
8.5.2 Catalytic oxidation
8.5.3 Air stripping
8.5.4 Adsorption
8.5.5 Membrane technology
8.5.6 Wet oxidation
8.5.7 Ozone oxidation
8.5.8 Advanced oxidation processes
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
9. Project engineering
Aims
Objectives
Notation and units
9.1 Feasibility study
9.1.1 Identification of target recalcitrant hazardous
compounds
9.1.2 Identification of possible pre-treatments
9.1.3 Identification of most adequate photocatalytic
process
9.1.4 Determination of optimum process parameters
9.1.5 Post-treatment process identification
9.1.6 Determination of treatment factors
9.2 Feasibility study example
9.2.1 Background
9.2.2 Experimentation. TiO2-Persulphate tests
9.2.3 Photo-Fenton tests
9.2.4 Conclusions and Treatment Factors
9.3 Preliminary design
9.4 Preliminary design example
9.5 Final design and project implementation
9.6 Example of final design and project
implementation
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
10. International collaboration
Aims
Objectives
Notation and units
10.1 International Energy Agency: The SolarPACES
Program
10.2 The European Union
10.3 The CYTED Program
10.4 Main research activities
10.4.1 United Stated
10.4.2 Spain
10.5 Guidelines to successful water treatment projects
in developing countries
Summary of the chapter
Bibliography and references
Self-assessment questions
Answers
SOLAR DETOXIFICATION
1. INTRODUCTION
AIMS
This unit describes an alternative source of energy that combines sunlight and chemistry to
produce chemical reactions. It outlines the basic chemical and physical phenomena that are
related with solar chemistry. This chapter will review approaches that have been taken,
progress that has been made and give some projections for the near and longer term prospects
for commercialisation of solar photochemistry. It also introduces the focus of this book: Solar
Detoxification.
OBJECTIVES
By the end of this unit, you will understand the main factors causing the photochemical
reactions and you will be able to do five things:
1. Distinguish perfectly between thermochemical and photochemical processes.
2. Understand the impact of pollutants on the environment.
3. Calculate the energy flux of a light source and its relationship with semiconductor
excitation.
4. Understand the basic principles sustaining advanced oxidation processes.
5. Describe the most important features of heterogeneous photocatalysis making it applicable
to the treatment of contaminated aqueous effluents.
NOTATION AND UNITS
Symbol
Aλ
Absorbance at wavelength λ
AOPs
Advanced Oxidation Processes
c
Light speed
Concentration of component i
ci
EG
Semiconductor band-gap energy
Spectral irradiance
Eλ
o
Spectral irradiances incident into the medium
Eλ
l
Spectral irradiances at a distance l
Eλ
EC50
Concentration that produce an effect in 50% of a population
GAC
Granulated activated carbon
h
Planck’s constant
LC50
Concentration that produce death in 50% of a population
NOEL
No observed effect level
partial pressure of component i
pi
U
energy of a photon
Absorption coefficient
αλ
extinction coefficient
ελ
quantum yield
φ
Wavelength
λ.
Units
nm/s, m/s
moles
eV, J
W m-2 nm-1
W m-2 nm-1
W m-2 nm-1
mg/L, mg/kg
Js
mg/L, mg/kg
mg/kg/day
atm
eV, J
cm-1 atm-1
mol-1 cm-1
nm, µm
1
SOLAR DETOXIFICATION
1.1 SOLAR CHEMISTRY
The dramatic increases in the cost of oil beginning in 1974 focussed attention on the need to
develop alternative sources of energy. It has long been recognised that the sunlight falling on
the earth’s surface is more than adequate to supply all the energy that human activity requires.
The challenge is to collect and convert this dilute and intermittent energy to forms that are
convenient and economical or to use solar photons in place of those from lamps. It must be
kept in mind that today there is a clear world-wide consensus regarding the need for longterm replacement of fossil fuels, which were produced million of years ago and today are
merely consumed, by other inexhaustible or renewable energies. Under these circumstances,
the growth and development of Solar Chemical Applications can be of special relevance.
These technologies can be divided in two main groups:
1. Thermochemical processes: the solar radiation is converted into thermal energy that
causes a chemical reaction. Such a chemical reaction is produced by thermal energy
obtained from the sun for the general purpose of substituting fossil fuels.
2. Photochemical processes: solar photons are directly absorbed by reactants and/or a
catalyst causing a reaction. This path leads to a chemical reaction produced by the
energy of the sun’s photons, for the general purpose of carrying out new processes.
It should be emphasized, as a general principle, that the first case is associated with processes
that are feasible with conventional sources of energy. The second is related only to
completely new processes or reactions that are presently carried out with electric arc lamps,
fluorescent lamps or lasers.
Heat
Increase of
Temperature
Photons
Modification of
chemical bonds
Thermochemical Process
Photochemical Process
Steam reforming of methane
CH4 + H2O → CO + 3H2 - 206 kJ/mol
600º - 850ºC
Excitation of a semiconductor
hν + SC →e- + p+
hν ≥ EG of SC
Figure 1.1 Schematic view of Solar Chemical Applications
From the outset, it was recognized that direct conversion of light to chemical energy held
promise for the production of fuels, chemical feedstock, and the storage of solar energy.
Production of chemicals by reactions that are thermodynamically ‘uphill’ can transform solar
energy and store it in forms that can be used in a variety of ways. Wide ranges of such
chemical transformations have been proposed. A few representative examples are given in
Table 1.1 to illustrate the concept.
2
SOLAR DETOXIFICATION
∆H (kJ/mol)
CO2(g) → CO(g) + 1/2O2
286
CO2(g) + 2H2O(g) → CH3OH (l) + 3/2O2
727
H2O(l) → H2(g) + 1/2O2
286
CO2(g) + 2H2O(l) → 1/6C6H12O6 (s) + O2
467
Table 1.1 Representative chemical reactions that can store solar energy (Thermochemical
processes)
These processes generally start with substances in low-energy, highly-oxidized forms. The
essential feature is that these reactions increase the energy content of the chemicals using
solar energy. For such processes to be viable, they must fulfil the following requirements, as
outlined by NREL (1995) and slightly modified by the authors:
• The thermochemical reaction must be endothermic.
• The process must be cyclic and with no side reactions that could degrade the
photochemical reactants.
• The reaction should use as much of the solar spectrum as possible.
• The back reaction should be very slow to allow storage of the products, but rapid when
triggered to recover the energy content.
• The products of the photochemical reaction should be easy to store and transport.
The other pathway for the use of sunlight in photochemistry is to use solar photons as
replacements for those from artificial sources. The goal in this case is to provide a costeffective and energy-saving source of light to drive photochemical reactions with useful
products. Photochemical reactions can be used to carry out a wide range of chemical
syntheses ranging from the simple to the complex. Processes of this type may start with more
complex compounds than fuel-producing or energy-storage reactions and convert them to
substances to which the photochemical step provides additional value or destroy harmful
products. The principles of photochemistry are well understood and examples of a wide range
of types of synthetic transformations are known (Figure 1.2). Therefore, the problem becomes
one of identifying applications in which the use of solar photons is possible and economically
feasible. The processes of interest here are photochemical, hence, some component of the
reacting system must be capable of absorbing photons in the solar spectrum. Because photons
can be treated like any other chemical reagent in the process, their number is a critical element
in solar photochemistry (see Chapter 2).
hυ<700nm
O
CHO
Methylene blue/O2
O
O
CHO
O
hυ<390nm
C6Cl5OH+9/2O2+ 2H2O
TiO2
6CO2 + 5HCl
Figure 1.2 Furfural photo-oxidation and pentachlorophenol mineralization (Photochemical
processes).
3
SOLAR DETOXIFICATION
Because they are very technologically and environmentally attractive, solar chemical
processes have seen spectacular development in recent years. In the beginning, research in
solar chemistry was centered only on converting the solar energy into chemical energy, which
could then be stored and transferred over long distances. Together with this important
application, other environmental uses have been developed, so that today the entire range of
solar chemical applications has a promising future. In principle, any reaction or process
requiring an energy source can be supplied by solar energy.
1.2 WATER CONTAMINANTS
Environmental pollution is a pervasive problem with widespread ecological consequences.
Recent decades have witnessed increased contamination of the Earth’s drinking water
reserves. The inventory of priority pollutants compiled by the U.S. Environmental Protection
Agency provides a convenient frame of reference (in Table 1.2 only a partial list is shown) for
understanding the importance of removing such contamination from the Earth.
1,1,2,2-Tetrachloroethane
1,l -Dichloroethane
1,2,4-Trimethylbenzene
1,2-Dibromoethane
1,2-Dichlorobenzene
1,2-Dichloropropane
1,2-Dinitrotoluene
1,2-Diphenylhydrazine
1,4-Dioxane
2,2,4-Trimethylpentane
2,4,6-Trichlorophenol
2,4,6-Trinitrotoluene
2,4 Diaminoanisole
2,4-Dichlorophenol
2,4-Dinitrophenol
2,4-Dinitrotoluene
2,4-Toluene diamine
2-Chloroethyl vinyl ether
2-Chlorophenol
2-Nitropropane
4,4’-Diaminodiphenyl ether
4,4’-Methylenedianiline
4-Aminoazobenzene
4-Methylphenol
5-Nitro-o-anisidine
Acetaldehyde
Acetamide
Acetone
Acetonitrile
Acetophenone
Acrolein
Acrylamide
Acrylic acid
Acrylonitrile
Aldrin
Aniline
Anthracene
Atrazine
Benzamide
Benzene
Benzidine
Benzo(a)pyrene
Benzyl chloride
Benzenehexachloride)
Biphenyl
Bis(2Chloroethoxy)methane
Bromoethane
Captan
Carbaryl
Carbon disulfide
Carbon tetrachloride
Catechol
Chlordane
Chloroacetic acid
Chlorobenzene
Chlorodibenzodioxins,
various
Chlorodibenzofurans
o-,m-,p-Cresols
Cumene
Cyclohexane
Diazomethane
Dibenzofuran
Dichlorvos
Dicofol
Diepoxybutane
Diethanolamine
Dimethyl phthalate
Disulfoton
Endosulfan
Epichlorohydrin
Ethylbenzene
Ethylene glycol
Ethylene thiourea
Fluometuron
Formaldehyde
Hexachlorobenzene
Hexachloroethane
Hexane
Hydroquinone
Isophorone
Isopropyl alcohol
Lindane
Malathion
Maneb
Mechlorethamine
Melamine
Methanol
Methoxychlor
Methyl acrylate
Methyl isocyanate
Methyl tert-butyl ether
Methylene bromide
Methylhydrazine
Mirex
Mustard gas
Nitrilotriacetic acid
Nitrobenzene
Nitrofen
Nitrogen mustard
Nitroglycerin
Nitrophenol
n-Butyl alcohol
n-Dioctyl phthalate
N-Nitrosodiethylamine
N-Nitrosopiperidine
N-Nitroso-N-ethylurea
Octachloronaphthalene
Octane
Oxirane
o-Anisidine
hydrochloride
o-Nitroaniline
o-Toluidine
hydrochloride
Parathion (DNTP)
PCBs
Pentachlorobenzene
Pentachlorophenol
Phenanthrene
Phosgene
Phthalic anhydride
Polybrominatedbiphenyls
Beta-Propoxur
Pyrene
p-Chloro-m-cresol
Quinone
Quintozene
Safrole
Set-Butyl alcohol
Sevin (carbaryl)
Styrene
Terephthalic acid
Tert-Butyl alcohol
Tetrachlorvinphos
Tetrahydrofuran
Thioacetamide
Thiourea
Toluene
Toluene diisocyanate
Total xylenes
Toxaphene
Triaziquone
Trichlorfon
Trifluralin
Urethane (ethyl carbamate)
Vinyl bromide
Vinyl chloride
Vinylidene chloride
Xylene (mixed isomers)
Zineb
Table 1.2. Organic compounds that are included in various lists of hazardous substances identified by
the U.S. EPA
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SOLAR DETOXIFICATION
In any case, a consensus exists that the environmental impact of a given contaminant depends
on the degree of exposure (its dispersion and the resulting concentration in the environment)
and on its toxicological properties. The assessment of exposure involves comprehension of
the dispersion of a chemical in the environment and estimation of the predicted concentration
to which organisms will be exposed. For example, the pesticide fenaminphos oxidizes very
quickly (half-life 10 days) into sulphoxide and sulphone, while its pesticidal properties remain
unaffected. A half-life of 70 days has been found for degradation of fenaminphos and its two
metabolites. Furthermore, the two metabolites are more mobile (soluble) than fenaminphos
(Hayo and Werf, 1996). Assessment of the contaminant’s effect involves summarizing data
on the effects of the chemical on selected representative organisms and using these data to
predict a no-effect concentration on a specific niche. Organisms may consume chemicals
through ingestion of food and water, respiration and through contact with skin. When a
chemical crosses the various barriers of the body, it reaches the metabolic tissue or a storage
depot. Toxicity of a chemical is usually expressed as the effective concentration or dose of the
material that would produce a specific effect in 50% of a large population of test species
(EC50 or ED50). If the effect recorded is lethal, the term LC50 (or LD50) is used. The ‘no
observed effect level’ (NOEL or NOEC) is the dose immediately below the lowest level
eliciting any type of toxicological response in the study. For example, the pesticide
methamidophos, which has been classified as a Restricted-Use Pesticide (RUP) by the U.S.
EPA, is highly toxic for mammals (acute oral LD50 = 16 mg/kg in rats and 30-50 mg/kg in
guinea pigs), birds (bobwhite quail 8-11 mg/kg) and bees. The 96-hour LC50 is 25-51 mg/L in
rainbow trout, but concentrations as low as 0.22 ng/L are lethal to larval crustaceans in 96hour toxicity tests. A 56-day rat feeding study resulted in a NOEL of 0.03mg/kg/day (Tomin,
1994).
Decontamination of drinking water is mainly by procedures that combine flocculation,
filtration, sterilization and conservation, to which a limited number of chemicals are added.
Normal human sewage water can be efficiently treated in conventional biological processing
plants. But very often, these methods are unable to reduce the power of the contaminant. In
these cases, some form of advanced biological processing is usually preferred in the treatment
of effluents containing organic substances. Biological treatment techniques are well
established and relatively cheap. However, these methods are susceptible to toxic compounds
that inactivate the waste degrading microorganisms. To solve this problem, apart from
reducing emissions, two main water treatment strategies are followed: (i) chemical treatment
of drinking water, contaminated surface and groundwater and (ii) chemical treatment of waste
waters containing biocides or non-biodegradable compounds.
Chemical treatment of polluted surface and groundwater or wastewater, is part of a long-term
strategy to improve the quality of water by eliminating toxic compounds of human origin
before returning the water to its natural cycles. This type of treatment is suitable when a
biological processing plant cannot be adapted to certain types of pollutants that did not exist
when it was designed. In such cases, a potentially useful approach is to partially pre-treat the
toxic waste by oxidation technologies to produce intermediates that are more readily
biodegradable. Light can be used, under certain conditions, to encourage chemicals to break
down the pollutants to harmless by-products. Light can have a dramatic effect on a molecule
or solid, because, when it absorbs light, its ability to lose or gain electrons is often altered.
This electronically excited state is both a better oxidizing and a better reducing agent than its
counterpart in the ground. Electron transfer processes involving excited-state electrons and
the contact medium (for example water) can therefore generate highly reactive species like
5
SOLAR DETOXIFICATION
hydroxide (•OH) and superoxide (O2•-) radicals (see Table 1.3). These can then be used to
chemically decompose a pollutant into harmless end-products. Alternatively, light can be used
directly to break up pollutant molecule bonds photolytically. These processes are called
Advanced Oxidation Processes (abbreviated as AOPs). Many oxidation processes, such as
TiO2/UV, H2O2/UV, Photo-Fenton and ozone processes (O3, O3/UV, O3/H2O2) are currently
employed for this purpose.
Oxidizing reagent
Oxidation Potential, V
Fluorine
3.06
Hydroxide radical (•OH)
2.80
Ozone
2.07
Hydrogen peroxide
1.77
Chlorine dioxide
1.57
Chlorine gas
1.36
Oxygen
1.23
Hypochlorite
0.94
Iodine
0.54
Superoxide radical (O2•-)
-0.33
Table 1.3. Oxidation potentials of common substances and agents for pollution abatement.
The more positive the potential, the better the species is an oxidizing agent
1.3 PHODEGRADATION PRINCIPLES
1.3.1 Definitions
For the benefit of those who may have a limited background in photochemistry, a brief outline
of some basic concepts of photochemistry is presented here. In order for photochemistry to
take place, photons of light must be absorbed. The energy of a photon is given by
hc
U=
(1.1)
λ
where h is Planck’s constant (6.626 10-34 J s), c is the speed of light and λ is the wavelength.
For a molecule’s bond to be broken, U must be greater than the energy of that bond.
When a given wavelength λ of light enters a medium, its spectral irradiance Eλ (W m-2 nm-1)
is attenuated according to the Lambert-Beer law, which is expressed in two ways, one for gas
phase and the other for liquid phase:
ln( E λo / E λl ) = α λ p i l
6
gas phase
(1.2)
SOLAR DETOXIFICATION
log ( E λo / E λl ) = ε λ c i l
liquid phase (1.3)
Eλo and Eλl are the incident spectral irradiances and at a distance l into the medium, αλ is the
absorption coefficient (cm-1 atm-1), pi is the partial pressure (atm) of component i, ελ is the
extinction coefficient (M-1 cm-1), and ci is the concentration (M) of component i. The
absorbence Aλ at wavelength λ is the product ελcil. The photochemical quantum yield (φ) is
defined as the number of molecules of target compound that reacts divided by the number of
photons of light absorbed by the compound, as determined in a fixed period of time.
Normally, the unit is the maximum quantum yield attainable.
The term photocatalysis implies the combination of photochemistry with catalysis. Both light
and catalyst are necessary to achieve or to accelerate a chemical reaction. Photocatalysis may
be defined as the “acceleration of a photoreaction by the presence of a catalyst”.
Heterogeneous processes employ semiconductor slurries for catalysis, whereas homogeneous
photochemistry is used in a single-phase system. Any mechanistic description of a
photoreaction begins with the absorption of a photon, being sunlight the source of photons in
solar photocatalysis. In the case of homogeneous photocatalytic processes, the interaction of a
photon-absorbing species (transition metal complexes, organic dyes or metalloporphyrines), a
substrate (e.g. the contaminant) and light can lead to a chemical modification of the substrate.
The photon-absorbing species (C) is activated and accelerates the process by interacting
through a state of excitation (C*). In the case of heterogeneous photocatalysis, the interaction
of a photon produces the appearance of electron/hole (e- and h+) pairs, the catalyst being a
semiconductor (e.g. TiO2, ZnO, etc). In this case, the excited electrons are transferred to the
reducible specimen (Ox1) at the same time that the catalyst accepts electrons from the
oxidizable specimen (Red2) which occupies the holes. In both directions, the net flow of
electrons is null and the catalyst remains unaltered.
C
hν

→ C*
(1.4)
C* + R
→
R* + C*
(1.5)
R*
→
P
(1.6)
hν

→ C(e − + h + )
(1.7)
→ Ox 2
(1.8)
→
(1.9)
C
h + + Red 2
e − + Ox1
Red1
1.3.2. Heterogeneous photocatalysis
The concept of heterogeneous photocatalytic degradation is simple: the use under irradiation
of a stable solid semiconductor for stimulating a reaction at the solid/solution interface. By
definition, the solid can be recovered unchanged after many turnovers of the redox system.
When a semiconductor is in contact with a liquid electrolyte solution containing a redox
couple, charge transfer occurs across the interface to balance the potentials of the two phases.
An electric field is formed at the surface of the semiconductor and the bands bend as the field
forms from the bulk of the semiconductor towards the interface. During photoexcitation (a
photon with appropriate energy is absorbed), band bending provides the conditions for carrier
separation. In the case of semiconductor particles, there is no ohmic contact to extract the
majority carriers and to transfer them by an external conductor to a second electrode. This
7
SOLAR DETOXIFICATION
means that the two charge carriers should react at the semiconductor/electrolyte interface with
the species in solution. Under steady state conditions the amount of charge transferred to the
electrolyte must be equal and opposite for the two types of carriers. The semiconductormediated redox processes involve electron transfer across the interface. When electron/hole
pairs are generated in a semiconductor particle, the electron moves away from the surface to
the bulk of the semiconductor as the hole migrates towards the surface (see Figure 1.3). If
these charge carriers are separated fast enough they can be used for chemical reactions at the
surface of the photocatalyst, i.e., for the oxidation or reduction of pollutants.
hν
recombination
Red1
Oxid2
Oxid1
Red2
recombination
Figure 1.3. Fate of electrons and holes within a particle of illuminated semiconductor in
contact with an electrolyte.
Metal oxides and sulphides represent a large class of semiconductor materials suitable for
photocatalytic purposes. Table 1.4 lists some selected semiconductor materials, which have
been used for photocatalytic reactions, together with band gap energy required to activate the
catalyst. The final column in the table indicates the wavelength of radiation required to
activate the catalysts. According to Plank’s equation, the radiation able to produce this gap
must be of a wavelength (λ) equal or lower than that calculated by Eq. 1.10.
λ=
hc
EG
(1.10)
where EG is the semiconductor band-gap energy, h is Planck’s constant and c is the speed of
light.
8
Material
Band gap (eV)
Wavelength corresponding to band gap (nm)
BaTiO3
3.3
375
CdO
2.1
590
CdS
2.5
497
CdSe
1.7
730
Fe2O3
2.2
565
SOLAR DETOXIFICATION
GaAs
1.4
887
GaP
2.3
540
SnO2
3.9
318
SrTiO3
3.4
365
TiO2
3.0
390
WO3
2.8
443
ZnO
3.2
390
ZnS
3.7
336
Table 1.4. Selected properties of several semiconductors
Summarizing, a semiconductor particle is an ideal photocatalyst for a specific reaction if:
• The products formed are highly specific.
• The catalyst remains unaltered during the process.
• The formation of electron/hole pairs is required (generated by the absorption of photons
with energy greater than that necessary to move an electron from the valence band to the
conduction band)
• Photon energy is not stored in the final products, being an exothermic reaction and only
kinetically retarded.
1.3.3. Homogeneous photodegradation
The use of homogeneous photodegradation (single-phase system) to treat contaminated waters
dates back to the early 1970s. The first applications concerned the use of UV/ozone and
UV/H2O2. The use of UV light for photodegradation of pollutants can be classified into two
principal areas:
• Photooxidation. Light-driven oxidative processes principally initiated by hydroxyl
radicals.
• Direct photodegradation. Light-driven processes where degradation proceeds following
direct excitation of the pollutant by UV light.
Photooxidation involves the use of UV light plus an oxidant to generate radicals. The
hydroxyl radicals then attack the organic pollutants to initiate oxidation. Three major oxidants
are used: hydrogen peroxide (H2O2), ozone and Photo-Fenton reaction. H2O2 absorbs fairly
weakly in the UV region with increasing absorption as the wavelength decreases. At 254 nm,
ελ is 18 M-1 cm-1, whereas at 200 nm is 190 M-1 cm-1. The primary process for absorption of
light below 365 nm is dissociation to yield two hydroxyl radicals:
hν
H 2 O2 →
2 • OH
(1.11)
The use of hydrogen peroxide is now very common for the treatment of contaminated water
due to several practical advantages: (i) the H2O2 is available as an easily handled solution that
can be diluted in water to give a wide range of concentrations; (ii) there are no air emissions;
(iii) a high-quantum yield of hydroxyl radicals is generated (0.5). The major drawback is the
low molar extinction coefficient, which means that in water with high UV absorption the
fraction of light absorbed by H2O2 may be low unless very large concentrations are used.
Furthermore, especially as concerns the focus of this text, H2O2 absorption is very low in the
Solar UV range (up 300 nm).
9
SOLAR DETOXIFICATION
Ozone is generated as a gas in air or oxygen in concentrations generally ranging from 1 to 8%
(v/v). It has a strong absorption band centered at 260 nm with ελ = 3000 M-1 cm-1. Absorption
of light at this wavelength leads to formation of H2O2:
hν
O3 →
O (1D ) + O2
(1.12)
O ( 1D ) + H 2 O 
→ H 2 O2
(1.13)
Hydroxyl radicals are then formed by reaction of ozone with the conjugate base of hydrogen
peroxide:
H 2 O2 + H 2 O 
→ HO2− + H 3 O +
(1.14)
HO2− + O3 
→ O3− + HO2•
(1.15)
O3− + H 2 O 
→ HO3• + OH −
(1.16)
HO3• 
→ • OH + O2
(1.17)
Since the net result of ozone photolysis is the conversion of ozone into hydrogen peroxide,
UV-ozone would appear to be only a rather expensive method of making hydrogen peroxide.
However, there are other oxidation-related processes occurring in solution, such as the direct
reaction of ozone with a pollutant (see Table 1.3). Ozone may have advantages in water with
high inherent UV absorbence, but it involves the same problem as hydrogen peroxide for use
in solar energy processes.
The essential step of the Fenton reaction is the same as for all AOPs. Highly reactive radicals
(like HO• and HO2•) oxidize nearly all organic substances to yield CO2, water and inorganic
salts. In the case of Photo-Fenton, Fe2+ ions are oxidized by H2O2 while one •OH is produced
(1.18), and the Fe3+ or complexes obtained then act as the light absorbing species that produce
another radical while the initial Fe2+ is recovered (1.19 and 1.20).
Fe 2+ + H 2 O2 → Fe 3+ + OH − + OH •
(1.18)
Fe 3+ + H 2 O + hν → Fe 2 + + H + + OH •
(1.19)
[ Fe(OOC − R)] 2 + + hν → Fe 2 + + CO2 + R •
(1.20)
Note that in equation (1.20) the ligand R-COO– can be replaced by other organic groups
(ROH, RNH2 etc.). Compared to other homogeneous photooxidation processes, the
advantages of Photo-Fenton are the improved light sensitivity (up to a wavelength of 600 nm,
corresponding to 35% of the solar radiation). On the other hand, disadvantages, such as the
low pH values required (usually below pH 4) and the necessity of removing iron after the
reaction, remain.
Some pollutants are able to dissociate only in the presence of UV light. For this to happen, the
pollutant must absorb light emitted by a lamp (or the sun) and have a reasonable quantum
yield of photodissociation. Organic pollutants absorb light over a wide range of wavelengths,
but generally absorb more strongly at lower wavelengths, especially below 250 nm (Figure
1.4). In addition, the quantum yield of photodissociation tends to increase at lower
wavelengths, since the photon energy is increasing (eq. 1.1). The net chemical result of
photodissociation is usually oxidation, since the free radicals generated can react with
dissolved oxygen in the water. In practice, the range of waste waters that can be successfully
treated by UV alone is very limited. This defect is more relevant when solar energy is used
(see Figure 1.4) because only photons up 300 nm are available.
10
SOLAR DETOXIFICATION
Figure 1.4. UV spectra between 200 and 400 nm of Acrinathrin and sunlight.
1.4 APPLICATION TO WATER TREATMENT
As mentioned above, UV light can be used in several ways. But direct photolysis can occur
only when the contaminant to be destroyed absorbs incident light efficiently. In the case of
UV/ozone and UV/hydrogen peroxide this does not happen. But here too, absorption by some
sensitizer must initiate the reaction, and limited absorption by the solute or the additive
restricts efficiency. Furthermore, these mixtures often still require large quantities of added
oxidant. By contrast, in heterogeneous photocatalysis, dispersed solid particles absorb larger
fractions of the UV spectrum efficiently and generate chemical oxidants in situ from
dissolved oxygen or water (see Figure 1.5). These advantages make heterogeneous
photocatalysis a particularly attractive method for environmental detoxification. The most
important features of this process making it applicable to the treatment of contaminated
aqueous effluents are:
• The process takes place at ambient temperature.
• Oxidation of the substances into CO2 is complete.
• The oxygen necessary for the reaction is obtained from the atmosphere.
• The catalyst is cheap, innocuous and can be reused.
• The catalyst can be attached to different types of inert matrices.
hν ≥ 3.0eV
O2-•
O2
e-
TiO2 Particle
h+
H2O
WATER
•
OH + H+
Figure 1.5. Effect of UV radiation on a TiO2 particle dispersed in water
For all of the above reasons, from now on only this method is dealt with in this text.
Whenever different semiconductor materials have been tested under comparable conditions
for the degradation of the same compounds, TiO2 has generally been demonstrated to be the
11
SOLAR DETOXIFICATION
most active. Only ZnO is as active as TiO2. TiO2’s strong resistance to chemical and
photocorrosion, its safety and low cost, limits the choice of convenient alternatives (Pelizzetti,
1995). Furthermore, TiO2 is of special interest since it can use natural (solar) UV. This is
because it has an appropriate energetic separation between its valence and conduction bands
which can be surpassed by the energy content of a solar photon (see Table 1.4). Other
semiconductor particles, e.g., CdS or GaP absorb larger fractions of the solar spectrum and
can form chemically activated surface-bond intermediates, but unfortunately, these
photocatalysts are degraded during the repeated catalytic cycles involved in heterogeneous
photocatalysis. Therefore, degradation of the organic pollutants present in waste water using
irradiated TiO2 suspensions is the most promising process and R&D in this field has grown
very quickly during the last years.
hν
TiO2 →
e − + h + + TiO2
(1.21)
e − + h + + TiO2 
→ TiO2 + heat and / or hν '
(1.22)
+
(TiO IV −O 2 − −Ti IV ) −OH 2 + hBV

→ (TiO IV −O 2 − −Ti IV ) −OH • + H +
−

→ O2−(•ads )
O2( ads ) + eBC
(1.23)
(1.24)
To date, evidence supports the idea that the hydroxyl radical (•OH) is the main oxidizing
specimen responsible for photooxidation of the majority of the organic compounds studied.
The first effect, after absorption of near ultraviolet radiation, λ<390 nm, is the generation of
electron/hole pairs, which are separated between the conduction and valence bands (Eq. 1.21).
In order to avoid recombination of the pairs generated (Eq. 1.22), if the dissolvent is
oxidoreductively active (water) it also acts as a donor and acceptor of electrons. Thus, on a
hydrated and hydroxylated TiO2 surface, the holes trap •OH radicals linked to the surface (Eq.
1.23). In any case, it should be emphasized that even trapped electrons and holes can rapidly
recombine on the surface of a particle (Eq. 1.22). This can be partially avoided through the
capture of the electron by preadsorbed molecular oxygen, forming a superoxide radical (Eq.
1.24).
Whatever the formation pathway, it is well known that O2 and water are essential for
photooxidation with TiO2. There is no degradation in the absence of either. Furthermore, the
oxidative species formed (in particular the hydroxyl radicals) react with the majority of
organic substances. For example, in aromatic compounds, the aromatic part is hydroxylated
and successive steps in oxidation/addition lead to ring opening. The resulting aldehydes and
carboxylic acids are decarboxylated and finally produce CO2. However, the important issue
governing the efficiency of photocatalytic oxidative degradation is minimizing electron-hole
recombination by maximizing the rate of interfacial electron transfer to capture the
photogenerated electron and/or hole. This issue is discussed in more detail later.
Degradation by photocatalysis has been most investigated in monoaromatics and
consequently, these pollutants appear as model compounds in dozens of scientific papers.
Some monoaromatics investigated have been benzene, dimethoxybenzenes, halobenzenes,
nitrobenzene, chlorophenols, nitrophenols, benzamide, aniline, etc., most of which are
recognised as priority pollutants (see Table 1.2). In addition to these, several other types of
molecules have also been investigated as substrates for photocatalytic degradation:
• Haloaliphatics (trichloroethylen, tetrachloromethane, etc.). Important because so many of
these compounds have been released into the environment and contaminate waters. Some
also originate during water treatment by chlorinating.
12
SOLAR DETOXIFICATION
•
•
•
•
Water-miscible solvents (ethanol, alkoxyethanol, etc.). These compounds are very
difficult to detoxify since they are resistant to treatment and are poorly adsorbed on GAC.
Pesticides. Contaminate waters where agricultural runoff is important. Among the recently
investigated compounds are triazines, organophosphorous, carbamates, phenoxyacids,
organochlorines, chloronicotinics, etc.
Surfactants. Surface active agents enter domestic and industrial waste waters in increasing
amounts. Because their biodegradability may be one of the more important constraints in
their use, photocatalytic degradation has received increasing attention.
Dyes. Strongly colored compounds can be removed by adsorption but it is always better to
destroy them by oxidation.
Three exhaustive reviews by Blake (1994, 1995, 1997) describe almost 1800 studies carried
out before 1996.
Despite encouraging laboratory-scale data and some industrial-scale tests, chemical oxidation
detoxification is still restricted to a few experimental plants. The broader application of those
technologies requires: i) reactor optimization and modeling and ii) assessment of the
efficiency of oxidation technology to reduce the toxicity of effluents. The following chapters
of this book will attempt to highlight these matters.
1.5 GAS-PHASE DETOXIFICATION
Airborne pollutants (such as volatile compounds) can be treated during the gas phase with the
UV/TiO2 process. Gas-phase treatment offers several advantages. In general, substrate masstransport is an order of magnitude faster in the gas phase than in the liquid phase. This in turn
leads to much faster reaction rates. Oxidant starvation (such as O2 supply) may be less of a
problem in the gas-phase than in water. There is also no interference on the photocatalytic
surface from other species that are invariably present in aqueous treatment media (for
example anions). In addition, photocatalysis separation after use is not a problem unlike with
aqueous slurry suspensions. By using solar energy to drive the process, no fuel is required,
gaseous affluent volume is reduced, no NOx is generated, no products of incomplete
combustion are produced, CO2 emanating from fuel burning is avoided and substantial fuel
saving may be achieved. Since no burning takes place, oxygen is only necessary at
stoichiometric ratio. Solar concentrators provide the opportunity for small-size solar furnaces
and even mobile solar parabolic dishes for on-site destruction of low productions of highly
toxic compounds
On the other hand, there are indications that mineralization may not be complete with some
organic substrates in the gas-phase. The TiO2 photocatalyst loses its activity after prolonged
use and must be reactivated with moist air that presumably restores the original degree of
hydroxylation on the oxide surface. There are also indications that product (or intermediate)
adsorption on the TiO2 surface may be problematic during the course of the reaction.
Pollutant substrates like trichloroethylene, acetone, formaldehyde, m-xylene and Nox have
been treated with TiO2/UV in the gas-phase in bench-scale tests. Field tests have also been
conducted to treat effluent air emissions using this technique at different manufacturing plants
in the USA (Rajeshwar, 1996).
13
SOLAR DETOXIFICATION
Class of Compound
Chemicals Tested
Aromatics
Benzene, Toluene
Nitrogen-containing ring
Pyridine, Picoline, Nicotine
Aldehydes
Acetaldehyde. Formaldehyde
Ketones
Acetone
Alcohols
Methanol. Ethanol, Propanol
Alkanes
Ethylene. Propene, Tetramethyl Ethylene
Terpenes
α-Pinene
Sulfur-containing Organics
Methyl Thiophene
Chlorinated E!hylenes
Dichloroethylene.Trichloroethylene.Tetrachloroetylene
Acetyl Chlorides
Dichloroacetyl Chloride, Tetrachloroacetyl Chloride
Table 1.5. VOCs amenable to treatment via Photocatalytic Oxidation (Jakobi et al., 1996).
SUMMARY OF THE CHAPTER
A description is given of how solar chemistry could become a significant segment of the
chemical industry and how it can be used, under certain conditions, to provoke chemical
breakdown of pollutants into harmless by-products. The behaviour of contaminants in
environmental water is summarised. The basic concepts of photochemistry relating to
photolysis of chemical bonds, homogeneous photodegradation and heterogeneous
photocatalysis are reviewed. The use of semiconductors for wastewater treatment, with
particular reference to TiO2, has been discussed. Examples of the waste materials that have
been treated successfully using TiO2, have been presented. Gas-phase photocatalysis has also
been introduced.
BIBLIOGRAPHY AND REFERENCES
Blake, D.M.; Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds
from Water and Air. National Technical Information Service, US Depart. of
Commerce, Springfield, VA22161, USA, May 1994. Update Number 1 To June 1995,
October 1995. Update Number 2 To October 1996, January 1997.
Hayo, M.G. and van der Werf. Assessing the impact of pesticides on the environment. Agric.
Ecosys. Environ., 60, 81-96, 1996.
Jacoby, W.A., Blake, D.M., Fennell, J.A., Boulter, J.E., Vargo, L.M., George, M.C. and
Dolberg, S. K. Heterogeneous Photocatalysis for Control of Volatile Organic
Compounds in Indoor Air. J. Air Waste Manage. Assoc. 46, no. 9, 891-8, 1996.
14
SOLAR DETOXIFICATION
National Renewable Energy Laboratory, Solar Photochemistry-Twenty Years of Progress,
What’s Been Accomplished, and Where Does It Lead? Report NREL/TP-433-7209,
Golden, Colorado, USA, 1995.
Pelizzetti, E. Concluding Remarks on Heterogeneous Solar Photocatalysis. Solar En. Mat.
Sol. Cells, 38, 453-457, 1995.
Rajeshwar, K. Photochemical Strategies for Abating Environmental Pollution. Chemistry &
Industry, 17, 454-458, 1996.
Tomin, C. The Pesticide Manual, a World Compendium. 10th Edition. British Crop Protection
Council. Croydon, UK, 1994.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. The solar energy is useful only to substitute fossil fuels converting it into thermal energy
thus provoking chemical reactions.
2. Toxicity of a chemical is the same for all the species.
3. Biological treatment techniques are the cheapest wastewater treatment methods.
4. The energy of a photon depends of the ambient temperature.
5. Heterogeneous photocatalysis employs liquid catalysts.
6. Light driven oxidative processes are initiated by excited electrons of the catalyst surface.
7. Ozone can be produced from air.
8. The most important characteristics of a photocatalysts are: stability to chemical and
photocorrosion, safety, cost and band-gap.
9. The electron/hole recombination can be avoided increasing reaction temperature.
10. Heterogeneous photocatalysis can be applied only to monoaromatics.
PART B.
1. Which is the most important difference between thermochemical and photochemical solar
processes?
2. Which are the usual ways to express the toxicity of a chemical in the environment?
3. Why biodegradation, which is a major mechanism in wastewater treatment, is quite
inefficient to treat certain types of wastewater?
4. What is the percentage of absorbed photons in a solution with the following
characteristics: extinction coefficient = 1327 cm-1 M-1, concentration of substrate 0.01 M,
illuminated pathlength = 5.6 cm? And if the extinction coefficient is 0.3?
5. What is the wavelength able to excite a semiconductor which band-gap is 4.0 eV?
6. Name three important characteristics of heterogeneous photocatalysis to be used as water
treatment process.
7. Why TiO2 is the most suitable photocatalyst for wastewater treatment?
8. Which is the more important electron acceptor in water?
9. Which is the most important product of photocatalytic degradation with organic
contaminants?
10. Why hydroxyl radicals react with organic substances?
Answers
Part A
15
SOLAR DETOXIFICATION
1. False; 2. False; 3. True; 4. False; 5. False; 6. False; 7. True; 8. True; 9. False; 10. False.
Part B
1. In thermochemical processes solar radiation is converted into thermal energy, in
photochemical processes the solar photons are absorbed directly by the reactants giving
rise to the reaction.
2. Toxicity of a chemical is usually expressed as the effective concentration or dose of the
material that would produce a specific effect in 50% of a large population of test species
(EC50 or ED50).
3. Because when compounds are very toxic, the micro-organisms need an extended period of
adaptation, when they are not invaible.
4. 100% and 3.8 %.
5. λ ≤ 310 nm
6. The process takes place at ambient temperature, the oxygen necessary for the reaction is
obtained from the atmosphere, the catalyst is cheap, innocuous and can be reused.
7. It has exhibited the highest activity. It is high stable to chemical and photocorrosion. It
can use natural UV.
8. Dissolved oxygen.
9. Carbon dioxide.
10. Because of its very high oxidation potential.
16
SOLAR DETOXIFICATION
2. SOLAR IRRADIATION
AIMS
This unit describes the power of light as a source of energy. It outlines the basic principles
that are related to the light spectrum and specifically to the solar spectrum. This chapter
discusses solar UV radiation and its photon flux in more detail, because this part of the solar
spectrum is the most important for driving chemical processes. Moreover, the major
atmospheric variables determining the amount of UV solar radiation on the earth’s surface are
discussed. A method for calculating UV attenuation at a given site is presented. Finally, solar
radiation measurement systems are described.
OBJECTIVES
At the end of this unit, you will understand the main factors affecting solar radiation
behaviour and you will be able to do six things:
1. Discriminate between the different components of solar radiation and their principal
characteristics.
2. Recognize typical solar spectra and understand the effect of sun position on the solar
power reaching the earth’s surface.
3. Find the photon flux of a polychromatic source of energy with simple calculations.
4. Describe the most important components of the earth’s atmosphere and the consequences
for power and spectral distribution of the solar radiation.
5. Understand the procedures that permit solar power to be calculated from available
radiation at any given site.
Comprehend the basic principles on which solar radiation measurement is based.
NOTATION AND UNITS
Symbol
AM
Air mass ratio
fn
Clouds factor
Fraction of power associated with a wavelength
fλ
Radiance exposure monthly average
H
TBDUV radiance exposure
H TBD
I
Photon flux density
Na
Quantity of photons absorbed by the system
N0
Avogadro’s number, 6.023 x 1023
Nλ
Number of photons supplied by a source of light of wavelength λ
Qλ
Energy of a monochromatic source of light of wavelength λ
T
Transmittance
Transmittance of direct-bean solar radiation under cloudless skies
Tλ
at a specific wavelength
Transmittance related to absorption and dispersion by aerosols
Ta,λ
Transmittance resulting from absorption of atmospheric gases
Tg,,λ
Transmittance related to the effect of the ozone layer
To,λ
Transmittance related to the molecules of air
TR,λ
Transmittance resulting from absorption by steam.
Tv,,λ
Units
nm-1
kJ m-2
kJ m-2
Einstein s-1 m-2
Photons s-1
Photons mol-1
Photons s-1
W m-2 µm-1
17
SOLAR DETOXIFICATION
TBDUV Typical “best day”. Completely clear sky during all the hours of
sunlight
Energy of one photon
Uλ
UVD
Direct ultraviolet light
UVG
Global ultraviolet light
Ultraviolet irradiance associated with a wavelength
UVλ
Quantum yield
Φ
Wavelength
λ.
eV, J
W m-2
W m-2
W m-2 nm-1
No units
nm, µm
2.1 THE POWER OF LIGHT
Light is just one of various electromagnetic waves present in space. The electromagnetic
spectrum covers an extremely broad range, from radio wavelengths of a meter or more, down
to x-rays with wavelengths of less than one billionth of a meter. Optical radiation lies between
radio waves and x-rays on that spectrum and has a unique combination of ray, wave, and
quantum properties. At x-ray and shorter wavelengths, electromagnetic radiation tends to be
quite particle-like in its behaviour, whereas toward the long wavelength end of the spectrum
behaviour is mostly wavelike. The UV-visible portion occupies an intermediate position,
having both wave and particle properties in varying degrees (See Figure 2.1a).
a)
X-rays
M icrowaves
40
Vis0ibl 77
UV
100-400 nm e 0
nm
100
b)
Infrared
770-10 6 nm
1000
W avelength λ , nanometers
10000
λ
Figure 2.1
The optical portion of the electromagnetic spectrum (a)
and light wave front modelled as a straight-line (b).
Like all electromagnetic waves, light waves can interfere with each other, become
directionally polarised, and bend slightly when passing through an edge. These properties
allow light to be filtered by wavelength or amplified coherently as in a laser. In radiometry,
light’s propagating wave front is modelled as a ray travelling in a straight line (See Figure
2.1b). Lenses and mirrors redirect these rays along predictable paths. Wave effects are
insignificant in a large-scale optical system, because the light waves are randomly distributed
and there are plenty of photons.
2.1.1 Ultraviolet light
Short wavelength UV-light exhibits more quantum properties than its visible or infrared
counterparts. Ultraviolet light is arbitrarily broken down into three bands, according to its
anecdotal effects. UV-A (315-400 nm), which is the least harmful type of UV light, because it
has the least energy (recall Eq. 1.1), is often called black light, and is used for its relative
harmlessness and its ability to cause fluorescent materials to emit visible light – thus
appearing to glow in the dark. UV-B (280-315 nm) is typically the most destructive form of
18
SOLAR DETOXIFICATION
UV light, because it has enough energy to damage biological tissues, yet not quite enough to
be completely absorbed by the atmosphere. UV-B is known to cause skin cancer. Since the
atmosphere blocks most of the extraterrestrial UV-B light, a small change in the ozone layer
could dramatically increase the danger of skin cancer. UV-C (100-280 nm) is almost
completely absorbed in air within a few hundred meters. When UV-C photons collide with
oxygen atoms, the energy exchange causes the formation of ozone. UV-C is never observed in
nature, however, since it is absorbed so quickly. Germicidal UV-C lamps are often used to
purify water because of their capability to kill bacteria.
2.1.2 Visible light
Visible light is concerned with the radiation perceived by the human eye. The lumen (lm) is
the photometric equivalent of the watt, weighted to match the eye response of the “standard
observer”. Yellowish-green light receives the greatest weight because it stimulates the eye
more than the blue or red light of equal radiometric power (1 W at 555 nm = 683.0 lumens).
To put this into perspective: the human eye can detect a flux of about 10 photons per second
at 555 nm; this corresponds to a radiant power of 3.58 x 10-18 W (or J s-1). Similarly, the eye
can detect a minimum flux of 214 and 126 photons per second at 450 nm and 650 nm,
respectively.
Blue
400
Green Yellow Red
500
600
700 nm
Figure 2.2
Visible light colours distribution
2.1.3. Infrared light
Infrared light contains the least amount of energy per photon of any other band and is unique
in that it has primarily wave properties. This can make it much more difficult to manipulate
than ultraviolet and visible light. Infrared is more difficult to focus with lenses, refract with
lenses, diffracts more, and is difficult to diffuse. Since infrared light is a form of heat, far
infrared detectors are sensitive to environmental changes – such as a person moving in the
field of view. Night vision equipment takes advantage of this effect, amplifying infrared to
distinguish people and machinery that are concealed in the darkness.
2.2 THE SOLAR SPECTRUM
All the energy coming from that huge reactor, the sun, from which the earth receives
1.7x1014 kW, meaning 1.5x1018 kWh per year, or approximately 28000 times world
consumption for one year (Figure 2.3a). Radiation beyond the atmosphere has a wavelength
of between 0.2µm and 50µm, which is reduced to between 0.3 µm and 3.0 µm when reaching
the surface due to the absorption of part of it by different atmospheric components (ozone,
oxygen, carbon dioxide, aerosols, steam, clouds). The solar radiation that reaches the ground
without being absorbed or scattered is called direct radiation; radiation that reaches the
ground but has been dispersed is called diffuse radiation, and the sum of both is called global
radiation. In other words, it is the direct radiation that produces shadow when an opaque
19
SOLAR DETOXIFICATION
object blocks it; diffuse radiation does not. In general, the direct component of global
radiation on cloudy days is minimum and the diffuse component is maximum, and the
opposite on clear days.
2000
1500
1500
Extraterrestrial
Direct Air Mass 1.5
Global 37º Air Mass 1.5
-2
Irradiance, W m µm
-1
2000
1000
1000
500
500
0
0
0.6
1.2
1.8
2.4
3.0
3.6
4.2
Wavelength, µ m
Figures 2.3a and 2.3b
(a) World solar irradiance, MWh m-2 year-1)
(b) Spectral solar radiation plotted from 0.2 to 4.5 µm
Figure 2.3b shows the standard solar radiation spectra (Hulstrom et al., 1985) at ground level
on a clear day. The dotted line corresponds to the extraterrestrial radiation in the same
wavelength interval. When this radiation enters the atmosphere, it is absorbed and scattered
by atmospheric components, such as air molecules, aerosols, water vapor, liquid water
droplets and clouds. The spectral irradiance data are for the sun at a solar zenith angle of
48.19º. This zenith angle corresponds to an air mass of 1.5, which is the ratio of the directbeam solar-irradiance path length through the atmosphere at a solar zenith angle of 48.19º to
the path length when the sun is in a vertical position. AM =1 when the sun is directly
overhead (zenith). As air mass increases, the direct beam traverses longer path lengths in the
atmosphere, which results in more scattering and absorption of the direct beam and a lower
percentage of direct-to-total radiation (for the same atmospheric conditions).
20
SOLAR DETOXIFICATION
Z enith
A
AM
Sunrise
Sunset
60º º
2
48.
M
1.
2 .0
5
A tm osphere
D iffuse
radiation
D irect
radiation
G lobal radiation
EARTH
Figure 2.4
Air mass and solar components
The AM 1.5 global irradiance is shown for a flat surface facing the sun and tilted 37º from the
horizontal. The 37º tilt angle is used because it is the latitude of the Plataforma Solar de
Almería, where most of the research presented here was done. The scarce part of the solar
spectrum that can be used in photocatalysis with TiO2 may be clearly seen (See Table 1.4)
but, as the energy source is so cheap and abundant, even under these limitations its use is of
interest.
2.3 SOLAR ULTRAVIOLET IRRADIATION
Solar ultraviolet radiation is, as explained above, only a very small part of the solar spectrum,
between 3.5% and 8% of the total of the solar spectrum, as demonstrated by measurement,
although this ratio may be different for a given location on cloudy and clear days. The
percentage of global UV radiation (direct + diffuse) generally increases with regard to total
global when atmospheric transmissivity decreases mainly because of clouds, but also because
of aerosols and dust. In fact, the average percentage ratio between UV and total radiation on
cloudy days is up to two percentage points more than values on clear days.
The efficiency of a chemical reaction is calculated from the ratio between the products and the
departing reactants. In photochemistry, it is very common to use the quantum yield concept,
which is calculated from a known amount of photons absorbed in the reaction. Quantum yield
(Φ) is defined as the ratio between the number of reacting molecules (∆n) and the quantity of
photons absorbed by the system (Na):
∆n
Φ =
(2.1)
Na
Experimentally, the quantum yield is expressed as the number of moles of reactant in an
interval of time t, divided into the number of moles of photons absorbed during the same
period. Knowledge of the quantum yield is rather important for an understanding of the
mechanism of a photochemical reaction. If every absorbed photon produces a molecular
transformation, Φ = 1. If it is less than 1, it means that deactivation processes or other
reactions competing with the one studied exist. Over 1 indicates a series of reactions the
promoter of which has been excited by a photon. In the case of photocatalysis by UV
radiation, the number of photons that reach the reacting mixture and are thereby susceptible to
21
SOLAR DETOXIFICATION
being absorbed, will be in relation to the UV solar spectrum. For reactors using solar
radiation, knowledge of the solar UV spectrum is important for the following reasons:
• The radiation (sunlight) that reaches them is not constant. This prevents correct
comparison between experiments carried out at different times of the day or seasons of the
year or under different atmospheric conditions.
• The extensive bibliography on photocatalytic decomposition of organic compounds
indicates that the majority of the experiments in which the photon flux is known are
carried out in laboratory reactors illuminated by lamps. In order to compare these results
with solar radiation or to use the information contained in those reports, it is necessary to
know the photon flux inside the solar reactor.
• The quantum yield of the reaction tested under a given experimental condition provides
information on the optimum conditions for decomposition of the contaminant. Knowledge
of the photon flux in this situation is basic to the determination of the efficiency of the
solar reactor components (reflective surface, absorber tube, control system, concentration
factor, etc.) and any possible modification, in each case, to improve photodegradation
conditions.
• Any economic comparison between solar radiation and electric lamps as the UV photon
source requires knowledge of the photon flux incident on the solar reactor.
The two spectra shown in Figure 2.5 correspond to the same spectra shown in Figure 2.3 for
the solar UV spectrum range at ground level. The shorter of them (direct UV) reaches 22 W
m-2 between 300 and 400 nm, the longer (global UV) reaches 46 W m-2.
Photon flux, 10-20 photons m-2 s-1 µm-1
The number of photons, Nλ, supplied by a monochromatic source of light with wavelength λ
and energy Qλ is related to the energy of one photon, Uλ, by Planck’s equation (Eq.1.1):
Qλ
λ
= Qλ
(2.2)
Nλ =
hc
Wλ
When a source of light is polychromatic as is solar radiation, the number of photons is given
by an integral covering the whole range of wavelengths of that source:
1 λ2
λ
N = ∫λ 2 N (λ )dλ =
(2.3)
∫ Q( λ )λdλ
1
hc λ1
20
20
15
UVG
10
8.4 x 1019 photons m-2 s-1
5
15
10
5
UVD
3.6 x 1019 photons m-2 s-1
0
0.30
0.32
0.34
0.36
0.38
0
0.40
Wavelength, µm
Figure 2.5
Ultraviolet spectra on the earth surface (standard ASTM)
Equation 2.3 gives the ratio between photonic and radiometric quantities, defining from here
22
SOLAR DETOXIFICATION
the photon flux density I [Einstein s-1 m-2] as the number of incident photons per unit of
surface and time:
d2 N
I =
(2.4)
N 0 dt dA
where N0 is Avogadro’s number (6.023 x 1023). 1 Einstein = 1 mol of photons = 6.02 x 1023
photons.
Using the spectrum data and the above equations in congruent units [S.I], it is possible to
determine the photon flux density I (ID = 3.6 x 1019 photons m-2 s-1 = 6 x 10-5 Einstein m-2 s-1,
IG = 8.4 x 1019 photons m-2 s-1 = 14 x 10-5 Einstein m-2 s-1). These two values give an idea of
the energy coming from the sun and available for photocatalytic reactions with TiO2, which
only uses the part of the UV spectrum up to 390 nm, as explained below. In any case, the UV
radiation values described vary from one location to another, and obviously, at different hours
of the day and in different seasons, making it necessary to know these data for the particular
location and in real time. This data will be very useful in those cases where this is not
possible.
2.4 ATMOSPHERIC ATTENUATION OF SOLAR RADIATION
A general expression for transmittance (T) of direct-bean solar radiation under cloudless skies
at a specific wavelength (λ) is (Iqbal, 1983):
Tλ = TR ,λ Ta ,λ To ,λ Tg ,λ Tv ,λ
(2.5)
TR,λ is the spectral transmittance resulting from the dispersion produced by molecules of air
(dimensions of many of which are ≈1Å, Raleigh dispersion). Ta,λ is the spectral transmittance
related to absorption and dispersion by aerosols (solid or liquid particles suspended in the air).
To,λ corresponds to the effect of the ozone layer. Tg,,λ is the transmittance resulting from
absorption of atmospheric gases (such as carbon dioxide and oxygen). Tv,,λ corresponds to the
absorption by water vapour. The effect of each of these parameters within the range in
question (300 nm-400 nm) would be the following (Riordan et al., 1990):
-4.08
• TR,λ = exp (-0.008735 λ
M’), where M’ is the air mass corrected according to its
density, which depends on the pressure and, therefore, the altitude. In agreement with
this, this factor would be practically constant for a given site.
-α
• Ta,λ = exp (-λ β M) where M is the mass of air, β is the coefficient of turbidity,
which usually varies between 0 and 0.5 and is a reflection of the amount of aerosols
in the air and α is an index of the size of the aerosol molecules. Depending on the
atmospheric contamination at the site, β varies differently. Where there is no
contamination it varies only slightly. The same reasoning is valid for α.
• To,λ is constant for a specific site since the ozone layer has a practically constant
thickness (for now).
• Tg,λ only influences wavelengths over UV.
• Tv,λ does not affect the UV spectrum.
Keeping in mind then, the different transmittances, it may be assumed that the solar UV
spectrum does not vary substantially within a specific site throughout the year, unless
atmospheric conditions (except clouds) do so. The dominating attenuator of solar radiation is
clouds. Under overcast skies there is no direct-beam radiation, and under partly cloudy skies
there is intermittent direct-beam radiation when clouds are not obscuring the sun’s disk.
Clouds are often assumed to have a wavelength-independent attenuation function in the UV
range; but in the near-infrared region (See Figure 2.6) they cause increased absorption due to
water vapour and liquid water (Tv,λ).
23
SOLAR DETOXIFICATION
0.7
1.2
0.6
1.0
0.5
Clear sky
Clear sky
-1
0.4
W m nm
0.6
-2
-2
W m nm
-1
0.8
0.3
0.4
0.2
0.2
0.1
Clouds
Clouds
0.0
400
600
800
1000
0.0
300
1200
325
Wavelength, nm
350
375
400
Wavelength, nm
Figure 2.6
Solar spectra on the earth surface (Plataforma Solar de Almería) between 300 and 1100 nm.
Clouds modify the total UV energy reaching the earth’s surface, but the wavelength
distribution is not affected. This cannot be guaranteed, however, if the data for all the spectra
shown in Figure 2.6 are not represented in a standardized manner as in Figure 2.7. This can be
done for any wavelength interval by the following operation. Summations have been used to
treat the discrete values nm to nm:
fλ =
UVλ
, therefore
λ = 400 nm
∑ UV
λ = 300 nm
λ = 400 nm
∑f
λ = 300 nm
λ
λ
=1
(2.6)
where fλ, is the fraction of power associated with wavelength λ and UVλ is the irradiance,
W m-2 nm-1 corresponding to each wavelength and measured with a spectroradiometer. In
Figure 2.7, the homogeneity of all the spectra recorded may be observed. If the spectrum of
UV radiation is assumed to have a fixed form, then standardized spectrum can be considered
as standard for each site. Therefore, the number of photons corresponding to this range of
wavelengths is only a function of the intensity (Measurable in real time with the radiometers,
see the following section in this chapter).
0.06
0.05
fλ, nm
-1
0.04
fλ =
UV
λ
λ = 400nm
∑ UVλ
λ
, therefore
λ = 400nm
∑ fλ = 1
λ = 300nm
= 300nm
0.03
0.02
0.01
0.00
300
Area below each curve = 1
325
350
375
400
Wavelength, nm
Figure 2.7
Normalised solar UV spectra shown in Figure 2.6.
24
SOLAR DETOXIFICATION
2.4.1 Annual available ultraviolet radiation
A general index of atmospheric transmittance due to all the processes described above is the
so-called “cloud factor” (fn). For the calculation of fn the annual average of ultraviolet
radiation must be found. Knowledge of this factor enables the amount of energy that reaches
the earth’s surface to be predicted for a given place at any time of the year. It should be noted
that the “cloud factor” for global radiation is always lower than the direct, as the diffuse
component of solar radiation is maximum when direct radiation is absent (global = direct +
diffuse). This factor is calculated (Eq. 2.7) from the ratio between average radiation (affected
by all atmospheric phenomena) and the highest attainable radiation at all times of the year.
This is usually calculated for each month separately. To find out the highest UV radiation
available each month, a typical “best day” (completely clear sky during all daylight hours) is
selected for each month (TBDUV) from among all the days for which average radiation is
calculated. The parameters taken into account in selecting the TBDUV are absence of clouds
and proximity to the 15th day of each month (See Figure 2.8). Several days per month
(corresponding to each year for period to be analyzed) must be chosen and compared to find
the day with the maximum H TBD .
 H 

f n = 1 − 
(2.7)
 H TBD 
where H TBD (kJ m-2) is the TBDUV radiance exposure (direct or global). It has been obtained
integrating the UV irradiance along the day. In this case, being discrete values, it has been
calculated integrating numerically the irradiance from sunrise to sunset. H (kJ m-2) is the
average of the month. This was calculated by multiplying the monthly average irradiance by
the monthly average of hours of sunlight. The “cloud factor” for UV radiation has to be
calculated from data collected with radiometers which are UV-specific. The UV-radiation
data base must be large enough to be considered statistically correct (at least 4-5 years).
50
June
April
UV, W m-2
40
October
30
20
January
10
0
6
8
10
12
14
16
18
20
SOLAR HOUR
Figure 2.8
TBDUV of different periods of the year at Plataforma Solar de Almería (37º N)
2.5 SOLAR RADIATION MEASUREMENT
Detectors translate light energy into an electrical current. Light striking a silicon photodiode
causes a charge to build up between internal “P” and “N” layers. When an external circuit is
connected to the cell, an electrical current is produced. This current is linear with regard to the
25
SOLAR DETOXIFICATION
incident light over a dynamic 10-decade range. A wide dynamic range is a prerequisite for
most applications. The radiometer should be able to cover the entire dynamic range of any
detector that will be plugged into it. This usually means that the instrument should be able to
cover at least 7 decades of dynamic range with minimal linearity errors. The current or
voltage measurement device should be the least significant source of error in the system. The
second thing to consider when choosing a radiometer is the type of features offered. Ambient
zeroing, integration ability, and a “hold” button should be standard. The ability to multiplex
several detectors to a single radiometer or control the instrument remotely may also be
desired. Lastly, portability and battery life may be an issue for measurements made in the
field.
Light is all around us every day, yet it remains the most elusive form of energy to measure
accurately. A single photon of light travels in a straight line in one direction, at a given
wavelength. A light bundle consists of a jumbled mixture of billions and billions of photons at
different wavelengths, going in different directions, at different moments in time. The watt
(W), the fundamental unit of optical power, is defined as a rate of energy of one joule (J) per
second. Optical power is a function of both the number of photons and the wavelength. Each
photon carries an energy that is described by Planck’s equation (Eq. 1.1). All light
measurement units are spectral, spatial or temporal distributions of optical energy. The
biggest hurdle in light measurement is the very spatial nature of light. Irradiance is a measure
of the energy density received from a light source. Since light expands outward from a point
source, the irradiance decreases with distance. The irradiance also decreases with incident
angle. Carefully designed input optics cannot prevent measurement errors caused by lax
attention to the measurement geometry upon which the units systems are based. Spectral
responsivity and detectivity present a very different problem. Many properties of light are
dependent on wavelength and the energy of one photon is inversely proportional to its
wavelength. Since a detector measures only absorbed light, it cannot differentiate between 1
photon (200 nm) and 10 photons (2000 nm). The light must be filtered by wavelength before
it reaches the detector.
Sensitivity to the band of interest is a primary consideration when choosing a detector. You
can control the peak responsivity and bandwidth through the use of filters, but you must have
an adequate signal to start with. Filters can suppress out-of-band light but cannot boost signal.
Another consideration is blindness to out-of-band radiation. If you are measuring solar
ultraviolet in the presence of massive amounts of visible and infrared light, for example, you
would select a detector that is insensitive to the long wavelength light that you intend to filter
out. Lastly, linearity, stability and durability are considerations. Some detector types must be
cooled or modulated to remain stable. High voltages are required for other types. In addition,
some can be burned out by excessive light, or have their windows permanently ruined by a
fingerprint.
2.5.1 Detectors
Planar-diffusion-type silicon photodiodes are perhaps the most versatile and reliable sensors
available. The P-layer material at the light sensitive surface and the N material at the substrate
from a P-N junction which operates as a photoelectric converter, generating a current that is
proportional to the incident light. Silicon cells operate linearly over a ten-decade dynamic
range, and remain true to their original calibration longer than any other type of sensor. For
this reason, they are used as transfer standards at the NIST (National Institute of Standards
and Technology, USA).
26
SOLAR DETOXIFICATION
The phototube is a light sensor that is based on the photoemissive effect. The phototube is a
bipolar tube which consists of a photoemissive cathode surface that emits electrons in
proportion to incident light, and an anode which collects the electrons emitted. The anode
must be biased at high voltage (50 to 90 V) in order to attract electrons to jump through the
vacuum of the tube. Some phototubes use a forward bias of less than 15 volts, however. The
cathode material determines the spectral sensitivity of the tube. Solar-blind vacuum
photodiodes use Cs-Ta cathodes to provide sensitivity only to ultraviolet light, providing as
much as a million to one long wavelength rejections. A UV glass window is required for
sensitivity in the UV down to 185 nm, with fused silica windows offering transmission down
to 160 nm.
The thermopile is a heat sensitive device that measures radiated heat. Infrared light contains
the least amount of energy per photon of any other band. Because of this, an infrared photon
often lacks the energy required to pass the detection threshold of a quantum detector. Infrared
is usually measured using a thermal detector such as a thermopile, which measures
temperature change due to absorbed energy. While these thermal detectors have a very flat
spectral responsibility, they suffer from temperature sensitivity, and usually must be
artificially cooled. The sensor is usually sealed in a vacuum to prevent heat transfer except by
radiation. A thermopile consists of a number of thermocouple junctions in series, which
convert energy into a voltage using the Peltier effect. Thermopiles are convenient sensors for
measuring the infrared, because they offer adequate sensitivity and a flat spectral response in
a small package. More sophisticated bolometers and pyroelectric detectors need to be chopped
and are generally used only in calibration labs.
The best method of operating a thermal detector is by chopping incident radiation, so that drift
is zeroed out by the modulated reading. The quartz window in most thermopiles is adequate
for transmitting from 200 to 4200 nm, but for long wavelength sensitivity out to 40 microns,
Potassium Bromide windows are used. Another strategy employed by thermal detectors is to
modulate incident light with a chopper. This allows the detector to measure differentially
between the dark (zero) and light states. Quantum-type detectors are often used in the near
infrared, especially below 1100 nm. Specialized detectors such as InGaAs offer excellent
responsivity from 850 to 1700 nm. Typical silicon photodiodes are not sensitive above 1100
nm. These types of detectors are typically employed to measure a known artificial near-IR
source without including long wavelength background ambient. Most radiometric IR
measurements are made without lenses, filters, or diffusers, relying on just the bare detector to
measure incident irradiance.
27
SOLAR DETOXIFICATION
100
80
70
%
60
50
40
30
20
Solar-blind va
cuum photodio
de
90
Silicon photodiode
e
pil
o
erm
Th
1.0
Detector
0.8
Filter
0.6
0.4
0.2
Combined responsivity
0.0
220
10
240
260
280
300
320
Wavelength, nm
0
200
400
600
800
1000
1200
Wavelength, nm
Figure 2.9
Responsivities of three detectors.
In the inset is shown a schematic of the effect of a filter on detector responsivity.
2.5.2 Filters
A detector’s overall spectral sensitivity is equal to the product of the responsivity of the
sensor and the transmission of the filter. Given a desired overall sensitivity and a known
detector responsivity, you can then solve a transmission curve for the ideal filter. Filter
bandwidth decreases with thickness according to Lambert-Beer’s law (see Eqs. 1.2 and 1.3),
so by varying filter thickness, you can selectively modify the spectral responsivity of a sensor
to match a particular function. Multiple filters cemented in layers give a net transmission
equal to the product of the individual transmissions. Filters operate by absorption or
interference. Colored glass filters are doped with materials that selectively absorb light by
wavelength, and obey Lambert-Beer’s law. The peak transmission is inherent to the additives,
while bandwidth is dependent on thickness. Sharp-cut filters act as long pass filters, and are
often used to subtract out long wavelength radiation in a secondary measurement. Interference
filters rely on thin layers of dielectric to cause interference between wave-fronts, providing
very narrow bandwidths. Any of these filter types can be combined to form a composite filter
that matches a particular photochemical process.
2.5.3 Input Optics
When selecting input optics for a measurement application, consider both the size of the
source and the viewing angle of the intended real-world receiver. Suppose, for example, that
you were measuring the erythemal (sunburn) effect of the sun on human skin. While the sun
may be considered very much a point source, skylight, refracted and reflected by the
atmosphere, contributes significantly to the overall amount of light reaching the earth’s
surface. Sunlight is a combination of a point source and a 2π steradian area source. The skin,
since it is relatively flat and diffuse, is an effective cosine receiver. It absorbs radiation in
proportion to the incident angle of the light. An appropriate measurement system should also
have a cosine response. If you aimed the detector directly at the sun and tracked the sun’s
path, you would be measuring the maximum irradiance. If, however, you wanted to measure
the effect on a person lying on the beach, you might want the detector to face straight up,
regardless of the sun’s position. This example can be extended to solar collectors (see Chapter
6).
28
SOLAR DETOXIFICATION
Different measurement geometries necessitate specialised input optics. Radiance and
luminance measurements require a narrow viewing angle (< 4º) in order to satisfy the
conditions underlying the measurement units. Power measurements, on the other hand,
require a uniform response to radiation regardless of input angle to capture all light. There
may also be occasions when the need for additional signal or the desire to exclude off-angle
light affects the choice of input optics. A high-gain lens, for example, is often used to amplify
a distant point source. A detector can be calibrated to use any input optics as long as they
reflect the overall goal of the measurement.
30º
30º
100%
60º
60º
50%
30º
100%
30º
±1.5%
60º
60º
50%
Figure 2.10
Relative spatial response of an ideal cosine diffuser (up)
and a radiance lens barrel (down).
Cosine Diffusers. A bare silicon cell has a near perfect cosine response, as do all diffuse
planar surfaces. As soon as you place a filter in front of the detector, however, you change the
spatial responsivity of the cell by restricting off-angle light. Fused silica or optical quartz with
a ground (rough) internal hemisphere makes an excellent diffuser with adequate transmission
in the ultraviolet. Teflon is an excellent alternative for UV and visible applications, but is not
an effective diffuser for infrared light.
Figure 2.11
Solar Global UV detector (tilted 37º and facing south) with a cosine diffuser
Radiance Lens Barrels. Radiance and luminance optics frequently employ a dual lens system
that provides an effective viewing angle of less than 4º. The trade-off of a restricted viewing
angle is a reduction in signal. Radiance optics merely limit the viewing angle to less than the
extent of a uniform area source. This input optic is used to measured direct sunlight, but
29
SOLAR DETOXIFICATION
mounted on a mobile sun-tracking platform (one loop per day) to follow the sun from sunrise
to sunset.
Figure 2.12
Solar Direct UV detector installed on a solar tracking system
Fibre Optics. Fibre optics allow measurements in tight places or where irradiance levels and
heat are very high. Fibre optics consist of a core fibre and a jacket with an index of refraction
chosen to maximise total internal reflection. Glass fibres are suitable for use in the visible, but
quartz or fused silica is required for transmission in the ultraviolet. Fibres are often used to
continuously monitor UV curing ovens, due to the attenuation and heat protection they
provide. Typical fibre optics restrict the field of view to about ±20º in the visible and ±10º in
the ultraviolet.
Integrating Spheres. An integrating sphere is a hollow sphere coated inside with Barium
Sulfate, a diffuse white reflectance coating that offers greater than 97% reflectance between
450 and 900 nm. The sphere is baffled internally to block direct and first-bounce light.
Integrating spheres are used as sources of uniform radiance and as input optics for measuring
total power. Often, a lamp is place inside the sphere to capture light that is emitted in any
direction.
High Gain Lenses. In situations with low irradiance from a point source, high gain input
optics can be used to amplify the light by as much as 50 times while ignoring off angle
ambient light. Flash sources such as tower beacons often employ fresnel lenses, making near
field measurements difficult. With a high gain lens you can measure a flash source from a
distance without compromising signal strength. High gain lenses restrict the field of view to
±8º, so cannot be used in full immersion applications where a cosine response is required.
SUMMARY OF THE CHAPTER
The three principal components of light (ultraviolet, visible and infrared) and their
wavelength distribution have been described. Typical solar spectra and air mass effect (sun
position) have been shown. The calculation of photonic fluxes (Einstein) from radiometric
measurement (W) and spectral data (nm-1) has been introduced. The attenuating components
of the atmosphere and their effect on UV radiation have been discussed to achieve a final
conclusion: UV spectrum is constant at a definite emplacement under certain circumstances.
This characteristic permits the standardisation of the solar-UV spectrum, which is very
helpful for finding a standard photon flux. The “cloud factor” index has been described and its
calculation from an UV-radiation database has been explained. Finally, solar radiation
measurement systems have been described, with special emphasis on their main components.
Their correct combination will permit accurate analysis of solar radiation and correct
evaluation of the quantum yield of photochemical reactions.
BIBLIOGRAPHY AND REFERENCES
30
SOLAR DETOXIFICATION
Hulstrom, R.; Bird, R.; Riordan, C. Spectral Solar Irradiance Data Sets for Selected
Terrestial Conditions. Solar Cells, 15, 365-391, 1985.
Iqbal, M. An Introduction to Solar Radiation. Academic Press, Canada. 1983.
Riordan, C.J.; Hulstrom, R.L.; Myers, D.R.. Influences of Atmosferic Conditions and Air
Mass on the Ratio of Ultraviolet to Total Solar Radiation. Solar Energy Research
Institute (SERI)/TP-215-3895. 1990.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. The Visible portion of the light is more powerful than the UV portion.
2. The most important portion of Solar UV light at earth’s surface is between 100 and 280
nm.
3. The solar radiation that reaches the ground level without being absorbed or scattered, is
called direct radiation.
4. Any economic comparison desired between solar radiation and electric lamps, as the UV
photon source, requires knowledge of the photon flux incident on the solar reactor.
5. The UV photon flux density on earth’s surface is in the range of thousands of photons per
square meter.
6. Clouds absorb UV light.
7. Clouds scatter UV light.
8. The “cloud factor” for UV light has to be determined using specific UV radiometers.
9. The best detector to measure infrared light is a phototube.
10. A detector’s overall spectral sensitivity is equal to the product of the transmission of the
sensor and the responsivity of the filter
PART B.
1. Which is the most important difference between ultraviolet, visible and infrared light?
2. Which are the usual units to express the solar spectrum power?
3. Why the Air mass at Sun zenith is called AM 1?
4. Cite at least two reasons to justify the importance of knowing the solar spectrum in
photochemistry?
5. Which is the typical unit in photochemistry? Why?
6. Why clouds do not absorb UV light?
7. Which is the usefulness of Equation 2.6?
8. Convert the following fraction of power associated with each wavelength (corresponding
to a standardised spectrum between 300 and 400 nm) in solar spectrum power knowing
that the total measured power is 40 W/m2.
λ, nm
300
302
304
306
308
310
312
fλ
0.0002
0.0004
0.0007
0.0011
0.0017
0.0023
0.0032
9. Why is it very difficult the measurement of infrared light measured with quantum
radiometer?
31
SOLAR DETOXIFICATION
10. How is it possible to vary the spectral responsivity of a sensor without changing its type of
filter?
Answers
Part A
1. False; 2. False; 3. True; 4. True; 5. False; 6. False; 7. True; 8. True; 9. False; 10. False.
Part B
1. Wavelength, ultraviolet light between 100 and 400 nm, visible light between 400 and 770
nm and infrared light between 770 and 106 nm.
2. W m-2 nm-1 or W m-2 µm-1
3. Because this is the direct beam solar irradiance minimum path length through the
atmosphere.
4. (a) The radiation that reaches the solar reactors is not constant. (b) In order to compare
laboratory results with solar results or to use the information obtained with lamps. (c) The
determination of the efficiency of the components of the solar reactor and the possible
modifications to improve the conditions of photodegradation. (d) To perform economic
comparisons between solar radiation and electric lamps.
5. Einstein. Because it is 1 mol of photons and therefore, the quantum yield of a
photochemical reaction (rate 1 M/min) that absorbs 1 Einstein L-1 min-1 is 1.
6. Because pure water does not absorb UV light.
7. To represent spectra in a standardised manner. This will permit the comparison between
spectra recorder at different sites, hour of the day and/or different seasons.
8.
λ, nm UVλ, W m-2 nm-1
300
302
304
306
308
310
312
0.0080
0.0160
0.0280
0.0440
0.0680
0.0920
0.1280
9. Infrared light contains the least amount of energy per photon of any other band. Because
of this, an infrared photon often lacks the energy required to pass the detection threshold
of a quantum detector.
10. Varying filter thickness.
32
SOLAR DETOXIFICATION
3. EXPERIMENTAL SYSTEMS
AIMS
This unit describes the experimental systems necessary for performing pilot-plant-scale solar
photocatalytic experiments. Various laboratory set-ups are also shown in order to emphasise
the big differences between these small photoreactors and pre-industrial-scale pilot plants. It
outlines the basic components of these pilot plants and the different possibilities for operating
them. This chapter will review modelling of the experiments from a chemical engineering
point of view. Finally, an extensive overview of relationships between radiometer
measurements and the photons that actually reach the photoreactor is presented.
OBJECTIVES
By the end of this unit, you will know the most important features of large-scale outdoor
photocatalytic experiments and you will be able to do six things:
1. Design a simple, versatile solar detoxification pilot system to fit the present and future
necessities of the research to be performed in it.
2. Understand pilot plant operation and decide among different options.
3. Calculate the kinetic constant using the appropriate method depending on pilot plant
characteristics and operation.
4. Perform on-site calibration of solar-UV radiometers.
5. Find a relationship between solar radiometer measurements and the photons that actually
reach the reaction.
6. Employ chemical actinometers to validate all the calculations performed to obtain the
photon flux inside photoreactors.
NOTATION AND UNITS
Symbol
C or Ci Reactant concentration
EUV
Accumulated energy (per unit of volume) incident on the
reactor.
I
Photon flux density
IE
Effective photon flux corresponding to the UV inside the
absorber of a photoreactor.
I*E
Volumetric effective photon flux corresponding to the UV
inside the absorber of a photoreactor.
Fraction of power associated with a wavelength
fλ
k
First order kinetic constant
N
Number of incident photons
Q
Flow rate
r
Reaction rate
Sp
Surface area of the reflector of a photoreactor capturing the
radiation.
ST
Surface area of the tube of a photoreactor.
texp
Experimental time
tR
Residence time
UV
Measurements provided by a radiometer
Ultraviolet irradiance associated with a wavelength
UVλ
Spectral data calculated with the standard spectrum and
UV*λ
radiometric data
Units
M or mg L-1
kJ L-1
Einstein s-1 m-2
Einstein s-1 m-2
Einstein L-1s-1
nm-1
min-1, h-1
Photons s-1 m-2
L min-1, m3 h-1
M min-1, mg L-1 min-1
m2
m2
min, s, h
min, s, h
W m-2
W m-2 nm-1
W m-2 nm-1
33
SOLAR DETOXIFICATION
UVΣ
V
VTOT
x
ΦE
η
λ
µλ
τ
Summation of the measurements provided by a
spectroradiometer
Volume
Volume of the entire pilot plant.
Conversion
Estimated quantum yield using moles of incident photons
instead of moles of photons absorbed by the catalyst.
Loss factors affecting the photon flux
Wavelength
Coefficient of absorptivity
One residence time
W m-2
L, m3
L, m3
µm, nm,
cm-1
min, s, h
3.1 LABORATORY SYSTEMS
The treatment of contaminated water necessarily includes the design of an efficient
photoreactor. Basic laboratory research on the process has mostly been performed with
experimental devices in which efficiency was not as important as obtaining appropriate
conditions that would permit reproducibility of the results and exhaustive knowledge of the
effects of all the important parameters. This is correct when the goal is a fundamental
knowledge of the process, but not always sufficient to attempt a change of scale. Two
different concepts of laboratory photoreactors are presented here. One is a continuos stirred
tank reactor (Figure 3.1) and the other a recirculating system (Figure 3.2).
Figure 3.1
Laboratory typical stirred tank reactor.
Figure 3.1 shows a basic scheme of a microphotoreactor used in laboratory experiments. The
photoreactor consists of a flask made of Pyrex with a flat bottom window constituted by an
optical filter generally made of quartz (fused silica) to let enter all the UV-visible radiation
with λ ≥ 220 nm. For an isothermal reaction, or when one wants to change the temperature of
the medium for determining the activation energy of the reaction (see section 4.7), the system
is equipped with a jacketed envelope through with flows a temperature-constant fluid (e.g.
water) delivered by a thermostat. There are several upper apertures to introduce the reacting
mixture, the catalyst, a controlled gas atmosphere and also to withdraws samples, either from
the slurry or from the gas phase through the ground top and the valve for analysis by GC (gas
chromatography) or GC/MS (gas chromatography-mass spectrometry, see chapter 5). A good
stirring is obtained with a magnetic stirrer, curiously working perpendicularly to the rotating
magnet stirrer, that it is at the right level. The determination of gaseous CO2 is made in
oxygen confined atmosphere by sampling amounts of supernatant gas phase through the
ground top and the valve, linked to a GC by a stainless steel tubing.
34
SOLAR DETOXIFICATION
The UV-light is provided by a xenon lamp introduced in a water-cooled envelope that is
closed by a silica optical disk and used for preventing the formation of ozone in the laboratory
atmosphere. All the IR beams, which could heat the slurry suspensions, are removed by a
water cell, which contains an optical disk transparent to the wavelength domain desired. The
radiant flux (in W/m2) is measured for each experiment by a radiometer whose head is settled
in the place of the photoreactor. The photograph represents a simple photoreactor, without a
cooled jacket and open to the air for a routine experiment of water detoxification by
illuminated slurry of titania. A mere mixing is sufficient to provide gaseous oxygen to the
medium (see section 4.2).
Figure 3.2
Laboratory typical recirculating system.
Figure 3.2 shows a schematic and a photo of the recirculating system. Water, contaminants
and titanium dioxide are continuously recirculated through a 250 mL Pyrex reactor. During
photocatalytic degradation experiments some parameters as pH, CO2 and O2 can be monitored
in situ by specific electrodes (probes). Air is introduced inside the reactor flask through a flow
cell and distributed via a sintered-glass tip to maintain a constant oxygen concentration in the
system. Irradiation is carried out by a solar simulator equipped with a high-pressure xenon arc
lamp and a parabolic reflector. Since solar simulators contain a significant thermal
component, it is necessary to decrease the temperature using a small heat exchanger. The
entire reactor is fabricated with chemically resistant materials (Pyrex, Teflon and Viton).
3.2 SOLAR DETOXIFICATION PILOT PLANTS
The first outdoor engineering-scale reactor developed was a converted solar thermal
parabolic-trough collector in which the absorber/glazing-tube combination had been replaced
by a simple Pyrex glass tube through which contaminated water could flow (Goswami and
Blake, 1996). Since that time, research all over the world has advanced a number of reactor
concepts and designs, including concentrating and non-concentrating reactors. The catalyst
can be deployed in several ways, including as a fixed catalyst, slurry, or neutral-density large
35
SOLAR DETOXIFICATION
particles. As mentioned above, a simple modification of the parabolic-trough solar thermal
collector was successfully developed and operated for experiments in which the catalyst was
deployed in slurry. Parabolic concentrating-type reactors have been used for applications such
as groundwater remediation and removal of metals from water. A disadvantage of
concentrating reactors is that they cannot concentrate diffuse solar radiation. This is not a
major problem for solar thermal applications, because diffuse radiation forms a small fraction
of the total solar radiation. However, solar photocatalytic detoxification with TiO2 as a
catalyst uses only the ultraviolet portion of solar radiation, and as much as 50 percent or more
of this may be present in diffuse form, especially at humid locations and during cloudy or
partly cloudy periods. Therefore, concentrating reactors would be more useful at dry, high,
direct-insolation locations. Another disadvantage of concentrating reactors is that the quantum
efficiency is low, due to a square root rather than linear dependence of rate on light flux (see
Chapter 4). These disadvantages tend to favour the use of non-concentrating reactors. In any
case, these are aspects which will be discussed in detail in Chapter 6 and, whatever type of
collector, solar detoxification pilot plants have several components in common and general
design characteristics which can be discussed here.
The design procedure for a pilot solar detoxification system requires the selection of a reactor,
catalyst operating mode (slurry or fixed matrix), reactor-field configuration (series or
parallel), treatment-system mode (once-through or batch), flow rate, pressure drop, pretreatment, catalyst and oxidant loading method, pH control, etc., so a pilot plant has to be as
versatile as possible to allow for these variables and, at the same time, provide sufficient
confidence in the experiments carried out in it. This is the crucial difference between pilot and
process plants. A pilot plant must fulfil all the present and future requirements of the research
to be performed in it.
In Figure 3.3 a detailed drawing of a plant is given. Usually, a detoxification pilot plant is
constructed with several solar collectors. All the modules are connected in series, but with
valves that permit to bypass any number of them. Sampling valves are in the outlet of each of
the modules. All the tubes and valves are black HDPE, material chosen because it is strongly
resistant to chemicals, weather-proof and opaque, in order to avoid any photochemical effect
outside of the collectors. There are storage-feeder tanks available, also made of HDPE and
having different capacities, where the test mixtures are prepared. Four different operating
modes are possible: recirculation, once-through, partial recirculation, and system cleaning.
SOLAR COLLECTOR
n
modules
in series
T
Closed valve
Opened valve
Sensors
Pump
T
Discharge
tank
Refrigeration system (optional)
O2
Pr
PIC
GAC
filter
Disposal
Flow control
FIC
O2
Contaminant
+ TiO2
Holding
tank
FCV
Batch tank
Pr
FI
Clean water
Figure 3.3
Photocatalytic Detoxification Pilot Plant (the batch mode is shown).
36
SOLAR DETOXIFICATION
When concentrating solar collectors are used, the temperature of the water which flows
through them rises considerably. Obviously, the slower the flow rate used in once-through
experiments and the longer recirculation experiments, the greater the increase of temperature
is. Therefore, to avoid evaporation and damage to plastics, cooling is necessary, and a closedcircuit water-cooling system has to be installed.
A centrifugal pump with an electric motor (calculated to provide sufficient flow when the
maximum length of the system is used) has to be installed to move the treatment water
through the reactor. The flow rate (in batch mode) has to be such that it guarantees only a
small amount of reactant is converted each time through the reactor, and the concentration
throughout the system remains relatively constant (this reasoning will become clear below).
Either a flow-rate control loop made up of a flow meter connected to a controller, which in
turn governs an automatic electric valve, or an electric pump with a speed controller has to be
installed to regulate the flow to the rate desired. The most important sensors required for the
system are temperature, pressure and dissolved oxygen (at least in the reactor outlet). Other
sensors, such as pH, selective electrodes, etc., could be useful depending on the type of
experiments to be carried out. As constant pure oxygen is required for the oxidation of
organics, an injection system at the reactor inlet allows oxygen to be added to the reactor
either at planned intervals by opening and closing in a predefined cycle, or continuously.
Atmospheric oxygen can be also stirred into the reaction medium in the reservoir tank. In this
case, the dissolved O2 is kept at a concentration of around 6-8 mg/L. An UV-radiation sensor
must be placed in a position where the solar UV light reaching the photoreactor can be
measured, permitting the evaluation of the incident radiation as a function of hour of the day,
clouds, and atmospheric or other environmental variations. All these data have to be sent by
an appropriate transmitter from the sensors to a computer, which stores the results for later
evaluation.
To clean the system, a drainage tube, with an active carbon filter to retain any organic
compound not decomposed during the experiments, must be hooked up to the sewage
pipelines. Use of demineralised water (conductivity less than 10 µS, organic carbon content <
0.5 mg/L) is recommended for cleaning as well as in the tests themselves.
3.3 OPERATION OF PILOT PLANTS
3.3.1 Once-through operation
Experimental procedure begins when the pump is turned on and the system is filled with clean
water. Those modules necessary are selected and the corresponding valves are set to bypass
the rest. Then the water is pumped through the circuit and the modules are covered.
Obviously, the maximum pump flow rate is necessary for this procedure. The amounts
required to obtain the initial concentrations of catalyst, contaminant and any other ingredient
in the experiment are added to the holding tank. When the time needed for mixture to be
completed has expired, this is verified by taking samples at two different points in the reactor
at the same time for analysis. A few minutes later two more samples are taken and, if the four
coincide, the concentration of the reactives may be considered to be the same throughout the
reactor. Simultaneously, the automatic control sets the flow rate (Q) which will then be kept
constant during the experiment, oxygen injection is activated and valves are adjusted so that
the fluid goes to the discharge tank. After that the modules to be used are put into operation.
This marks the beginning of the experiment.
37
SOLAR DETOXIFICATION
The modules are kept illuminated a little longer (experimental time) than necessary to allow
the water in the holding tank to go through the reactor and approach the discharging tank.
This time (texp) is:
V + nVmod
t exp = tube
(3.1)
Q
where Q is the flow rate, Vtube is the volume in the pipes between the modules and the tank
and Vmod is the volume in each module, with n the number of modules in series. At this time,
samples are taken at all the valves in the outlets of each of the modules in the experiment.
This provides “n” number of samples with different residence or illumination times (tR,i) to
enable determination of kinetics. Under these conditions, the reactor behaves according to the
ideal plug-flow model as explained later. The residence time corresponding to each sample
collected at the end of the experiment is calculated with the following equation:
nV
t R ,i = i illu
(3.2)
Q
where i is the number of modules through which the samples have passed before being
collected and Villu is the volume in the illuminated section of each module. When the test is
over, n samples have been obtained with a reactor residence time that is a function of the flow
rate. Thus, if the procedure is repeated at a different flow rate, that group of samples has a
different tR and the number of points (tR,i, Concentration) necessary to evaluate any
experiment can be obtained.
3.3.2 Batch operation
Solar detoxification pilot plants are frequently operated in a recirculating batch mode as
depicted in Figure 3.3. In this scheme, the fluid is continuously pumped between a reactor
zone and a tank in which no reaction occurs, until the desired degradation is achieved. The
systems are operated in a discontinuous manner by recirculating the slurry solution with an
intermediate reservoir tank and centrifugal pump. This type of operation differs little from the
previous one.
When concentration of the reactives is the same throughout the reactor, oxygen injection (if
necessary) is activated and the position of the valves is maintained so that the fluid begins and
ends up in the holding tank (now called the batch tank). The automatic control sets the
maximum flow rate, which has to be such that it guarantees that only a small amount of
reactant is converted each time it goes through the reactor. Then the modules that are going to
be used are put into operation. This begins the experiment. Recirculation is continued and the
test lasts however long required, even up to several days. Samples may now be taken at any of
the sampling ports, since as the system is in recirculation mode, tR is the same for samples
taken at any point in the system. The (t R,i, Concentration) pairs are thus obtained (Eq. 3.3).
V
t R ,i = illu t exp,i
(3.3)
VTOT
where VTOT is the volume of the entire pilot plant. Villu and VTOT are defined at the beginning
of the experiment by the number of modules used and the level of water in the batch tank. The
experimental time (texp) is the difference in time between the initial sample (initial
concentration of the pollutant, t = 0) and samples collected during the experiment (t > 0).
38
SOLAR DETOXIFICATION
SOLAR REACTOR (n modules)
Holding
Tank
C0
Q(L/min)
n1
Villu
C1
ni
Ci
Discharge
Tank
Recirculation line
Batch
Tank
C2(t)
SOLAR REACTOR
C1(t)
Figure 3.4
Schematic of two pilot plant operation concepts: A once-through operation (top) and a batch
operation (bottom).
3.3.3 Modelling once-through and batch operation
In general, once-through operation can be modelled as plug-flow. The variation in
concentration (dC) over time (dt) for once-through experiments and for first order reactions
(typical in photocatalysis) would be:
C
dC
r=−
= kC , which integrated is : ln i = − k t R ,i
(3.4)
dt
C0
where tR,i is calculated by Equation 3.2. For most laboratory-scale batch systems, the amount
of conversion of reactant that passes through the reactor is small, and the concentration
throughout the system is relatively constant. In that case, the entire system can easily be
modelled as a simple batch system in which only a fraction (i.e., the part in the reactor) of the
total mixture is undergoing reaction. In this case, Equation (3.4) above can also be applied,
but using Equation 3.3 to calculate tR,i.
In a large field system, the amount of conversion each time the mixture passes through the
reactor is noticeable. As the relatively clean water in the reactor is mixed with the “dirty”
water in the batch tank, the water sent from it to the reactor has a lower and lower
concentration. Because the rate usually decreases with concentration, the overall rate in the
reactor responds likewise. Thus, unless properly accounted for, the presence of the tank will
alter the perceived performance of the photoreactor. Two solutions are available to solve this
problem. The first, and also the easier of the two, is to use a very high flow rate to achieve
low conversion each time through. This high flow rate (Eq. 3.5) must allow more than 1 %
conversion per pass (C1(t) ≈ C2(t)) to be avoided. This 1% has been selected because is very
far from the error associated to any chemical method applied for Ci analysis.
V
r
Q ≥ TOT
(3.5)
C0 x
where r is the reaction rate, C0 the initial concentration of the reactant and x the conversion
(0.01). For example, if the initial reactant concentration is 50 mg L-1, the reaction rate is 0.1
mg L-1 min-1, and VTOT is 250 L, the flow rate must be over 50 L/min. When this is not
possible because the reaction rate is very high, initial concentration very low and/or the pump
is not strong enough, the mathematical formulation is the following:
In steady-state, the concentration in the reactor outlet at time t, C1(t) is determined from the
39
SOLAR DETOXIFICATION
inlet concentration (which does not change with time) and the reaction kinetics. However,
because the system is a transient process, the normal steady-state plug flow reactor equation
cannot be used to model the photoreactor. Because the inlet concentration changes with time,
C2(t) is defined by what went into the reactor one residence time prior to t, C2(t-τ), and the
kinetics. The batch tank is still modelled as a well-mixed tank. Solving these equations is
more difficult than the low-conversion-per-pass case and a numerical routine is required to fit
the data from the batch test. This routine also takes into account the volume of the piping
between the reactor outlet and the batch tank.
The defining equations for the system components shown in Figure 3.4 are given by basic
chemical engineering principles. For the plug flow reactor:
C1 dC
τ*
(3.6)
∫C2 r = −∫0 dτ
In the batch process, both concentrations vary with time, that is, C1 = C1(tR) and C2 = C2(tR).
A solution for C1(tR) and C2(tR) for all tR can be obtained if three different time regimes are
defined as follows:
• Regime 1: tR = 0 (Reactor exposed to sunlight at t = 0). By definition we have C1=C2=C0
• Regime 2: 0 < tR <τ. Under these conditions, the fluid exiting the reactor has been
illuminated for less than one complete reactor residence time. In Eq. 3.6, the time spent in
the reactor is equal to tR, and the starting concentration is C0, thus:
C1 dC
tR
=
−
(3.7)
∫C0 r ∫0 dτ
• Regime 3: tR≥τ. Now the fluid exiting the reactor has been illuminated for one complete
residence time and τ*= τ. However, to solve for the reactor outlet concentration at time t,
it must be known what went into the reactor at time tR-τ. This defines the lowest limit on
the rate integral:
C1 ( t R ) dC
τ
=
−
(3.8)
∫C2 (tR −τ ) r ∫0 dτ
Having defined these three regimes, the balance of material in the well-mixed tank may now
be written:
dC 2 (t R )
Vbatch = Q[C1 (t R ) − C 2 (t R )]
−
(3.9)
dt
where Vbatch is the volume in the batch tank. Now, only linking Eq. 3.9 with the different
forms of Eq. 3.6 remains to be done. To do this, we must define an expression for the rate. A
simple first order rate expression (typical in photocatalysis) can be used as an example (see
Eq. 3.4). The algorithm that solves Eq 3.6 for C1(t) as a function of C2, at the different time
regimes using first-order kinetics is the solution:
• Regime 1: C1=C2=C0
• Regime 2: C1(t)= C0 exp(-kt)
• Regime 3: C1(t)= C2(t-τ) exp(-kτ)
This expression for C1(t) is substituted into Eq 3.9 and the value for C2(t) predicted. This
prediction is then compared to the experimental values for the concentration in the batch tank,
with iteration until the k value which provides the best fit to the data is obtained. This
methodology can be used to solve numerically for the concentration profile in any batch
process. Although, in any case, this method is only really necessary when the recirculation
flow is not high enough (See Eq. 3.5).
40
SOLAR DETOXIFICATION
3.4 EVALUATION OF SOLAR UV RADIATION INSIDE PHOTOREACTORS
Solar ultraviolet radiation is an essential parameter for the correct evaluation of data obtained
during photocatalytic experiments in a solar water decontamination pilot plant. The kinetic
constants of photocatalytic processes can be obtained by plotting substrate concentration as a
function of three different variables: time, incident radiation power inside the reactor and
photonic flux absorbed by the catalyst. Depending on the procedure, the complexity of
obtaining these constants, as well as their applicability, vary. When the photonic flux
absorbed by the catalyst is used as an independent variable, extrapolation of the results is
better. However, many parameters (incident photons passing through the reactor without
interacting with the catalyst, the directions in which light scatters, distribution of the sizes of
TiO2 particle suspended in the liquid, etc.) must be known for this, making it impractical in a
reactor used for pilot plant experiments.
Use of the experimental time as the calculation unit could give rise to misinterpretation of
results, because the reactor consists of illuminated and non-illuminated elements. Large
experimental reactors require much instrumentation and the reactor must also be as versatile
as possible, substantially increasing the non-illuminated volume. With use of residence time,
that is, the time the water has been exposed to the radiation, the conclusions would also be
erroneous. This is because when time is the independent variable, the differences in the
incident radiation in the reactor during an experiment are not taken into account. Furthermore,
the more different the environmental conditions of the experiments to be compared (different
days, different periods of the year or atmospheric variations), the more critical this becomes.
The only way to avoid this problem is to use a relationship between experimental time, plant
volume, collector surface and the radiant power density measured by radiometers. This
permits extrapolation of known data from one scale to another, as well as avoiding the
problem arising from a variable source of radiation (sunlight).
Recalling Eq. 2.4, the photon flux density I [Einstein s-1 m-2] is the number of incident
photons per unit of surface and time:
2
d N
I=
(3.10)
N 0 dt dA
where N0 is Avogadro’s number (6.023 x 1023). Using Solar spectrum data (See Figures 2.6
and 2.7) and the former equations in congruent units [S.I], it is possible to determine the
photon flux density. In any case, the UV radiation values vary from one location to another,
and obviously, during the day and from season to season, so that it is necessary to know these
data for a given location and in real time. The calculation of the photon flux in any
photochemical reactor could be undertaken following the flow diagram shown in Figure 3.5.
41
SOLAR DETOXIFICATION
SPECTROPHOTOMETRIC MEASUREMENT OF UV
IT IS IMPOSSIBLE TO
EVALUATE THE PHOTON
FLUX WITHOUT ON-LINE
SPECTRUM
MEASUREMENTS
RADIOMETERS CALIBRATION
NO
IS IT THE UV SPECTRUM CONSTANT?
YES
IT IS POSSIBLE TO OBTAIN AN EQUATION WHICH CORRELATE THE
RADIOMETRIC AND SPECTROPHOTOMETRIC DATA
DETERMINATION OF SPECTRAL COLLECTOR
EFFICIENCIES (REFLECTIVITY, TRASMISSIVITY, ETC..)
EQUATION TO OBTAIN PHOTON FLUX INSIDE THE PHOTOREACTOR
AS FUNCTION OF ON-LINE UV MEASUREMENTS.
ACTINOMETRIC EXPERIMENTS
EFFICIENCIES
CALCULATION
NOT CORRECT
NO
EXPERIMENTAL ACTINOMETRIC QUANTUM YIELD =
CALCULATED (BY PREVIOUS EQUATION) QUANTUM YIELD?
YES
END
Figure 3.5
Calculating procedure to find out the photon flux inside a solar reactor.
3.4.1 Radiometers calibration
Solar UV radiometers (See Chapter 2.5) provide data in terms of the incident W m-2 on each
of them, which gives an idea of the energy reaching any surface in the same position as they
are with regard to the sun. According to the manufacturer, UV instruments are sensitive to
ultraviolet radiation (See Figure 2.9). But in their technical description, the interval of
wavelengths covered in their calibration is not usually reported. Therefore, that specification,
so important in the calculation of photonic flux, is not certifiable. In order to resolve this
uncertainty, on-site calibration is necessary. This is performed by comparing the data
provided by the UV radiometers with those from a spectroradiometer, which gives data in
terms of W m-2 nm-1, in the same position as each of the radiometers. This gives:
UVΣ =
λ =n
∑UV
λ = 300 nm
(3.11)
λ
where UVΣ (W m-2) is the summation of the radiation measurements (above 300 nm up to n in
1 nm intervals) provided by the spectroradiometer. An example (with only a few measured
data) is shown in Table 3.1. The two ways of measuring UV at different times of day are
compared considering n = 400 nm, which is the ultraviolet-visible threshold, after which the
radiometers should not measure.
Local time
UV, W m-2
UVΣ, W m-2
UV-UVΣ, W m-2
100(UV-UVΣ)/UVΣ, %
10:31
13.69
11.68
2.01
+17.2
11:00
17.43
16.01
1.42
+8.9
12:59
26.96
25.42
1.54
+6.1
15:34
17.68
16.00
1.68
+10.5
Table 3.1
Radiometric and spectroradiometric UV measurements at different times of day compared
considering n = 400 nm
42
SOLAR DETOXIFICATION
In view of these results, the conclusion is that the UV radiometer measures beyond 400 nm. In
order to find the real interval, the same procedure is carried out for n values over 400 nm at 1
nm intervals. The results of the previous example are given in Table 3.2, together with the %
of error at each interval.
UVΣ(1), W m-2
UV, W m-2
n=401
n=402
n=403
n=404
n=405
13.69
11.99(+14.2)
12.33(+11.0)
13.30(+2.9)
13.53(+1.2)
13.10(+4.5)
17.43
16.42(+6.2)
16.87(+3.3)
17.33(0.0)
17.79(-2.1)
18.25(-4.7)
26.96
26.01(+3.6)
25.4(+6.1)
27.32(-1.3)
27.0(0.0)
28.65(-6.3)
17.68
16.39(+7.6)
16.81(+5.2)
17.24(+2.5)
17.67(0.0)
18.51(-4.4)
Table 3.2
Radiometric and spectroradiometric UV measurements at different times of day compared
considering n > 400 nm.
It seems that, taking as the upper limit a wavelength of 403-404 nm, the radiation measured
by both instruments is in agreement. The radiometer measurement interval is fundamental for
the calculation of the photon flux that reaches the interior of the reactor. The alternative might
be on-line measurement of the solar spectrum with a solar spectroradiometer, but this is very
often not possible because of the price and/or the manpower required for a spectroradiometer.
3.4.2 Correlation between radiometric and spectroradiometric data
Now, recalling Eq. 2.6, it is possible to standardise the UV spectrum up to the radiometer cutoff wavelength. In this case, 404 nm is used. It is also used as an example in the above
calculations to clarify the explanation, but similar reasoning could be applied to any
wavelength interval.
λ = 404 nm
UV
f λ = λ = 404 nmλ , therefore ∑ f λ = 1
(3.12)
λ = 300 nm
∑UVλ
λ = 300 nm
If the UV radiation spectrum is assumed to have a fixed shape similar to that in Figure 2.6, a
standardised spectrum is available in the wavelengths measured by the radiometer. Therefore,
using the standardised spectrum and the irradiance data (W m-2) measured by the radiometer,
the spectral distribution can be calculated for all of these data:
UVλ* = f λ UV
(3.13)
*
where UV λ are the spectral data calculated with the standard spectrum (fλ) and the radiometer
data. Therefore, the number of photons is only a function of the intensity (measurable in real
time with the radiometers). Once the spectral distribution of the radiometer measurements is
known, the number of photons incident per unit of time and surface (N) corresponding to
those measurements can be found. Recalling Eq. 2.3, which relates the number of photons
from a given polychromatic source of light to the energy corresponding to each wavelength,
this can be transformed for this case into the following (using summations of discrete values):
1 λ = 404 nm *
N 300− 404 nm =
(3.14)
∑UVλ λ
hc λ =300 nm
43
SOLAR DETOXIFICATION
where N300-404nm are incident photons. But, as the spectral distribution is assumed to be
constant, Eqs. 3.13 and 3.14 yield:
UV λ = 404 nm
N 300− 404 nm =
(3.15)
∑ fλ λ
hc λ =300 nm
Therefore, the wavelength considered equivalent to the summation would be:
λ = 404 nm
∑f
λ =300 nm
λ
λ = 368.79 nm
To calculate the values corresponding to other intervals, the same procedure is followed.
Returning to Eq. 3.15, the number of photons corresponding to the average radiation at any
given instant for the radiometer is:
N 300 −404 nm = 1.856 × 10 18 UV
(3.16)
2
where N300-404nm is the incident photons, between 300 and 404 nm, per m and second, when
UV is measured in W m-2 and λ in nm, with speed of light c = 2.988 × 1017 nm s-1 and the
Planck’s constant h = 6.626 × 10-34 J s. If Einstein (moles of photons) is used as the unit, the
result is:
N
I 300 − 404 nm = 300 − 404 nm = 3.083 × 10 −6 UV
(3.17)
N0
3.4.3 Collector efficiency
All the factors contributing to collector efficiency are shown in Figure 3.6. With the
combination of these factors as defined below and Eq. 3.17, the photon flux may be
calculated.
• ηC includes all the errors produced while the collector was built up.
• ηR,λ is the reflectivity of the parabolic mirror surface (usually aluminium, see Chapter 6).
Since the reflection spectrum does not vary over a wide range and that variation is
uniformly distributed over the whole interval (300-400 nm), an average spectral
distribution may be considered. However, as the surface is outdoors and could get dirty or
damaged, it has to be measured periodically, in which case the value is ηR,i. With frequent
cleaning (before every experiment), it may be considered constant and is then ηR.
• ηT,λ is the spectral transmissivity of the absorber tube.
1
η R,λ
1'
2
3
4
ηT ,λ
1+1’ I
2
Reflected I
3+4 I inside photoreactor, IE
η
Efficiency factors
ABSORBER
MIRROR
Figure 3.6
Drawing of the various loss factors (η) affecting the photon flux (I) inside a photoreactor.
The solar radiation that reaches ground level without being absorbed or scattered is called
direct radiation, the radiation that has been dispersed but reaches the ground is called diffuse
radiation and the sum of both is called global radiation (see Chapter 2). Global radiation is
collected directly by the transparent absorber tube without intervention of the collector. In
44
SOLAR DETOXIFICATION
Figure 3.6 the path followed by I until it arrives inside the absorber tube is shown. It must
arrive at the surface and be reflected (part is lost due to ηR,i) in the right direction by the real
mirror surface (ηC), before penetrating (ηT,λ) in the tube. Furthermore, the parabolic
concentration factor must also be considered (ratio of surface area of the parabola capturing
the radiation and surface area of the tube, Sp/ST) if it is a concentrating solar collector (see
Chapter 6). Therefore, the effective photon flux corresponding to the UV inside the absorber
(IE) is:
 S

I E = f  I , P ,η C ,η R ,i ,ηT ,λ 
(3.18)
 ST

Only the transmissivity of the glass, ηT,λ, affects the radiation reaching the absorber without
being reflected by the mirror (1’). In this case Eq. 3.18 is simplified:
I E = f ( I ,η T , λ )
(3.19)
Since ηT,λ depends on the wavelength, in order to evaluate IE, Eq. 3.15 must be recalculated as
follows:
S
 UV λ = 404 nm
I E =  P × η C × η R ,i 
(3.20)
∑ f λ ηT , λ λ
 ST
 hc λ =300 nm
where, if UV is W m-2, the units of IE are Einstein m-2 s-1 incident in the inside of the tube.
Sp/ST, as accurate as possible, is used to determine IE in each case. For this it is necessary to
make the corresponding trigonometric calculations based on the collector characteristics.
Once this is known, the same ratio can be calculated for the reactor volume. Therefore, Sp/ST
in Eq. 3.20 is substituted by this ratio (collector area/collector volume). In this way, photon
flux is obtained in units congruent with the reaction rate (M s-1) so that an estimated quantum
yield (ΦE) similar to that in Eq. 2.1 can be obtained, but using moles of incident photons
instead of moles of photons absorbed by the catalyst.
reaction rate
(3.21)
ΦE =
I E∗
where I*E are Einstein L-1s-1 of UV irradiance incident inside the tube. To calculate the values
corresponding to other wavelength intervals, the same procedure is followed.
3.4.4 Actinometric experiments
In a chemical actinometer, the photochemical conversion is directly related to the amount of
photons absorbed. This method has been used since the 30’s but due to recent progress in
radiation sensors, semiconductor and electronic equipment development, physical measuring
devices have become more popular for photochemistry. In the case of reactors with simple
geometries, they are preferable because they are very quick, simple and precise. In the case in
hand, the chemical actinometer is needed for validation (see outline of work in Figure 3.5) of
all the calculations performed to obtain the equations detailed above. A good chemical
actinometer meets the following specifications:
• The photochemical system should be simple and the reaction should be reproducible, under
well-defined and easily controlled conditions. The quantum yields should be well known
for a wide range of wavelengths, if polychromatic wavelength radiation has to be
measured.
• The quantum yield should be independent of the intensity of radiation, actinometer
concentration and temperature (large pilot plants cannot be thermostatised.)
• The reagents and products should be reasonably stable, so errors do not arise between the
time the sample is taken and the time it is analysed.
45
SOLAR DETOXIFICATION
• The analytic methods should be simple and the reagents should be easily synthesised and,
even better, commercially available. This is, if possible, much more important in the case
of pilot plants, because of the large volumes of actinometer that have to be prepared.
• The system should be sufficiently sensitive for low radiation intensities and the evaluation
of photons absorbed should be simple.
For this case, a common uranyl-oxalate system is explained as an example. Very complete
information on actinometric systems has been summarised by Kuhn et al. (1989):
∗
hv
UO22 + → UO22 +
(3.22)
2+ *
2+
UO2
+ H 2 C2 O4 → UO2 + CO + CO2 + H 2 O
(
)
(
)
Absorptivity of this solution may be estimated from:
Absorbed Photons
= 1 − exp(− µλ 2b)
(3.23)
Incident Photons
where b is the inside reactor radius, 2b is the light path length and µλ is the coefficient of
absorptivity. General characteristics of the actinometer and experimental details may be found
in Curcó, et al. (1996) and references therein.
Photon flux inside the reactor during the actinometric experiments may be calculated from
Eq. 3.20. However, in this case the range of wavelengths is widened to 536 nm (actinometer
activity cut-off) and solar spectra up to 536 nm is used. If the photon flux inside the reactor
and the characteristics of the actinometer are known, the oxalic acid degradation rate can be
calculated from IE(300-536), µλ, oxalic acid quantum yield in the mixture and the area/volume
ratios in each case. The comparison between this calculated acid decomposition rate and the
rate measured in the actinometric experiments gives an idea of the validity of the equations in
point 3.4.3. The oxalic acid degradation rate calculated by the experiments or by Eq. 3.21 has
to be quite close. Therefore, all the equations developed are assumed to be valid. However,
actinometry would not be useful for finding the photon flux, at any given time, inside a
photoreactor illuminated by solar radiation. The variations in solar intensity due to changes in
weather, and the impossibility of using an actinometer inside the reactor while the
photocatalytic experiments are being performed, make it impossible.
3.5 SIMPLIFIED METHOD FOR THE EVALUATION OF SOLAR UV RADIATION
INSIDE PHOTOREACTORS
The above procedure is usually the most appropriate but, several parameters are very often not
available: on-site solar spectrum, collector reflectivity, absorber transmissivity, etc. In these
cases, a shorter procedure can be used. Although not the best solution, it is frequently very
useful. Nevertheless, an UV radiometer mounted at the same angle as the solar collector is
always necessary for data evaluation. This radiometer sends a signal to a computer in which
the data (UV) are stored. As radiation data are collected continuously, it is very easy to
calculate the average incident radiation on the collector surface ( UV n ), for each period of t,
and apply Eq. 3.24 to that average. Consequently, the amount of energy collected by the
reactor (per unit of volume) from the start of the experiment until each sample is collected
may be found by:
S
EUV,n = EUV,n-1 + ∆ t n UV n P
VTOT
(3.24)
∆ t n = t n - t n −1
46
SOLAR DETOXIFICATION
-r, µM min-1
where tn is the experimental time at which each sample was taken, SP is the collector surface,
VTOT is the total plant volume and EUV,n is the accumulated energy (per unit of volume, kJ L-1)
incident on the reactor for each sample taken during the experiment. This procedure is correct
only if the rate of the photoprocesses under examination is linear with the intensity of
radiation (see Chapter 4). For this purpose, several different experiments must be carried out
in the photoreactor to determine this effect at very different solar radiation conditions
(morning, noon, with and without clouds, summer, winter, etc) without changing the kinetic
order of the reaction.
UV, W m-2
Figure 3.7
Typical photocatalytic degradation (-rDCA) in a solar pilot plant under different UV solar light
intensities.
The results of a typical test are shown in Figure 3.7. The reaction rate is calculated at the
beginning of the experiment (zero order) and UV corresponds to exactly the same period
used for the calculation of –r. Therefore, the rate is linear with regard to the radiation intensity
under the same experimental conditions and in the same reactor where Eq. 3.24 is going to be
applied. Figure 3.8 shows the improvement obtained using this equation to calculate the
reaction rate in a two-day photocatalytic degradation experiment. Obviously, UV power
changes during the day and clouds, on the first day, make this variation still more noticeable,
but with Eq. 3.24, the data for both days can still be combined and compared with other
photocatalytic experiments. Consequently, with EUV, the reaction rate (-r) is expressed in
terms of mass of reactant degraded per kJoule of UV incident on the collector surface. If r is
expressed in these units, collector efficiency is already included in it through the use of
incident surface radiation, since different r with the same substance and different solar
collectors, means collector efficiency is different.
47
SOLAR DETOXIFICATION
2nd day
UV, W m-2
C, mg L-1
1st day
2nd day
1st day
C, mg L-1
t, min
EUV, kJ L-1
Figure 3.8
Photocatalytic degradation in a solar pilot plant. Plots of concentration (z) as a function of
experiment time (up) and accumulated energy (down). Solar UV power throughout the
experiment is also shown.
SUMMARY OF THE CHAPTER
A pilot plant has to be as versatile as possible in order for any photocatalytic experiment to be
performed with sufficient confidence. The pilot plant must fulfil all the actual and future
necessities of the research to be performed in it. They may be operated in once-through or
recirculation mode. The calculation of residence time in the reactor is different in each case
and thereby, the conversion. In any case, the use of residence time to calculate reaction rates
(very common in chemical engineering) is not always recommendable because solar power is
not constant. The knowledge of the wavelength interval at which the UV radiometers used are
active is basic to data treatment. On-line measurement of UV power is essential and reactor
efficiency must always be calculated considering, at least, the amount of energy incident on it.
Therefore, it is necessary to find a relationship between the radiometer measurements and the
photons that actually reach the reaction. The estimation of quantum yields using Eq. 3.21
enables results obtained in the photoreactor to be compared with others and thereby, make use
of existing literature on the compounds that are going to be tested. This is very important
when working with a large reactor, where any test means a considerable outlay of time and
expense. So the more information available “a priori”, the fewer experiments are necessary
and the faster useful conclusions may be arrived at. Actinometric experiments have been
shown to be useful to contrast the validity of all those equations related to photon flux
calculation.
BIBLIOGRAPHY AND REFERENCES
.
Curcó, D.; Malato, S.; Blanco, J. and Gimémez, J. Photocatalysis and Radiation Absorption
in a Solar Plant. Sol. En. Mat. Sol. Cells, 44, 199-217, 1996.
Goswami D. Y. and Blake D. M. Cleaning up with Sunshine. Mechanical Engineering,
August, 56-59, 1996.
Kunh, H.J., Braslavsky, S.E. and Schmidt, R. Chemical actinometry. Pure &Appl. Chem.,
Vol. 61, 2, 187-210, 1989.
48
SOLAR DETOXIFICATION
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. The photocatalytic laboratory microreactors must be open to the atmosphere.
2. The power of the pilot plant pump should be low for avoiding high cost.
3. The residence time corresponding to each sample collected at the end of once-through
experiments is calculated with the following equation:
nV
t R ,i = i illu
VTOT
4. In the general case once-through operation system can be modelled as a plug flow reactor.
5. The kinetic constants of photocatalytic processes can be only obtained by plotting
substrate concentration as a function of time.
6. The interval of wavelengths covered in their calibration is not usually reported in UV
instruments technical description.
7. The spectral transmissivity of the photoreactor absorber tube does not affect the
photocatalytic reaction rate.
8. Quantum yield estimated using moles of incident photons is always less that quantum
yield calculated using of moles of photons absorbed by the catalyst.
9. The quantum yield of a chemical actinometer should increase when the intensity of
radiation does.
10. The amount of energy collected by the reactor (per unit of volume) from the start up of the
experiment until each sample is collected, calculated by the following equation, is only a
simplified procedure.
S
EUV,n = EUV,n-1 + ∆ t n UV n P
VTOT
∆ t n = t n - t n −1
PART B.
1. Why is it necessary filtering IR in laboratory photoreactors?
2. Why a pilot plant has to be as versatile as possible?
3. Which are the main characteristics of the material to be used for connecting the pilot plant
photoreactors?
4. Which are the most important sensors to be installed throughout and at a photocatalytic
pilot plant?
5. Why is it very important the flow rate in batch experiments?
6. Why residence time is not the best variable for obtaining kinetic constants in solar
photocatalysis?
7. Which are the essential data, necessary for obtaining an equation similar to the following,
for correlating radiometric measurements and photonic flux inside a photocatalytic solar
photoreactor?
N
I λ1− λ 2 = λ1− λ 2 = f (UV )
N0
8. Which is the wavelength considered equivalent to the following normalised spectrum?
λ, nm
300
310
320
fλ
0.0002
0.0023
0.0063
49
SOLAR DETOXIFICATION
330
340
350
360
370
380
390
0.0109
0.0118
0.0124
0.0127
0.0159
0.0156
0.0160
9. If Einstein (moles of photons) is used as the unit, which is the number of photons
corresponding to the average radiation at any given instant in the previous example?
10. Which are the essential data for radiation evaluation in a solar photocatalytic reactor?
Answers
Part A
1.False; 2. False; 3. False; 4. True; 5. False; 6. True; 7. False; 8. True; 9. False; 10. True.
Part B
1. Because IR beams could heat the slurry suspensions very quick.
2. Because the necessity of accomplishing all the experiments with enough confidence and
fulfilling all the actual and future necessities of the research to be performed in it.
3. All the tubes and valves must be strongly resistant to chemicals, weather-proof and
opaque, in order to avoid any photochemical effect outside of the photoreactors.
4. Temperature, pressure, dissolved oxygen and solar irradiation.
5. Because it is directly related with the conversion per pass and, therefore, with the
modelling of the photoreactor.
6. Because when time is the independent variable, the differences in the incident radiation in
the reactor during an experiment are not taken into account.
7. Solar spectra.
8.
λ = 390 nm
λ = 390 nm
λ = 300 nm
λ = 300 nm
∑
fλ = 1 ;
∑f
λ
λ = 373.6 nm
9. 3.135 x 10-6 UV (Einstein s-1 m-2)
10. Radiation data from a radiometer mounted on the same angle, as the solar collector.
50
SOLAR DETOXIFICATION
4. FUNDAMENTAL PARAMETERS IN PHOTOCATALYSIS
AIMS
This unit describes the fundamental parameters related to heterogeneous photocatalysis
reactions. Examples for better comprehension of oxygen, pH, catalyst concentration, initial
substrate concentration, radiation intensity and effect of temperature are also shown. It
outlines the basic tests for understanding experimental system behaviour when these
parameters change and why these changes affect the photocatalytic reaction rate. Finally, an
overview of the different ways of evaluating quantum yields is presented.
OBJECTIVES
By the end of this unit, you will know the main aspects of experimental photocatalysis
variables and you will be able to do 7 things:
1. Understand why blank experiments are always necessary in photocatalysis.
2. Design experiments that avoid the effects of oxygen and pH and, thereby, reduce the
number of variables affecting the tests.
3. Comprehend the influence of catalyst concentration and how to find the optimum catalyst
mass in an experimental reactor.
4. How a Langmuir Hinshelwood (L-H) model, which, although it does not explain the
photocatalytic mechanisms, may be used in heterogeneous photocatalysis to obtain the
kinetic constants for reactor optimisation.
5. Arrange experiments to obtain L-H parameters.
6. Describe the interdependence of the photocatalytic reaction rate and illumination intensity
and when this relationship is directly proportional.
7. Determine useful parameters for describing photocatalysis efficiency without knowing the
amount of photons absorbed by the system and, why quantum yield is almost impossible
to be calculated in heterogeneous photocatalysis experimental systems.
NOTATION AND UNITS
Symbol
C or Ci Reactant concentration
C0
Initial reactant concentration
CS
Solvent concentration
E
Activation energy
I
Photon flux corresponding to the UV inside a photoreactor.
k’
Apparent reaction rate constant
kr
Reaction rate constant
K
Reactant adsorption constant
KS
Solvent adsorption constant
L-H
Langmuir Hinshelwood model
Number of photons absorbed
Na
pKa
-log(acid ionisation constant)
pO2
Oxygen partial pressure
r
Reaction rate
Number of reacting molecules
∆n
Quantum yield
Φ
Fraction of surface covered by the substrate
θx
Relative photonic efficiency
ζr
Units
M or mg L-1
M or mg L-1
M or mg L-1
kJ mol-1
Einstein L-1 s-1
min-1
M min-1
M-1
M-1
Photons s-1
atm
M min-1
Molecules s-1
51
SOLAR DETOXIFICATION
4.1 DIRECT PHOTOLYSIS
As mentioned in Chapter 1, some pollutants can only be dissociated in the presence of UV
light. For this, the pollutant must absorb the light of the lamp (or the sun) with a reasonable
photodissociation quantum yield. Although organic pollutants absorb light over a wide range
of wavelengths, this is generally stronger at the lower wavelengths. However, such natural
photodegradation is usually very slow. For example, at least 10 days under perfectly sunny
conditions is necessary to reduce 50 mg/L acrinathrin to half. One half of 100 mg/L
pentachlorophenol at pH 7.3 decomposes in 48 hours. So the photolytic reaction rate is usually
different from one substance to another even in the same experimental device. The photolytic
half-lives of hundreds of substances has been summarised by Tomin (1994).
In any case, the focus here is on fundamental photocatalytic parameters and therefore the
photolytic effect will be discussed from this point of view. These tests have to be performed in
order to find out the decomposition rates without the semiconductor. As TiO2 readily sticks to
the glass in the photoreactors, it is necessary to carry out these tests at the beginning, before
the catalyst comes into contact with the photoreactors. In pilot-plant-scale experiments,
removal of the thin coating of catalyst on the tubes after TiO2 suspensions have circulated
through them is a very hard, complex and expensive task. In laboratory tests, TiO2 can only be
removed with an ultrasonic cleaner or by abrasion. This type of experiment will focus on
demonstrating the absence or evaluating the importance of the following effects:
• The treatment is not feasible without a catalyst.
• Increase in temperature (due to illumination) in the reactor does not cause product loss.
• There is no adsorption of pollutant or its metabolites in the materials of the pilot plant.
After these tests have been performed, the photocatalytic experiment results may be
considered accurate and the kinetic parameters can be determined properly. Any side effect of
the photocatalytic reaction rate can be quantified and subtracted from the global rate, resulting
in the real photocatalytic reaction rate.
4.2 INFLUENCE OF OXYGEN
In semiconductor photocatalysis for water purification, the pollutants are usually organic and,
therefore, the overall process can be summarised by Eq.4.1. Given the reaction stoichiometry
of this equation, there is no photomineralization unless O2 is present. The literature provides a
consensus regarding the influence of oxygen. It is necessary for complete mineralization and
does not seem to be competitive with other reactives during the adsorption on TiO2 since the
places where oxidation takes place are different from those of reduction (See Figure 1.5). The
O2 avoids the recombination of e-/h+ (Eq. 1.24) and, photoactivated oxygen (O2•-) also reacts
directly (Table 1.3).
organic pollutant + O2
 
→ CO
semiconductor
2
+ H 2 O + mineral acids
(4.1)
ultra − bandgap energy
The concentration of oxygen also affects the reaction rate, which is faster when the partial
pressure of oxygen (pO2) in the atmosphere in contact with the water increases. In any case, it
seems that the difference between using air (pO2 = 0.21 atm) or pure oxygen (pO2 = 1 atm) is
not drastic (See Figure 4.1). In an industrial plant it would be purely a matter of economy of
design. In Figure 4.1 it is clear that when all the oxygen contained in the water has been
consumed, photodecomposition of TOC comes to a halt. At the moment injected oxygen
reaches the reactor, photodecomposition continues. Therefore, injection of pure O2 becomes
necessary in once-through experiments (See Figure 3.3 and Figure 3.4) at low flow rates. At
52
SOLAR DETOXIFICATION
high flow rates or with recirculation, the addition of oxygen is not always necessary since the
illumination time per pass is short. The water again recovers the oxygen consumed when it
reaches the tank (open to the atmosphere and stirred).
3.0
2.5
reaction rate
O2
C/C0
2.0
1.5
air
1.0
0
20
40
60
pO2
80
100
0.5
TOC
0.0
0
10
20
30
40
Illumination time, min
Figure 4.1
Effect of the concentration of dissolved oxygen on photocatalytic mineralization. [O2] 0 = 8.5
mg L-1. In the inset, the usual effect of partial pressure of oxygen on the photocatalytic
reaction rate is shown.
4.3 pH INFLUENCE
The pH of the aqueous solution significantly affects TiO2, including the charge of the particle
and the size of the aggregates it forms. The pH at which the surface of an oxide is uncharged
is defined as the Zero Point Charge (pHzpc), which for TiO2 is around 7. Above and below this
value, the catalyst is negatively or positively charged according to:
− TiOH 2+ ↔ TiOH + H +
(4.2)
−
+
− TiOH ↔ −TiO + H
(4.3)
The equilibrium constants of these reactions (Kormann et al. 1991) are pKTiOH2+ = 2.4 and
pKTiOH = 8.0, the abundance of all the species as a function of pH: -TiOH ≥ 80% when
3<pH<10; -TiO- ≥ 20% if pH>10; -TiOH2+ ≥ 20% when pH<3. Under these conditions, the
photocatalytic degradation of the ionisable organic compounds is affected by the pH. At first
sight, and for pollutants for which pKa is outside the range of 1-13, a very acidic solution
appears to be detrimental and a very basic solution to be favourable, since the variations are
modest or non-existent around neutrality. Because even at extreme pHs the change in the
photocatalytic rate is generally less than one order of magnitude, the TiO2 water treatment
definitively possesses an advantage over other processes. In many cases, a very important
feature of photocatalysis is not taken into account when it is to be used for decontamination of
water, is that during the reaction, a multitude of intermediate products are produced that may
behave differently depending on the pH of the solution. To use only the rate of decomposition
of the original substrate could yield an erroneous pH as the best for contaminant degradation.
53
SOLAR DETOXIFICATION
Therefore, a detailed analysis of the best pH conditions should include not only the initial
substrate, but also the rest of the compounds produced during the process.
Mean particle-size measurements (presented in Figure 4.2) have been found to be constant at
pH far from pH ≈ 7. 300 nm sizes increase to 2-4 µm when dispersion reaches pHzpc. The zero
surface charge yields zero electrostatic surface potential that cannot produce the interactive
rejection necessary to separate the particles within the liquid. This induces a phenomenon of
aggregation and TiO2 clusters become larger. The large mean size in suspension at pH≈7
becomes much smaller at pH far from 7. This effect is clearly related to the capability of the
suspension for transmitting and/or absorbing light. Furthermore, larger clusters sediment more
quickly than small particles, thus the agitation necessary to maintain perfect homogeneity
must be more vigorous. In contrast, these variations in particle size could be an advantage for
separating the catalyst from water (by sedimentation and/or filtration) at completion of
photocatalytic treatments.
3000
pHZPC= 6.9
Mean size, nm
2500
2000
1500
1000
500
0
2
4
6
pH
8
10
12
Figure 4.2
Mean particle size of TiO2 (P-25) suspended in water versus pH. [TiO2]=0.2 g/L.
To unambiguously exclude the pH effect, the reagents used to change the pH must contain
counterions that have no effect on the rate of water treatment. Sodium hydroxide and
hydrogen chloride or sulphuric acid have generally been chosen to produce basic or acid pH,
respectively. Organic buffers must be avoided because they are potential consumers of •OH,
and toxic inorganic acids or bases the same, for evident reasons.
4.4 INFLUENCE OF CATALYST CONCENTRATION
Whether in static, slurry or dynamic flow photoreactors, the initial reaction rates were found
to be directly proportional to catalyst mass. This indicates a truly heterogeneous catalytic
regime. However, above a certain value, the reaction rate levels off and becomes independent
of catalyst mass. This limit depends on the geometry and working conditions of the
photoreactor and is for a definite amount of TiO2 in which all the particles, i.e. the entire
surface exposed, are totally illuminated. When catalyst concentration is very high, after
travelling a certain distance on an optical path, turbidity impedes further penetration of light in
the reactor. In any given application, this optimum catalyst mass ([TiO2]OPT) has to be found
in order to avoid excess catalyst and ensure total absorption of efficient photons.
54
SOLAR DETOXIFICATION
UV LAMP
UV path length
LAMP
PROTECTION
CHAMBER
Cooling
water
REACTION CHAMBER
UV LAMP
UV
UV
UV
UV
UV
UV
UV
UV
UV
a)
b)
UV
c)
Figure 4.3
Different laboratory photoreactor designs and zones of radiation penetration when
illuminated in different ways.
There are a number of studies in the literature on the influence of catalyst concentration on
process efficiency. The results are very different, but from all of them it may be deduced that
radiation incident on the reactor and path length inside the reactor are fundamental in
determining the optimum catalyst concentration:
•
•
•
If the lamp is inside of the reactor and coaxial with it (see Figure 4.3a), [TiO2]OPT is very
high (on the order of several grams per litre) if the path length is short (several mm). On
the other hand, [TiO2]OPT is low (hundreds of mg per litre) if several centimetres are
crossed (see Figure 4.4 solid circles).
If the lamp is outside (see Figure 4.3c), but the path length is short (1-2 cm max.), the
maximum rate is obtained with 1-2 g L-1 of TiO2 (see Figure 4.4 solid squares).
If the lamp is outside, but the path length is several centimetres long, similar to a reactor
illuminated by solar radiation (see Figure 4.3b and 3.6), the appropriate catalyst
concentration is several hundreds of milligrams per litre. An example of this is shown in
Figure 4.4, where open circles corresponds to a photoreactor with a large diameter and
solid triangles to one with a smaller diameter, but both several centimetres. In this case, it
is very clear than the optimum rate is attained at lower catalyst concentrations when the
photoreactor diameter is wider.
In all the cases described above, a “screening” effect is produced when the TiO2 concentration
is very high. The reaction rate diminishes due to the excessive opacity of the solution, which
prevents the catalyst farthest in from being illuminated. Besides, the larger the size (see Figure
4.2), the less the opacity of the suspension. When the radiation comes from a parabolic trough
collector, something similar to what is shown in Figure 4.3b occurs. In any case, these are
only approximations based on the results obtained by different authors and, because of all of
the above, it is always necessary to find out, experimentally, the optimum catalyst
concentration for the plant studied. That is, the minimum concentration at which the
maximum reaction rate is obtained. But it does not seem to be necessary to test a very wide
range of concentrations. This effect is shown in Figure 4.4. Usually, the reaction rate increases
very quickly with TiO2 concentration, but only at low catalyst concentrations (usually below
100 mg/L). After that, the reaction rate stabilises and at very high catalyst concentrations the
55
SOLAR DETOXIFICATION
reaction rate decreases. When this point is found, it is not really necessary to continue
checking any higher because no more useful information is going to be obtained.
1.2
1.0
0.6
TOC
1.0
relative rate
relative rate
0.8
0.4
0.5
0.2
0.0
0.0
0.2
0.0
0.0
0.4
0.8
TiO2, g L
1.2
0.4
-1
TiO2, g L
1.6
0.6
0.8
2.0
-1
Figure 4.4
Influence of catalyst concentration on the rate of photocatalysis (normalised rates have been
used to make it more easily understood) in different reactors (see text). The continuous line is
only to clarify the tendency. In the inset, relative reaction rates of degradation (two different
photoreactors) and mineralization (TOC) of the same contaminant are shown. See text for
comments.
As shown in Figure 4.4, initial substrate behaviour is not always the same as that of TOC.
This is due to the influence of intermediate products generated during the photocatalytic
reaction. That is, the screening effect influences the decomposition of the original product in a
different way than the other organic species present in the reaction and, in the example
presented, to mineralise the contaminant it is unnecessary to use more than 0.2 g L-1 of TiO2.
4.5 INITIAL CONTAMINANT CONCENTRATION INFLUENCE
As oxidation proceeds, less and less of the surface of the TiO2 particles is covered as the
contaminant is decomposed. Evidently, at total decomposition, the rate of degradation is zero
and a decreased photocatalytic rate is to be expected with increasing illumination time (or the
accumulated energy, see Eq. 3.24). Most authors agree that, with minor variations, the
expression for the rate of photomineralization of organic substrates with irradiated TiO2
follows the Langmuir Hinshelwood (L-H) law for the same saturation-type kinetic behaviour
in any of four possible situations: (i) the reaction takes place between two adsorbed
substances; (ii) the reaction occurs between a radical in the solution and the adsorbed
substrate; (iii) the reaction takes place between the radical linked to the surface and the
substrate in the solution; and (iv) the reaction occurs with both species in solution. In all
cases, the expression of the equation rate is similar to the L-H model. From kinetic studies
only, it is therefore not possible to find out whether the process takes place on surface or in
solution. Although the L-H isotherm has been rather useful in modelling the process, it is
generally agreed that both rate constants and orders are only "apparent". They serve to
describe the rate of degradation, and may be used for reactor optimization, but they have no
physical meaning, and may not be used to identify surface processes. Thus, while not a useful
56
SOLAR DETOXIFICATION
tool for describing the active species involved in oxidation, engineers and solar designers
seem to have a common understanding on the usefulness of the unmodified L-H model.
(C0 - C) = krt
2.5
0.8
ln (C0/C) = k’t
2.0
0.6
ln (C0/C)
Normalised Concentration
1.0
0.4
1.5
1.0
0.5
0.0
0.2
0
5
10
15
EUV, kJ/L or time, min
0.0
0
5
10
15
20
25
EUV, kJ/L or time, min
Figure 4.5
Typical photocatalytic degradation. The insert shows data adjusted to Eq. 4.6.
Due to the above, for L-H standard data treatment, it is assumed that the reaction occurs on
the surface, which is also the assumption most widely accepted as possible. Under these
conditions, two extreme situations are defined to illustrate the adsorption on the catalyst
surface: (I) substrate and water compete for the active catalyst sites and (II) the reactant and
the solvent are adsorbed on the surface without competing for the same active catalyst sites.
According to the L-H model, the reaction rate (r) is proportional to the fraction of surface
covered by the substrate (θx). In each case the following expression can be obtained:
dC
k r KC
r== krθ x =
(4.4a)
dt
1 + KC + K s C s
dC
k KC
r== krθ x = r
(4.4b)
dt
1 + KC
where kr is the reaction rate constant, K is the reactant adsorption constant, C is he
concentration at any time, KS is the solvent adsorption constant and CS is its concentration (in
water CS ≈ 55.5 M). As CS >>C and, CS remains practically constant, the part of the catalyst
covered by water is unalterable over the whole range of C and the previous equations can be
integrated:
K
K
C
( C 0 - C) = k r
t
ln 0 +
(4.5a)
C 1+ K s Cs
1+ K s C s
C
ln 0 + K( C 0 - C) = k r Kt
(4.5b)
C
When C0 is very small, both equations can be reduced to an order one-reaction rate equation:
C
ln 0 = k ′t
(4.6)
C
So, if ln (C0/C) is represented versus t (or the accumulated energy, see Eq. 3.24), a line, the
slope of which is the apparent reaction rate constant k’, should be obtained (see Figure 4.5).
Likewise, at higher concentrations, both equations can be simplified by adjusting them to zero
order, (C0 - C) = krt, as might be the case at the beginning of the experiment represented in
Figure 4.5.
57
SOLAR DETOXIFICATION
r0
r0
-1
t1/2
Using an L-H model, graphics similar to those depicted in Figure 4.6 may be obtained from
the experimental data and from the linearisation of the previous equations. The effect of the
initial concentration on the degradation rate is shown in Figure 4.6a, where, due to the
saturation produced on the semiconductor surface as the concentration of the reactant
increases, it reaches a point at which the rate becomes steady. Figure 4.6b shows a
linearisation of Eqs. 4.4, where the slope of that straight line is (1+KSCS)/krK, but could also
be 1/krK. Finally, Eqs. 4.7 are obtained from Eqs. 4.5 when the concentration is half of the
initial (C/C0 = 0.5):
0.693(1 + K S C S ) 0.5 C 0
+
(4.7a)
t 1/2 =
kr K
kr
0.693 0.5 C 0
+
(4.7b)
t 1/2 =
kr K
kr
C0
C0
-1
C0
a)
b)
c)
Figure 4.6
Graphics related to the adjustment of data to a L-H type kinetic model
It should be emphasised that photodecomposition gives rise to intermediates, which could also
be adsorbed competitively on the surface of the catalyst. The concentration of these
intermediates varies throughout the reaction up to their mineralization and thus, Eq. 4.4 may
also take the following form:
k r KC
r=
(4.8)
n
1 + KC + ∑ K i C i (i = 1, n )
i =1
where i is the number of intermediates formed during degradation (the solvent is also included
in the summation).
An understanding of the reaction rates and how the reaction rate is influenced by different
parameters is important for the design and optimisation of an industrial system. The L-H
reaction rate constants are useful for comparing the reaction rate under different experimental
conditions. Once the reaction constants kr and K have been evaluated, the disappearance of
reactant can be estimated if all other factors are held constant. Due to this, a series of tests at
different initial substrate concentrations has to be performed to demonstrate whether the
experimental results could be adjusted with this model. The concentration range has to be
wide enough to allow correct fit of the L-H linearisation. This means, from the lowest
concentration at which the initial rate could be determined until the limit where the
relationship between initial reaction and initial concentration remains constant (see Figure
4.6). The results shown in Figure 4.5 correspond to an example of each of the experiments to
be carried out for this purpose. From the slope of the line corresponding to the starting points
of each of the experiments, the initial degradation rate can been calculated. Figure 4.7 shows
58
SOLAR DETOXIFICATION
the initial rate calculated. From 0.2-0.4 mM of substrate the initial rate is steady. At this
concentration, catalyst saturation occurs and the reaction rate becomes constant.
2.0
2.0
1.5
1.5
2
1.0
1.0
1/r0
r0, mM h
-1
3
1
0.5
0.0
(kr)-1 = 0.638
0.0
(krK)-1 = 0.0381
0.5
0
0.2
0
25
0.4
1/C0
0.6
50
75
0.8
1.0
0.0
C0, mM
Figure 4.7
Initial degradation rate as function of the initial substrate concentration. The insert shows the
linear transformation of Eq. 4.4 from which the rate constant and the adsorption coefficient
can be estimated from the intercept and the slope, respectively.
The constants can be calculated from the graphic inserted in Figure 4.7 using the L-H model.
1 1
1 1
k r KC 0
→ = +
(4.9)
r0 =
1 + KC 0
r0 k r k r K C0
kr = 1.57 mM h-1
K = 16.75 mM-1
With known kr and K the time required to degrade a definite substrate concentration (C0)
down to a certain level (C) may be found from Eq. 4.5. With this, definition of reactor volume
and surface (see Eq. 3.2 or 3.3) is possible. If accumulated energy is used instead of time,
them Eq. 3.24 should be used for obtaining the kinetic constants and, afterwards, for reactor
design.
4.6 RADIANT FLUX INFLUENCE
Since 1990, the kind of solar technology, which should be involved in detoxification, has been
clarified. Initial experiments with parabolic troughs for water and dishes or furnaces for gasphase treatments have evolved to lower flux systems. The reason for using one-sun systems
for water treatment is firmly based on two factors, first the high percentage of UV photons in
the diffuse component of solar radiation and second the low order dependence of rates on light
intensity. It has been demonstrated by experiment that above a certain UV photon flux,
reaction rate dependency on intensity goes down from one to a half order (Ollis, 1991;
Herrmann, 1995). This does not seem to occur at a particular radiation intensity, as different
researchers obtain different results, but presumably is significantly affected by experimental
conditions. Some authors impute the transition of r = f (I1.0) to r = f (I0.5), to the excess of
59
SOLAR DETOXIFICATION
photogenerated species (e-, h+ and •OH). This can be demonstrated as follows. According to
section 1.4, the five basic simplified equations are:
TiO2 + hν → e − + h +
(4.10a)
−
+
e + h → N + energy
(4.10b)
−
−
A+e → A
(4.10c)
+
+
D+h → D
(4.10d)
−
+
A + D → Intermediates → Products
(4.10e)
and the rate-limiting step is the reaction in the adsorbed phase (Eq. 4.10e). Therefore:
[ ][ ]
r = re = k e A − D +
(4.11)
In an n-type semiconductor such as titania, the photo-induced holes are much less numerous
than electrons (photo-induced electrons plus n-electrons): [p+]<<[e-]. Therefore holes are the
limiting active species. Thence:
[ ]
r = re = rd = k d [D ] p +
(4.12)
At any instant, one has:
d p+
= ra − rb − rd = 0 = k a − k b e − p + − k d [D ] p +
(4.13)
dt
Thence:
ka I
p+ =
(4.14)
−
k b e + k d [D ]
and:
k a k d [D ]I
r=
(4.15)
k b e − + k d [D ]
From the above equation, it can be seen that the reaction rate is directly proportional to light
flux. In the case of high fluxes, the instantaneous concentrations [e-] and [p+] become much
larger than kd[D] and [e-] ≈ [p+]. Therefore Eq 4.14 becomes:
k I
k I
2
p + ≈ a − ; Therefore p + ≈ a
(4.16)
kb
kb e
The reaction rate becomes:
[ ]
[ ][ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
[ ]
1/ 2
REACTION RATE
k I 
r = re = rd = k d [D ]  a 
(4.17)
 kb 
which means that r is proportional to I0.5 and that the rate of electron-hole formation is greater
than the rate of photocatalysis, which favours electron-hole recombination. Therefore
optimum use of light power corresponds to the region where r is proportional to I.
r = k I½
r=kI
FOTONIC FLUX
Figure 4.8
60
r=k
SOLAR DETOXIFICATION
Relation between the photocatalytic reaction rate and the intensity of the radiation received.
At higher radiation intensities, another transition from r = f (I0.5) to r = f(I0) is produced. At
this moment, the photocatalytic reaction is no longer dependent on the radiation received,
depending only on mass transfer within the reaction. So, the rate is constant although the
radiation increases. This effect may be due to different causes, such as a lack of electron
scavengers (i.e. O2), or organic molecules in the proximity of the TiO2 surface and/or excess
of products occupying active centres of the catalyst, etc. Actually, these phenomena appear
more frequently when working with a supported catalyst, and/or at slow agitation speeds,
which implies less catalyst surface in contact with the liquid and less turbulence. It does not
favour reactant contact with the catalyst or dispersion of products from the proximity of the
catalyst to the liquid.
Many articles on this aspect of photocatalysis provide information on the light intensity at
which the change of order is produced. The values found are very dissimilar. It may only be
intuited that at an intensity of several suns (1 sunUV = 22 WUV m-2), quantum yield diminishes.
This effect should be measured experimentally in each device, but this limit (several suns) is
usually accepted as a general rule.
4.7 TEMPERATURE EFFECT
Because of photonic activation, photocatalytic systems do not require heating and operate at
room temperature. The true activation energy Et is nil, whereas the apparent activation energy
Ea is often very low (a few kJ/mol) in the medium temperature range (20ºC-80°C ). However,
at very low temperatures (-40°C-0°C), activity decreases and activation energy Ea becomes
positive. By contrast, at "high" temperatures (>70-80°C) for various types of photocatalytic
reactions, the activity decreases and the apparent activation energy becomes negative. This
behaviour can be easily explained within the frame of the Langmuir-Hinshelwood mechanism
described above. The decrease in temperature favours adsorption, which is a spontaneous
exothermic phenomenon. In Eqs. 4.4 θ tends toward unity, whereas KC becomes >>1. In
addition, the lowering temperature also favours adsorption of the final reaction products,
desorption of which tends to be the rate-limiting step. To the contrary, when temperature
increases above 80°C, nearing the boiling point of water, the exothermic adsorption of
reactants is disfavoured and this tends to become the rate-limiting step.
In addition to these mechanical effects, other consequences of plant engineering must be
considered. If temperature is high, the materials (see point 3.2) used for the plant should be
temperature-resistant (more expensive), and oxygen concentration in water decreases.
Consequently, the optimum temperature is generally between 20 and 80°C. This absence of
need for heating is attractive for photocatalytic reactions carried out in aqueous media and in
particular for environmental purposes (photocatalytic water purification). There is no need to
waste energy heating water that already possesses a high thermal capacity.
4.8 QUANTUM YIELD
In photochemistry, a concept called quantum yield is used to evaluate the results obtained and
compare different experimental conditions. Recalling Eq. 2.1 (Φ = ∆n Na-1), the quantum
yield of a photochemical reaction is defined with regard to the number of reacting molecules
and the number of photons absorbed.
61
SOLAR DETOXIFICATION
In this book, a heterogeneous system made up of a suspended solid (TiO2), a gas (O2) in
bubbles and/or dissolved aqueous solution of a multitude of compounds (initial substrate,
intermediates, H+, anions…) is described. Finding out the amount of photons absorbed by the
catalyst, from the behaviour of the radiation incident on a suspension such as this, is very
difficult. In order to calculate this, if so desired, one would have to: a) evaluate the light
absorption of a very complex reactive mixture, which, moreover, changes its composition
throughout the reaction, b) from this basis, determine the photon flux that arrives at each
particle of the catalyst to photoactivate it, and c) estimate the photons absorbed and dispersed.
Furthermore, it seems that this, for the moment, is a difficult undertaking (Serpone et al.,
1996). Remember that, in heterogeneous catalysis, the reaction rate is usually expressed as a
function of the grams of catalyst. In photocatalysis, it should include the number of active
centres, as well as the surface area of catalyst. But as a consequence of the above comments,
the number of active centres is unknown and the surface of catalyst exposed to light is
undetermined.
This is therefore simplified by considering only the radiation of a certain wavelength (Eq.
3.21) incident on the inside of the reactor for calculation of Na. The value obtained from this
is called the estimated quantum yield: ΦE. No distinction is made between the photons
corresponding to each wavelength, assuming that all of them have the same effect on the
surface of the catalyst. In all cases, this simplification is accepted as valid by a multitude of
authors and widely used in the bibliography. Consequently, the reported “quantum yields”
have sometimes been reported as lower limits not allowing for scattered light. A simple means
of assessing process efficiencies for equal absorption of photons is therefore desirable in
heterogeneous photocatalysis (Eq. 4.18). The initial photoconversion of phenol has been
chosen as the standard process and Degussa P-25 titania as the standard photocatalyst. This
compromise has been adopted by a group of scientist (belonging to different research groups)
considered among the most important in the world (Serpone et al., 1996). The choice of
phenol was dictated by the recognition that the molecular structure of phenol is present in
many organic pollutants and, like many of theses, is essentially degraded by oxidation rather
than reduction.
rate of disappearance of substrate
ζr =
(4.18)
rate of disappearance of phenol
where ζr is called relative photonic efficiency. When the reaction rate for the test substances
and phenol (secondary actinometer) are obtained under identical experimental conditions there
is no need to measure the photon flux. The use of relative photonic efficiency renders
comparison of process efficiencies between studies carried out in different laboratories or
pilot plants possible because ζr is basically independent of the fundamental photocatalysis
parameters (light intensity, reactor geometry and TiO2 concentration for a given catalyst).
However, it depends on the initial concentration of substrate and on temperature. In any case,
based on initial rates of degradation, ζr illustrates only one aspect of photodegradation and is
also useful to compare different photocatalyst materials for water treatment purposes.
An example of an application of ζr to degradation of pesticides (lufenuron and propamocarb)
under sunlight is shown in the following paragraphs. Phenol (20 mg/L) has been used to
calculate ζr following the method proposed above. In Figure 4.9 the experiments carried out
with phenol are shown.
62
SOLAR DETOXIFICATION
1.0
Normalized Concentration
1.0
0.8
Average
Average
C/C0
0.8
TOC/TOC0
0.6
0.6
0.4
0.4
0.2
0.2
0.0
0
5
10
15
20
25
0.0
EUV, kJ/L
Figure 4.9
Plots of the normalised concentration as a function of the accumulated energy for the
photodegradation and mineralization of phenol. C0 = 20 mg/L , TiO2 = 200 mg/L, pH0 = 5.
The mineralization rate (measured by TOC analysis) is also included because efficiencies
based on the disappearance of organic carbon (ζr ,TOC, Eq. 4.19) provide more practical
information.
rate of disappearance of substrate TOC
ζ r ,TOC =
(4.19)
rate of disappearance of TOC from phenol
Phenol and pesticide experiments have been performed under the exact same conditions. With
sunlight, it is not possible to work under constant illumination conditions. Therefore, Eq. 3.24
is used to avoid this uncertainty and reaction rates used to determine ζr and ζr ,TOC are
calculated using EUV instead of time. Relative photonic efficiencies of the two pesticides
tested are reported in Table 4.1. All the efficiencies are lower than unity, indicating that the
initial photocatalytic oxidative degradations of the test substances, at the selected initial
concentration, are less efficient than for phenol.
Substrate
Rate µmol/kJ
ξr
ξr,TOC
Propamocarb
5.8
0.21
0.48
Lufenuron
8.8
0.32
0.73
Table 4.1 belongs here.
Relative photonic efficiencies for the two pesticides treated with phenol as the standard
reference (C0 = 20 mg/L).
SUMMARY OF THE CHAPTER
Photolysis tests have to be performed always before photocatalysis tests in order to find out
decomposition rates without the semiconductor. The water to be treated must contain enough
dissolved oxygen. To unambiguously exclude the effect of pH, the reagents used to modify the
pH must contain counterions that have no effect on the rate of water treatment. The optimum
catalyst concentration always depends on the experimental device used and, therefore, must be
63
SOLAR DETOXIFICATION
always tested first. The direct application of the Langmuir-Hinshelwood model produces an
empirical equation, which fits the degradation experimental data accordingly. This equation is
useful in a wide range of initial concentrations and is necessary for engineering plant design.
Experimentation at pilot plant level is essential to obtain these equations. Above a certain flux
of UV photons, reaction rate changes depending on intensity. It may only be intuited that at
intensities of several suns, quantum yield diminishes. This effect should be measured
experimentally in each device, but this limit is generally accepted. The optimum temperature
is generally between 20 and 80°C. The use of relative photonic efficiencies renders
comparison of process effectiveness between studies carried out in different reactors possible.
BIBLIOGRAPHY AND REFERENCES
Herrmann, J.M. Heterogeneous Photocatalysis: an Emerging Discipline Involving Multiphase
System. Catalysis Today, 24, 157-164, 1995.
Ollis, D.F. Solar-Assisted Photocatalysis for Water Purification: Issues, Data, Questions.
Photochemical Conversion and Storage of Solar Energy, 593-622, Kluwer Academic
Publishers, 1991.
Serpone, N., Sauvé, G., Koch, R., Tahiri, S., Pichat, P., Piccinni, P., Pelizzetti, E., Hidaka, H.
Standardisation protocol of process efficiencies and activation parameters in
heterogeneous photocatalysis: relative photonic efficiencies ζr J. Photochem.
Photobiol. A: Chem., 95, 191-203, 1996.
Tomin C. The Pesticide Manual, a World Compendium. 10th ed. British Crop Protection
Council and Royal Society of Chemistry. Croydon, UK, 1994.
Kormann, C., Bahnemann, D.W. and Hoffmann M. R. Photolysis of Chloroform and other
Organic Molecules in Aqueous TiO2 Suspensions. Environ. Sci. Technol., 25, 494500, 1991.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. Oxygen avoids recombination of e-/h+ and promotes •OH formation.
2. The reagents used to change the pH must contain organic buffers.
3. The optimum concentration of the catalyst is always around 1 g/L.
4. Initial substrate behaviour is always the only fundamental parameter for choosing
[TiO2]OPT.
5. By applying the Langmuir Hinshelwood (L-H) rate law it is possible to find out whether
the photocatalytic process is on the surface or in solution.
6. When C0 is very small, L-H isotherm equations can be reduced to an order one-reaction
rate equation.
7. It has been demonstrated that above a certain UV-photon flux, reaction rate changes from
one to half-order dependence to the intensity.
8. Temperature does not affect photocatalytic reaction rate.
9. The amount of photons absorbed by the catalyst must be known to compare photocatalytic
reactors.
10. Simplifications must be employed for calculating photocatalytic efficiencies.
PART B.
1. Why is it necessary to carry out a blank test before putting TiO2 in a photoreactor?
2. What are blank photocatalysis experiments mainly for?
64
SOLAR DETOXIFICATION
3. What is the main difference between bubbling oxygen and air inside a photoreactor?
4. To unambiguously exclude the pH effect, what are the reagents most often used to change
the pH?
5. Why are L-H constants called “apparent” in photocatalysis?
6. Calculate the L-H constant corresponding to the following experimental data:
Table 4.2 belongs here.
Experimental data from a series of photocatalytic experiments at different initial
concentration.
7. What is the most accepted reason for photocatalytic reaction rate dependence on change of
order of intensity?
8. What is the usual UV power of light intensity where the change of order is produced?
9. What is the optimum temperature range for heterogeneous photocatalytic reactions?
10. Why is use of a secondary actinometer necessary?
Answers
Part A
1.True; 2. False; 3. False; 4. False; 5. False; 6. True; 7. True; 8. False; 9. False; 10. True.
Part B
1. As TiO2 readily sticks to the materials of the photoreactors.
2. This type of experiment evaluates the importance of the treatment without catalyst, the
increase in temperature in the reactor and the adsorption of pollutant or its metabolites in
the materials of the pilot plant.
3. The concentration of O2 inside the reactor. By bubbling oxygen, the [O2] is almost five
times greater than by bubbling air.
4. Sodium hydroxide and hydrogen chloride or sulphuric acid.
5. Because with kinetic studies only, it is impossible to find out whether the process is on the
surface or in solution and therefore, the constants obtained have no physical meaning, and
may not be used to identify surface processes.
6. kr = 2.19 mg L-1 min-1; K = 0.0192 (mg/L)-1
7. The transition of r = f(I1.0) to r = f (I 0.5) is usually attributed to the excess of
photogenerated species (e-, h+ and •OH).
8. Above 50-75 WUV per square meter of photoreactor surface, the quantum yield
diminishes.
9. Between ambient temperature and 60-70 ºC.
10. Because ζr is basically independent of photocatalysis fundamental parameters (light
intensity, reactor geometry and TiO2 concentration for a given catalyst).
65
SOLAR DETOXIFICATION
5. WATER DECONTAMINATION BY SOLAR
DETOXIFICATION
AIMS
This unit describes the most important applications of photocatalysis for water
decontamination. It outlines the decomposition of organic and inorganic contaminants and the
usefulness of biological testing in addition to chemical analyses. Examples are also given to
facilitate comprehension of the degradation pathways involved in the decomposition of the
initial compounds. The use of modified catalysts and additional oxidants to increase process
efficiency by trapping the photogenerated electrons and/or by producing extra oxidising
species is discussed. Finally, an overview of the most common sophisticated analytical
techniques is presented.
OBJECTIVES
By the end of this unit, you will know why photocatalysis for the treatment of contaminated
water is important and the best analytical techniques for determining the degree of
decontamination during treatment. You will:
1. Be able to describe the stoichiometric parameters of the reactions involved in the
photocatalytic process.
2. Understand how it is possible to combine photocatalytic and biological processes.
3. See how photocatalysis can treat not only organic but also inorganic contaminants.
4. Find out different ways to improve process efficiency by modifying the catalyst and/or
adding other extra-electron acceptors.
5. Review the most common analytical tools used to determine the degradation rate of the
original contaminants.
6. Learn the most sophisticated analytical techniques used to describe the complicated
mechanisms involved in the photocatalytic degradation of organic substances.
7. Understand how to determine the toxicity of water and why this is so important for water
treatment.
NOTATION AND UNITS
Symbol
API
Atmospheric pressure ionisation
BOD
Biological Oxygen Demand
CI
Chemical ionisation
DPs
Degradation products
Eo
Oxidation potential
ECD
Electron capture detector
EI
Electron impact
FID
Flame ionisation detector
GC
Gas chromatography
HPLC High pressure liquid chromatography
LC50
Concentration of a chemical that is lethal to 50% of the exposed
population
LLE
Liquid-liquid extraction
Poct
Water/octanol partition coefficient
MS
Mass spectrometry
NMR
Nuclear magnetic resonance
66
Units
mg O2/L
V
mg/L
SOLAR DETOXIFICATION
NPD
SPE
TOC
Nitrogen phosphorus detector
Solid phase extraction
Total organic carbon
mg/L
5.1 DETOXIFICATION OF POLLUTANTS
Chemists play a significant role in devising new methods to solve environmental problems.
Existing methods deal with concentrated contaminants; what is still needed is an effective
method for handling toxic materials widely dispersed in the environment. Irradiated TiO2
efficiently degrades nearly every significant functional group, including the most
environmentally recalcitrant materials. The primary processes associated with semiconductorsensitised photodegradation of organic substances are illustrated in Figure 5.1. As shown by
Eqs. 1.21-1.24, the hydroxyl radical (•OH.) is the major intermediate reactive responsible for
organic substrate oxidation. The free radical HO2• and its conjugate O2•- are also involved in
degradation processes, but those radicals are much less reactive than free hydroxyl radicals.
These radicals react strongly with most organic substances by hydrogen abstraction or
electrophilic addition to double bonds. Free radicals further react with molecular oxygen to
give a peroxy radical, initiating a sequence of oxidative degradation reactions that may lead to
complete mineralization of the contaminant. In addition, hydroxyl radicals may attack
aromatic rings at positions occupied by a halogen, generating a phenol homologue. The
electrons of the conduction band can also degrade organic compounds by reductive pathways
as is shown in Figure 5.1 in the case of tetrachloromethane.
Figure 5.1 belongs here
Major general processes for the photo-oxidative or photo-reduction degradation of organic
compounds in aqueous solution sensitised by semiconductor particles. Examples of photooxidation (PCP) and photo-reduction (CCl4) are shown
In general, the types of compounds that have been degraded include alkanes, haloalkanes,
aliphatic alcohols, carboxylic acids, alkenes, aromatics, haloaromatics, polymers, surfactants,
herbicides, pesticides and dyes. A partial tabulation of organic compounds successfully
degraded by photocatalysis is provided in Table 5.1. Eq. 5.1 generally holds true for an
organic compound of general formula CnHmOp.
m
 (m − 2 p)

C n H mOp + 
+ n O2 → nCO2 + H 2 O
(5.1)
4
2


In the case of organic compounds containing halogens, Eq. 5.2 shows how the corresponding
halide is formed.
m−q
 (m − 2 p)

C n H mO p X q + 
H 2 O + qHX
+ n O2 → nCO2 +
(5.2)
4
2


Under photocatalytic oxidative conditions, sulphur is recovered as sulphate in sulphur
containing compounds according to Eq. 5.3
C n H m O p S r + xO2 → nCO2 + yH 2 O + zH 2 SO4
(5.3)
Table 5.1 belongs here.
Some examples of TiO2-sensitised photodegradation of organic substrates.
5.1.1 Total mineralization
In photodegradation, transformation of the parent organic compound is desirable in order to
eliminate its toxicity and persistence, but the principal objective is to mineralise all pollutants.
67
hν
68
CO2
Inorganic acids
Water
Oxidised
intermediate
-
TiIVOH
A-
TiIIIOH
A
TiIVOH
+
Organic
pollutant
TiIIIOH+••
If A = O2
O2-••
O2
HO2•, HO2-, H2O2, •OH
CCl4 + ecb-→CCl3•+Cl•+
CCl3
ecb-→:CCl2
OH
+
O
Cl
Cl
. OH
Cl
-H2O
Cl
Organic
pollutant
Cl-
Cl
Cl
Figure 5.1
Cl
Cl .
Cl
OH
.
Cl
Cl
Cl
-HCl
Cl
HO
Oxidised
intermediate
O
O
Cl
Cl
O
Cl
Cl .
Cl
Cl
O
H2O
OH
. Cl
OH
Cl
Cl
Cl
O
SOLAR DETOXIFICATION
The effectiveness of degradation is not demonstrated only because the entire initial compound
is decomposed. As observed in the HPLC chromatograms (Figure 5.2), oxamyl
(chromatogram A, retention time 4.45 min) disappears nearly completely (chromatogram B).
However, many new organic compounds appear.
Figure 5.2 belongs here
HPLC-UV chromatograms of photodegradation of oxamyl before photocatalytic treatment (A)
and when the oxamyl has completely disappeared (B).
Furthermore, the stoichiometry proposed for the general reactions (Eqs. 5.1-5.3) has to be
demonstrated in each case by a correct mass balance. Reactives and products might be lost
and this will originate not confident results. Mineralization rate is determined by monitoring
inorganic compounds, such as CO2, Cl-, SO42-, NO3-, PO43-, etc. When organics decompose, a
stoichiometric increase in the concentration of inorganic anions is produced in the water
treated and, likewise, very often an increase in the concentration of hydrogen ions (decrease in
pH). For this reason, the analysis of these two products of the reaction is of interest for the
final mass balance. However, the decrease in pH is not a very reliable parameter of this
balance, save in some cases, because it is influenced by other processes which take place in
the medium: the effect of the TiO2 suspension, the formation of CO2 and intermediates, etc.
In order to demonstrate that there are no product losses, the molar ratio must be in accordance
with the organic substrate structure. For example, the pentachlorophenol decomposition
reaction to which mass balances should be adjusted is shown in Eq. 5.4. Therefore, [Cl-] = 5
[PCP]0, but this only occurs at the end of the experiment, when the TOC is almost 0. During
the degradation, the formation of intermediates impedes this, since these intermediates contain
differing amounts of chlorides
hυ
9

→6CO2 + 5HCl
C 6 Cl 5 OH + O2 + 2 H 2 O TiO
(5.4)
2
2
In Figure 5.3, a complete pentachlorophenol degradation test is shown. The products obtained
are practically stoichiometrically correct. The slight difference may be attributed to the facts
that all the TOC is not completely decomposed and that results obtained with four different
analytical techniques were compared.
Figure 5.3 belongs here
Evolution of H+ and Cl- during pentachlorophenol degradation. To more clearly demonstrate
that reaction 5.4 is completed, the concentration of TOC in mM is calculated considering
1 mMol TOC = 6 mMol of C = 72 mg of C.
The oxidation of carbon atoms into CO2 is relatively easy. It is, however, in general, markedly
slower than the dearomatization of the molecule. Until now, the absence of total
mineralization has been observed only in the case of s-triazine herbicides, for which the final
product obtained was essentially 1,2,5-triazine-2, 4,6, trihydroxy (cyanuric acid), which is,
fortunately, not toxic. This is due to the strong stability of the triazine nucleus, which resists
most methods of oxidation. For chlorinated molecules, Cl- ions are easily released into the
solution. Nitrogen-containing molecules are mineralised into NH4+ and mostly NO3-.
Ammonium ions are relatively stable and the proportion depends mainly on the initial
oxidation degree of nitrogen and on the irradiation time. The pollutants containing sulphur
atoms are mineralised into sulphate ions. Organophosphorous pesticides produce phosphate
69
70
A
OXAMYL
B
min.
Figure 5.2
[PCP], TOC], mM
0.12
0.6
0.10
0.5
Cl-
0.08
H+
TOC
0.06
0.4
0.3
PCP
0.04
0.2
0.02
0.1
0.00
0.0
0
5
10
15
tR, min
71
Figure 5.3
20
25
30
35
SOLAR DETOXIFICATION
ions. However, phosphate ions in the pH range used remained adsorbed on TiO2. This strong
adsorption partially inhibits the reaction rate that, however, remains acceptable. Until now, the
analyses of aliphatic fragments resulting from the degradation of the aromatic ring have only
revealed formate and acetate ions. Other aliphatics (presumably acids, diacids, and
hydroxylated compounds) are very difficult to separate from water and to analyse. Formate
and acetate ions are rather stable, which in part explains why total mineralization takes much
longer than dearomatization.
5.1.2 Degradation pathways
As pointed out above, a variety of degradation products (DPs) are formed in photocatalytic
processes. Nevertheless, in most cases no attention is paid to the possible formation of these
DPs which, on the other hand, allow the degradation processes to be better understood and
evaluated and thereby comparison among the degradation pathways of different types of
compounds, functional groups, chemical oxidation parameters, etc. Cost-effective treatment to
complete compound mineralization is usually not feasible and the generation of by-products
appears to be unavoidable with photocatalytic degradation. Identification of those by-products
is the key to maximising overall process efficiency. Since hydroxyl radicals react nonselectively, numerous by-products are formed at low concentrations. On the other hand some
of the degradation products obtained are of interest, because they may be more toxic and
persistent than the parent compound. In some cases of photocatalytic TiO2 treatments, only
traces of these metabolites are detectable because they are degraded faster than the parent
compounds and mineralization is almost total in a short time. But in other cases, such as striazine herbicides mentioned above, overall conversion to the final degradation products
takes longer. For example, primary intermediates of the photocatalytic degradation of various
aromatic pollutants detected and identified correspond to hydroxylation of the benzene ring
(see Figure 5.1). Maximum transient concentrations are much lower for these intermediates
than the initial pollutants, since CO2, acetate and formate are formed in the initial stages of
degradation. The orientation of the hydroxylation of the aromatic ring depends on the nature
of the substituents. For instance, for chlorophenols and dimethoxybenzenes, the para and
ortho positions (with respect to OH for the chlorophenols) are favoured as is expected. By
contrast, for benzamide and nitrobenzene, the hydroxylation occurs at all free sites, whereas a
meta orientation is expected for electron-withdrawing substituents. A list of compounds
produced during photodegradation of a pesticide (pyrimethanil) with TiO2 is shown in Figure
5.4. It will give an idea of the complexity of the chemical reactions involved in photocatalytic
methods.
Figure 5.4 belongs here
Chemical structures of pyrimethanil and its degradation products obtained during a
photocatalytic treatment with TiO2.
5.1.3 Toxicity reduction
The success of photocatalytic degradation in wastewater treatment depends on how much the
cost of illumination, i.e., energy, can be brought down. This can be achieved by improving the
efficiency of photocatalytic degradation by modifying the catalyst, the photoreactor design, by
using additional oxidants, etc. It is also possible to envisage incomplete photocatalytic
degradation to detoxify the wastewater. The products of incomplete degradation and their
concentrations may be sufficiently innocuous as to be permissible for discharge directly into
the environment or for further biological treatment. Biological treatment of biodegradable
residual waters is presently the most compatible with the environment and the least expensive.
72
HIDROXY DERIVATIVES
ALIPHATIC
DERIVATIVES
PYRIMIDIL DERIVATIVES
OH
N
N
N
NH2
CH3
CH3
O
N
N
CH3
CH3
CH3
O
N
NH
CH3
N
CH3
OH
OH
CH3
O
NH2
NH2
C
CH3
CH NH2
N
N
NH
N
NH
O
C NH C
CH3
CH3
PYRIMETHANIL
N
CH3
PHENYL DERIVATIVES
OH
N
N
N
CH3
C NH C
O
N
73
Figure 5.4
NH2
N
C NH2
CH3
O
OH
CH3
OH
O
N
NH
O
NH
CH3
OH
O
C CH3
NH
C CH3
NH
OH
OH
N
OH
CH3
OH
O
OH
CH
SOLAR DETOXIFICATION
However, it is very difficult to identify all the intermediary compounds en-route to complete
mineralization. Toxicity testing of the photocatalytically treated wastewater is therefore
necessary, particularly when incomplete degradation is planned. Any number of wholeorganism bioassays could be used for the assessment of water quality before disposal. Since it
is not feasible to determine the specific toxicity of every toxic intermediary compound, wholeeffluent toxicity testing using aquatic organisms is a direct, cost-effective and relevant means
of determining effluent toxicity. Therefore, bioassays can provide a more direct and
appropriate measure of mixtures of toxins than chemical analyses alone, which are not
sufficient to assess potential effects on aquatic biota (Tapp et al., 1996). A review of these
tests is shown at the end of this chapter.
Another useful technique for evaluating the feasibility of disposing photocatalytically treated
waters before total mineralization is determining the biodegradability (BOD) in sludge from
biological activated sludge plants. This sludge is inoculated with the treated water and BOD
tests are carried out. In this case, the results are obtained in terms of “relative
biodegradability”. This means that once biodegradability is achieved, it can only be assured if
the same biological activated sludge used for the BOD tests is used for disposing the treated
water. This test produces non-universal results, but could be very useful if the biological
treatment plant that will receive the effluent from the photocatalytic treatment is known prior
to design of the photocatalytic plant. This permits disposal of the pre-treated water into a
biological (low cost) treatment.
5.1.4. Detoxification of inorganic pollutants
Photocatalytic processes may also transform inorganic substances from the environment.
Specifically, H2S and CN- can be converted into less toxic materials and various strategic
and/or toxic metals can be removed from waste effluents. The toxicity of H2S is comparable
to that of HCN. In the presence of light and a semiconductor catalyst, the products of
oxidation are molecular hydrogen and sulphur. Free cyanides are generated in large quantities
in heat-treating operations and in metal-finishing industries. The greatest amounts of cyanidecontaining wastes are produced by precious metals milling operations and coal gasification
processes. CNO- is the first product of photocatalytic oxidation of cyanides in the presence of
polycrystalline TiO2 in an aqueous medium. The proposed mechanism implies the oxidation
of cyanide by the photogenerated holes and the reduction of oxygen by electrons according to
Eqs. 5.5. The reacting mixture must be at pH 10 to avoid the formation of volatile HCN.
CN − + 2h + + 2OH → CNO − + H 2 O
(5.5)
Cyanate ions are photo-oxidised to nitrates and a satisfactory nitrogen balance is achieved
according to the overall reaction shown in Eq. 5.6. At strong oxidant conditions volatile
nitrogen-containing species are not produced or are quickly photooxidised to NO2- and NO3-.
CNO − + 4O2 + 2OH − + 3H 2 O → CO32 − + NO3− + 4 H 2 O2
(5.6)
It has now been discovered that ozonation (production of drinking water) generates low levels
of bromate ions, recognised as a suspect agent of cancer. Drinking water treated with ozone
will typically have in the range of tens of µg/L of bromate and TiO2 is able to sensitise the
photodecomposition of bromate (Eq. 5.7).
ν / TiO2
2 BrO3− h

→ 2 Br − + 3O2
(5.7)
Heavy metals are generally toxic and can be removed from industrial waste effluents as small
crystallites deposited on the photocatalyst as demonstrated by Eq. 5.8:
n
n
ν / TiO2
M n + + H 2 O h

→ M 0 + nH + + O2
(5.8)
2
4
74
SOLAR DETOXIFICATION
Under identical conditions, the following reactivity pattern has been found: Ag > Pd > Au > Pt
>> Rh >> Ir >> Cu = Ni = Fe = 0 for AgNO3, PdCl2, AuCl3, H2PtCl6 or Na2PtCl6, RhCl3,
H2IrCl6, Cu(NO3)2, Ni(NO3)2 and Fe(NO3)3. As the photodeposition conversion increases, the
metal particles form agglomerates, reaching several hundreds of nm. Silver photodeposition
has been applied in the recovery of Ag from used photographic baths in which the silverthiosulphate complex is decomposed, Ag+ being reduced to Ag°. Because of their favourable
redox potentials, only noble metals can be photodeposited. This property has been used to
selectively recover noble heavy metals. For instance, silver has been separated from copper in
solutions simulating industrial electrolytic baths. Other toxic, heavy non-noble metals could
be removed from water. Mercury, because of its favourable redox potential, was photoreduced
as a zero-valent metal. The cations Pb2+ and Tl3+ have been deposited on UV-irradiated TiO2
powder as PbO2 and Tl2O3. Similarly, uranium has been photodeposited on TiO2 as U3O8
from uranyl solutions. From an application point of view, the recovery of silver from
photographic baths seems the most promising issue, provided legislation on discharge water
containing Ag becomes stricter.
5.2 QUANTUM YIELD IMPROVEMENT BY ADDITIONAL OXIDANTS
One practical problem in using TiO2 as a photocatalyst is electron/hole recombination (reverse
of Eq. 1.21), which, in the absence of proper electron acceptors, is extremely efficient and
thus represents a major energy-wasting step and limitant to achieving a high quantum yield.
Oxygen has been chosen in most of the applications for this purpose, although its role is not
only related to electron scavenging (see Chapter 4). But with only dissolved oxygen as an
oxidant, low mineralization photo-efficiencies (production of CO2) are obtained (in the range
of 1-5%).
One strategy for inhibiting e-/h+ recombination is to add other (irreversible) electron acceptors
to the reaction. Outstanding enhancement of the rate of degradation of various organic
contaminants through the use of inorganic peroxides has been demonstrated (Pelizzetti et al.
1991, Malato et al. 1998). The addition of other oxidising species could have several different
effects:
• Increase the number of trapped e- in the e-/h+ pairs and, consequently, avoid
recombination.
• Generate more •OH and other oxidising species.
• Increase the oxidation rate of intermediate compounds.
• Avoid problems caused by a low O2 concentration.
It must be mentioned here that in highly toxic wastewater where degradation of organic
pollutants is the major concern, the addition of an inorganic anion to enhance the organic
degradation rate may often be justified. For better results, these additives should fulfil the
following criteria: dissociate into harmless by-products and lead to the formation of •OH or
other oxidising agents. There is another advantage related to the use of this type of oxidant
when solar energy is the photon source. Although scientific research on photocatalytic
detoxification has been conducted for at least the last three decades, industrial/commercial
applications, engineering systems and engineering design methodologies have only been
developed recently. In this type of installation, the photoreactor is by far the most expensive
component and a barrier to commercialisation. The increase of the photocatalytic reaction rate
with these additives would decrease photoreactor dimensions proportionally and dramatically
decrease overall costs.
75
SOLAR DETOXIFICATION
5.2.1. Hydrogen Peroxide
Hydrogen peroxide is the obvious candidate. It can increase the efficiency of the process at
high irradiance (see Section 4.6) and it has been tested with a large number of compounds.
Also, it is a very commonly used chemical and, therefore, very cheap. Being an electron
acceptor, hydrogen peroxide reacts with conduction band electrons (Eq. 5.9) to generate
hydroxyl radicals, which are required for the photomineralization of organic pollutants.
H 2 O2 + e − → • OH + OH −
(5.9)
•
The following reactions (Eq. 5.10 and 5.11) can also produce OH (reaction 5.11, which
requires λ < 300 nm, does not take place with solar radiation).
H 2 O2 + O2•− → • OH + OH − + O2
(5.10)
•
H 2 O2 + hν →2 OH
(5.11)
The effect of this electron acceptor deserves some further comment. In some cases, the
addition was found to be beneficial, increasing the degradation rate. The effect depends on the
H2O2 concentration, generally showing an optimum range of concentration. At higher
concentrations the improvement starts to lessen. Whereas this beneficial effect can easily be
explained in terms of prevention of electron/hole recombination and additional •OH
production through reactions 5.9, 5.10 and 5.11, inhibition could be explained in terms of
TiO2 surface modification by H2O2 adsorption, scavenging of photoproduced holes (Eq. 5.12)
and reaction with hydroxyl radicals (Eq. 5.13).
H 2 O2 + 2h + → O 2 + 2 H +
(5.12)
H 2 O2 + •OH → H 2 O + HO2•
(5.13)
The inhibition of adsorption not only depends on the characteristics of the pollutant, but also
on the hydrogen peroxide/organic concentration ratio. This may be explained in terms of
Langmuir-Hinshelwood kinetics, rC = krKC/(1+KC) and competitive adsorption (see Chapter
4). If pollutant concentration (C) is too low and H2O2 concentration too high, organic
adsorption decreases because of adsorption of hydrogen peroxide and, therefore, the additional
hydroxyl radicals generated by H2O2 do not react efficiently. When C is higher, the radicals
react more easily and rC increases and, when C is still higher, the reaction rate is not as
affected by adsorption (1+ KC ≈ KC; rC = kr). In the latter situation the reaction rate is only
dependent on •OH concentration related to favourable (Eqs. 5.9-5.11) and unfavourable
reactions (Eqs. 5.12-5.13). There is an optimum ratio of H2O2/C under these circumstances
whereby, organic material is sufficient to consume generated hydroxyls and, to avoid
detrimental reactions, if peroxide is not too high. At high molar ratios an inhibition effect
would be expected because the unfavourable reactions become more and more important. All
this may be summarised: (i) if pollutant concentration is low, the hydrogen peroxide easily
inhibits the degradation rate; (ii) if the molar ratio between H2O2 and pollutant is too high, the
same is true.
5.2.2. Persulphate
In homogeneous reactions, the persulphate ion accepts an electron and dissociates (Eq. 5.14).
The sulphate radical anion is also generated adequate thermal and photolytic (wavelength <
270 nm) conditions. This radical goes through the reactions explained below (Eq 5.16 and
5.17). Persulphate can therefore be a beneficial oxidising agent in photocatalytical
detoxification because SO4•- is formed from the oxidant compound by reaction with the
photogenerated semiconductor electrons (e-CB, Eq. 5.15). In addition, it can trap the
photogenerated electrons and/or generate hydroxyl radicals. The sulphate radical anion (SO4•-)
76
SOLAR DETOXIFICATION
is a very strong oxidant (Eo = 2.6 V) and engages in at least three reaction modes with organic
compounds: by abstracting a hydrogen atom from saturated carbon, by adding to unsaturated
or aromatic carbon and by removing one electron from carboxylate anions and from certain
neutral molecules.
−
S 2 O82− +eaq

→SO4−• + SO42−
(5.14)
−
S 2 O82− +eCB

→SO4−• + SO42−
−•
4
−
CB
(5.15)
2−
4
SO +e 
→SO
(5.16)
SO4−• + H 2O 
→ • OH + SO42− + H +
(5.17)
5.2.3. Other oxidants
Other compounds could also potentially increase the reaction rate because they are also
electron scavengers. The most frequently tested so far in heterogeneous photocatalysis are
described in Eqs. 5.18-5.21.
H+
IO4− + 8e − 8

→ 4H 2O + I −
(5.18)
+
H
ClO3− + 6e − 6

→ 3H 2 O + Cl −
(5.19)
+
H
BrO3− + 6e − 6

→ 3H 2 O + Br −
−
5
−
•
2−
4
−
5
−
(5.20)
−
−•
4
HSO + e 
or HSO + e 
(5.21)
→ OH + SO
→ OH + SO
Chlorate has been proven insufficient to improve effectiveness. However, both IO4- and
bromate increase the mineralisation rate in all cases tested. Nevertheless, these additives are
very expensive compared to hydrogen peroxide and peroxydisulphate, and their application
would dramatically increase treatment cost. Even more importantly, they do not dissociate
into harmless products (Br- and I-), because hundreds of mg/L of these anions are undesirable
in water. Potassium peroxymonosulfate (commercially called oxone) was also examined as an
irreversible electron acceptor. The formula of this salt is 2KHSO5.KHSO4.K2SO4, written in
aqueous solution as HSO5-. Upon accepting an electron from the conduction band, HSO5would dissociate into two different pathways (Eq. 5.21). The disadvantage is its high
molecular weight. Many grams of product are necessary for 1 active mol (HSO5-).
5.3 CATALYST MODIFICATION
The TiO2 band-gap represents only 5% of the solar spectrum (see Chapter 2). From the
standpoint of solar collecting technology, it is therefore a rather inefficient process even for a
high added-value application. In contrast to other Advanced Oxidation Technologies,
photocatalysis has the advantage of being solarizable and of being an environmentally friendly
technology. TiO2 is a cheap photostable catalyst, and the process may be run at ambient
temperature and pressure conditions. Furthermore, the oxidant, molecular oxygen (O2), is the
mildest one. Therefore, basically, TiO2 is a mild catalyst that works at mild conditions with
mild oxidants. However, as concentration and number of contaminants increase, the process
becomes more complicated and challenging problems, such as catalyst deactivation, slow
kinetics, low photoefficiencies and unpredictable mechanisms need to be solved. It is clear
that naked TiO2 needs extra help to undertake practical applications of industrial and
environmental interest and this could lead to the loss of some of the charm of its mild
operation. Moreover, even reactor set-ups using artificial light, and the cost of running the
lamps involved in them, will be much cheaper if visible radiant flux can be employed.
77
SOLAR DETOXIFICATION
The redox process is based on the migration of electrons and holes to the semiconductor
surface and two further oxidation and reduction steps (see Figure 5.1). Two basic lines of
R&D attempt to balance both half-reaction rates, one by adding electron acceptors (additional
oxidants, already commented on above) and the other by modifying catalyst structure and
composition. Both try to promote competition for electrons and avoid recombination of e-/h+
pairs. A third approach has focused not only on increasing quantum yield but finding new
catalysts with band-gaps that match the solar spectrum better. Unfortunately, the choice of
convenient alternatives for substituting titanium dioxide in photocatalytic detoxification
systems is limited. The appropriate semiconducting material should be:
• Non toxic.
• Stable in aqueous solutions containing highly reactive and/or toxic chemicals.
• Not photo-corrodible under band-gap illumination.
• Economical, that is, an increase in photocatalytic reaction rates must be always be
accompanied by a non-proportional increase in overall process costs.
Generally, metal oxides fulfil these criteria but most metal oxides are wide band-gap
semiconductors or insulators. Although iron (III) oxide is one of the few exceptions (see Table
1.4), it has demonstrated satisfactory activity in a limited number of cases. Other non-oxide
semiconductors (e. g. CdS) are usually unstable and photodegrade in time. A few examples
will be given in the following sections to illustrate the large body of work conducted in the
area of photocatalyst modification.
5.3.1. Metal semiconductor modification
The addition of noble metals to a semiconductor could modify the photocatalytic process by
changing the semiconductor surface properties. Figure 5.5 is an illustration of the capacity of a
metal in contact with a semiconductor surface to capture electrons. After excitation, the
electron migrates to the metal where it becomes trapped and e-/h+ recombination is avoided.
The hole is then free to migrate to the surface where oxidation of the organics can occur (see
Figure 5.1). The Pt/TiO2 system (Pt deposited on titania by the impregnation and reduction
method) is the metal-semiconductor system most commonly studied (see Figure 5.6). Loading
of Pt is optimum to achieve the maximum photocatalytic rate, affecting the distribution of
electrons in the system. Above the optimum metal content, the efficiency decreases because
once negatively charged, Pt particles become attractive for holes, which recombine with
electrons into inefficient thermal energy. Care must therefore be taken in studies conducted
with modified metal semiconductors to use the optimum quantity of metal.
Figure 5.5 belongs here
Electrons capture by a metal in contact with a semiconductor surface.
Figure 5.6 belongs here
Concentrating solar reactor with platinum/titanium dioxide catalyst on ceramic saddles.
Tested on air contaminated by spray paint at Fort Carson Army Base in Colorado (USA).
Courtesy of National Renewable Energy Laboratory (USA).
5.3.2. Composite semiconductors
Coupled semiconductors provide a way to increase the separation between charges and reduce
the energy (increasing wavelength) necessary to excite the system. Figure 5.7 shows CdSTiO2 as an example. The energy from light (λ<497 nm) is large enough to cause an electron to
leave the CdS valence band and go over to the conduction band. The hole remains in the CdS
78
Metal
Metal
ν
hν
e-
Metal
h+
Figure 5.5
79
80
Figure 5.6
SOLAR DETOXIFICATION
while the electron is transferred to the TiO2 conduction band. The separate charges are then
free to undergo electron transfer with the species adsorbed on the surface.
Figure 5.7 belongs here
The excitation process in a semiconductor-semiconductor photocatalyst.
5.3.3. Surface sensitisation
Surface sensitisation of a wide band-gap semiconductor via chemisorbed or physisorbed dyes
can increase the efficiency of the excitation process. The wavelength range can be expanded
by charge transfer from the sensitiser to the semiconductor. Figure 5.8 illustrates the
excitation, charge transfer and regeneration steps. If the oxidative energy level of the excited
state of the dye is more negative than the semiconductor conduction band, then the dye can
transfer the electron to the conduction band of the semiconductor. The electron in turn can be
transferred to reduce an organic acceptor adsorbed on the surface. Without the presence of a
redox couple, the dye-semiconductor system can also be used in oxidative degradation of the
dye itself. This is important due the large number of dye substances found in industrial textile
wastewater.
Figure 5.8 belongs here
Steps of excitation with a sensitizer in the presence of an adsorbed organic electron acceptor
(A).
5.4 RECOMMENDED ANALYTICAL METHODS
5.4.1 Original contaminants
The analyses performed on samples collected during photocatalytic treatment have to be as
complete as possible in order to adjust the mass balance for the photocatalytic decomposition
of the contaminants and assure that the analytical data are reliable and the organic compounds
have not disappeared in some other way (evaporation, adsorption in the reactor, adsorption in
the catalyst, etc.) besides photocatalysis. Liquid chromatography (HPLC) with UV detection is
the method of choice for analysis of samples from photocatalytic treatment since direct
injection of the aqueous sample into the analytical column is allowed, avoiding the necessity
of previous extraction. Samples require only filtering to remove TiO2 or any other particles
that could damage the chromatographic column before they are injected into the equipment. A
device made up of a syringe and 0.22 µm membrane filters is used for this purpose. Gas
chromatography is only suggested if HPLC is not viable or when a pre-concentration is
necessary due to the very low concentration of the parent compounds.
5.4.2 Mineralization measurements (TOC)
Total organic carbon analyses of samples taken during photocatalysis degradation experiments
are vital for the following reasons:
• Since identification of all the intermediates generated during photodecompositon is not
possible, pinpointing the moment at which only CO2 remains and water is considered
completely decontaminated is crucial.
• Determination of the CO2 produced might be reasonable, since this must be stoichiometric
with the organic carbon present at the beginning in the organic molecule. However, since
the reactors are usually large and not airtight, loss into atmosphere of the carbon dioxide
produced prevents this. For the same reasons, the samples might become contaminated by
atmospheric CO2 and falsify results.
81
82
ν
hν
e-
CdS
EG= 3.2 eV
e-
EG= 2.5 eV
h+
TiO2
Figure 5.7
e-
Dye*
CB
ν
hν
Dye
Figure 5.8
VB
A
CB
VB
A
CB
VB
A-
83
SOLAR DETOXIFICATION
•
It is a reliable, simple and rapid way to close the mass balance at any moment to get an
idea of the remaining amount of intermediates and check the extraction methods and
analysis of intermediates.
The basic techniques for the determination of TOC in water have remained relatively
unchanged for 20 years. Organic compounds are converted to CO2 using a combination that
may include chemical oxidizing agents, ultraviolet radiation or high-temperature combustion
(Wangersky, 1993). The CO2 is then measured using non-dispersive infrared absorption,
micro-coulometry or conductimetric techniques. Since many water samples contain inorganic
forms of carbon (HCO3- and CO32-), it is usually necessary to remove theses species, typically
using a gas stripping technique prior to measurement of TOC. Some part of the organic carbon
(VOCs) may also very often be removed by this procedure. In the case of heterogeneous
photocatalysis, it is necessary to use a TOC analyser able to manage non-filtered samples.
When TiO2 is filtered from the sample, there is an important loss of organic compounds
because they are adsorbed onto the solid retained in the filter.
5.4.3 Intermediates analysis (GC-MS/HPLC-MS)
In order to analyse a reaction mixture containing pesticides and their degradation products
(DPs), it is necessary to have analytical screening methods available that permit separation
and identification of compounds with very different hydrophilic-hydrophobic characteristics
and spanning very different concentration ranges. The use of gas chromatography with
classical detectors, like ECD, NPD or FID is obviously insufficient. But gas or liquid
chromatographic systems coupled to mass spectrometry (GC/MS and LC/MS) represent a
good fast alternative for achieving very useful structural information on the compounds
generated in such processes (Agüera and Fernández-Alba, 1998).
Important advantages of the GC/MS-based methods are: a) the large amount of structural
information they yield and the spectra libraries available either from data bases or from
research papers which make DP identification feasible; b) the durability and reliability of the
GC-MS interface; and c) the highly efficient sensitivity and separation that avoids overlapping
compounds with similar structures. However, GC-MS based methods have important
drawbacks because of their low capacity to analyse very polar, low volatile and thermally
unstable compounds. Identification of DPs is usually based on their EI (Electron Impact) mass
spectra, mainly by comparing the unknown compound spectrum with published spectra. A
disadvantage of EI is that it does not usually provide molecular weight information.
Additional and very useful structural information on DPs can be obtained by chemical
ionisation (CI). From the molecular weight and interpretation of fragmentation patterns, it is
possible to hypothesise a molecular structure. However, even an EI mass spectrum does not
provide enough information about the location of the functional groups (e.g., position of a
hydroxyl group on a benzyl ring). A comparison with a standard, when commercially
available, is required for unequivocal confirmation. When by-product standards are not
commercially available, the laboratory synthesis of DPs could be a solution.
Because GC methods require compounds with high vapour pressures. Derivation, of at least
the acid fraction (BF3/MeOH or diazomethane procedure), has become a typical procedure for
identifying the less volatile and polar compounds. Nevertheless, derivation is not easy and it is
also time-consuming. Because of this, LC-MS techniques are gaining in importance. LC
techniques present several advantages over GC: (i) little or no cleaning of the sample is
required, (ii) high polar, low volatile and thermally labile compounds are more easily
84
SOLAR DETOXIFICATION
analysed, (iii) direct analysis of the samples is possible, avoiding polar DPs to escape from
extraction procedures. The major role of LC-MS in the degradation processes studied is (i) to
check DP molecular weight and (ii) to detect DPs which are not directly amenable with GCMS techniques.
The ready availability of atmospheric pressure ionisation (API), electrospray (ESP)/ionspray
(ISP) and atmospheric pressure chemical ionisation (APCI) LC-MS interfaces that provide
general structural information and sensitivity, has expanded the applicability of LC-MS,
allowing identification of a large range of polar transformation products. LC-MS shows
important weaknesses since the information usually achieved lacks detailed structural data and
is less sensitive and discriminating than GC-MS. These disadvantages can frequently prevent
DP identification as a consequence of the low detection threshold or overlapping peaks. Since
the DPs obtained are very complex (see Figure 5.9), mass spectrometry-based techniques
therefore do not provide enough structural information for the unequivocal structural
elucidation of the DPs in many cases and it must be done by more complex time-consuming
techniques such as LC/ MS/MS or NMR. But the combination of both GC/MS and LC/MS is
very useful and, in many cases, enough structural information to evaluate the degradation
process may be found fast. In addition, other analytical measures such as Total Organic
Carbon (TOC) or Ion Chromatography (IC) can be of great help to assess the success of the
degradation process by evaluating the mineralization rate achieved or by establishing the mass
balance of the whole process.
Figure 5.9 belongs here
Degradation pathway proposed for pirimiphos-methyl dissolved in water when illuminated in
the presence of TiO2.
5.4.4 Extraction methods
Common handling procedures for analysis of samples of chemically treated water involve the
use of extraction methods because identification of degradation products has to be carried out
at sub-mg/L level. Trace organic compounds in wastewater are still typically enriched by
liquid-liquid extraction (LLE) using an appropriate solvent; however, solid-phase extraction
(SPE) is gaining in acceptance (Chiron et al., 1997), mainly because SPE generates less
matrix interference and a wide range of new adsorbents (able to trap DPs having a wide range
of polarities) are commercially available, including: alkylsilanes modified silica, e.g. C-18 and
end-capped C-18; porous polymers, e.g. poly (styrene-divinylbenzene) PRP-1 or PLRP-S; and
carbon modified materials, e.g. porous graphitic carbon (PGC). SPE materials are expected to
show different behaviours with respect to capacity and breakthrough volumes of both analyte
and matrix interferences. Because the retention of a compound is higher in its neutral form
than in its ionic form, phenol and benzoic acid extractions are better under acidic conditions
while amines are best recovered at alkaline conditions. Problems may arise with
multifunctional compounds. The retention of hydrophilic compounds (Log Poct <0) such as
aminophenols or cyanuric acid is poor but can be high with porous graphitic carbon. On the
other hand, short chain aliphatics such as oxalic or formic acids (usually the final organic
photooxidation degradation products) may not be recovered at all. In order to identify as many
chemicals as possible, a sequential extraction scheme, involving different SPE adsorbents,
could be proposed. A C-18 phase is used initially to select all neutral hydrophobic compounds
at pH7. The C-18 filtrates are then passed through a polymeric adsorbent also at pH7, where
medium polar compounds are retained. Subsequently filtrates are acidified to pH4.5 and
pH2.5, respectively, to extract the majority of acidic compounds with a polymeric or carbon85
86
N
CH3
N
CH2 CH3
CH2 CH3
CH3
N
H3CO
H3CO
O
OCH3 P OCH3
OCH3
CH2 CH3
CH2 CH3
R1 N
N
H3CO
P O
H3CO
S
P O
O
+
N
N
CH3
CH2 CH3
N
CH2 CH3
N
N
= R2
= R1
Pyrimiphos Methyl
+
SCH3 P OCH3
OCH3
R2 N
R2 N
CH2COOH
R2 N
HOH2C
OH
Figure 5.9
R2 N
CH3
CH3
N
R2 N
R2 N
CH2COOH
R2 N
CH2CH3
CH3
CH2CH3
CH2CH3
R1 N
H
CH2CH3
R1 N
H
CH2CH3
CH3
CH2CH3
CHO
CH2CH3
HOC
N
CH2COOH
CH2CH3
NH2
OH
CH2COH
CH2CH3
CH2CHO
H
CHO
R1 N
R1 N
CH2CHO
R2 N
CH2CH3
R1 N
CH2CHO
CH3
+R
O
OH
R2 N
CH2CH2OH
H
CH2CH3
N
O
N
O
O
P
S
OCH3
OCH3
1
N
CHOHCH3
CH2CH3
SOLAR DETOXIFICATION
type adsorbent. Polymeric adsorbents can be used over pH2-13 without decomposition of the
adsorbent material.
5.4.5 Toxicity analysis
The test organisms in these assays include representatives from four groups: micro-organisms,
plants, invertebrates and fish. The Organisation for Economic Cooperation and Development
(OECD) has suggested a set of minimum data to assess the effects of chemicals on the
environment. The OECD lists mortality in fish, impaired reproduction in crustaceans and
inhibition of growth in algae as examples of ecotoxicity tests needed to predict the impact of
released chemicals upon ecosystems. The two main invertebrate toxicity tests routinely used
are the 21-day Dapinia and the 7-day Ceriodaphinia survival and reproduction tests. Tests are
conducted by exposing the organisms to the toxins under control conditions and after the
required incubation period, live organisms are counted. Fish bioassays have been conducted
for decades. The distinct physiological and behavioural responses of fish to low levels of
pollutants has been employed in the development of fish monitors that act as indicators of
water quality (Tothill et al., 1996). The tests are usually based on larval growth and survival,
where newly hatched fish are exposed to a range of effluents for 1-2 days or up to 7 days. The
acute lethality test with fish measures the concentration of a chemical that is lethal to 50% of
the exposed population after 96 h (LC50). Species such as rainbow trout (Oncorhynchus
mykiss) and fathead minnow (Pimephales promelas) are commonly used. During recent years,
research has been carried out to reduce or replace acute fish tests with in vitro assays, using
cultured fish cell lines. The use of algae in bioassays has proved useful in detecting metals,
herbicides, pesticides and crude oil compounds. However, the culturing and preparation of the
algae suspension is a lengthy process and, further, it is difficult to maintain an identical
culture of algae each time the bioassay is conducted.
Many of the above tests require specialised equipment and operator skills and are time
consuming. The use of higher organisms such as fish may also be ethically undesirable. In the
last few years, there has been increased interest in bacterial screening to assess toxicity.
Studies of effects on microbial function or activity constitute a more direct, rapid and sensitive
approach to measure chemical stress. These can be classified by the type of measurement
used:
• Monitoring transformation of carbon, sulphur or nitrogen.
• Determination of the activity of microbial enzymes such as dehydrogenases, adenosine
triphosphatases and other enzymes.
• Measurement of growth, mortality and photosynthesis.
• Determination of glucose uptake activity using radioisotopes.
• Measurement of oxygen consumption using a dissolved oxygen electrode or respirometer.
• Measurement of luminescence using a photometer.
The development and applications of biological toxicity testing are rapidly increasing.
Numerous bioassay procedures are now available, however, it is difficult to state the
sensitivity of these tests, and therefore, a universal-monitoring device for toxicity testing is
unlikely to be available. In any case, most of the recent studies have dealt with the use of the
bacterial luminescence assay for toxicity screening. The use of toxicity tests for evaluation of
photocatalytic treatments is not very common for the moment, but several papers have already
been published (Jardim et al., 1997; Herrmann et al. 1999).
87
SOLAR DETOXIFICATION
SUMMARY OF THE CHAPTER
Photocatalytic degradation of organic and inorganic compounds follows a definite
stoichiometry that has to be determined by appropriate analytical techniques during
photocatalytic experiments in order to close the mass balance between reactives and products.
Furthermore, degradation pathways and by-products formed are so complex that analysis of
water toxicity during the treatment is recommended to assure that it decreases. The
photocatalytic degradation of contaminants by photocatalysis can be enhanced by the use of
additional oxidants like hydrogen peroxide and persulphate and/or the modification of the
catalyst using deposited metals, composite semiconductors or dye sensitisers.
The degradation of the original contaminants is monitored preferably by HPLC, and the final
mineralization by TOC and Ionic Chromatography, but identification of intermediate products
has to be by MS techniques (GC-MS and HPLC-MS). A combination of several MS
techniques with several extraction methods is better in order to assure the identification and
quantification of a significant number of DPs. As the detection of all DPs is almost
impossible, the use of different organism for determining toxicity is very important in order to
guarantee that the treatment is correct.
BIBLIOGRAPHY AND REFERENCES
Agüera, A., Fernández-Alba, A. R., GC-MS and LC-MS Evaluation of Pesticide Degradation
Products Generated from Advanced Oxidation Processes in Waters: An overview.
Analusis, 26, 123-130, 1998.
Chiron, S., Fernandez-Alba, A.R. and Rodriguez, A., Pesticide Chemical Oxidation
Processes: An Analytical Approach. Trends Anal. Chem., 16, 518-526, 1997.
Herrmann, J.M., Guillard, Ch., Argüello, M., Agüera, A., Tejedor, A., Piedra, L. and
Fernández-Alba, A. Photocatalytic Degradation of Pesticide Pyrimiphos-Methyl.
Determination of the Reaction Pathway and Identification of Intermediate Products by
Various Analytical Methods. Catalysis Today, 1999, in press.
Herrmann, J.M. Heterogeneous Photocatalysis: an Emerging Discipling Involving Multiphase
System. Catalysis Today, 24, 157-164, 1995.
Jardim, W.F., Moraes, S.G. and Takiyama, M.M.K. Photocatalytic Degradation of Aromatic
Chlorinated Compouns using TiO2: Toxicity of Intermediates. Wat. Res., 31, 17281732, 1997.
Malato, S., Blanco, J., Richter, C, Braun, B. and M. I. Maldonado. Enhancement of the Rate of
Solar Photocatalytic Mineralization of Organic Pollutants by Inorganic Oxidising
Species. Appl. Catal. B: Environ., 17, 347-360, 1998.
Pelizzetti, E., Carlin, V., Minero C. and M. Grätzel. Enhancement of the Rate of
Photocatalytic Degradation on TiO2 of 2-Chlorophenol, 2,7-Dichlorodibenzodioxin
and Atrazine by Inorganic Oxidizing Species. New J. Chem., 15, 351-359, 1991.
Tapp, J.F., Wharfe, J.R and Hunt, S.M. Toxic Impacts of Wastes on the Aquatic Environment,
Royal Soc. Chem., 1996.
Tothill, I.E. and Turner, A.P.F. Developments in Bioassay Methods for Toxicity Testing in
Water Treatment. Trends in Anal. Chem., 15, 178-187, 1996.
Wangersky, P.J. Dissolved Organic Carbon Methods: a Critical Review. Marine Chem., 41,
61-74, 1993.
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SOLAR DETOXIFICATION
SELF-ASSESSMENT QUESTIONS PART A. True or False?
1. Electrons in the conduction band cannot degrade organic compounds.
2. When organics decompose, an increase in the concentration of hydrogen ions is produced
in the water.
3. The final objective of photocatalytic treatment is always the total mineralization of the
organic contaminants.
4. Inorganic compounds are degraded through oxidative and/or reductive photocatalytic
pathways.
5. The addition of other oxidants in the photocatalytic process only produces additional
oxidant species.
6. Hydrogen peroxide always enhances the decomposition of organics.
7. Liquid chromatography (HPLC) with UV detection is the method of choice for analysing
samples from photocatalytic treatment because it is the easiest.
8. TOC analysis is a method for measuring CO2 production during experiments.
9. GC/MS and LC/MS are complementary methods.
10. There is a universal-monitoring device for toxicity testing.
PART B.
1. Which is the general stoichiometry for the total mineralization of methamidophos
(C2H8NO2PS)?
2. Why is the analysis of the initial compound not enough to determine the efficiency of
contaminant degradation?
3. Why is toxicity determination during a photocatalytic experiment very important?
4. What are the final products in photocatalytic treatment of CN-?
5. Which are the main advantages of an additional oxidant?
6. What are the most common oxidants used for photocatalytic degradation of contaminated
water?
7. What metals are used for doping TiO2 and how do they increase photoefficiency?
8. What are the commonest analytical techniques for determining the disappearance of the
initial compound during photocatalysis in water? Why?
9. What is the principal advantage of applying MS-based analytical methods to the
photocatalytic degradation of organic compounds?
10. What are the most common toxicity tests in use today?
Answers
Part A
1.False; 2. True; 3. False; 4. True ; 5. False; 6. False; 7. True; 8. False; 9. True; 10. False.
Part B
TiO / hυ
2
1. C 2 H 8 NO 2 PS + 7O 2 
→ 2CO 2 + H 3 PO 4 + H 2 SO 4 + HNO 3 + H 2 O
2. Because reactives and products could be lost causing results not to be reliable.
3. Because the products of incomplete degradation and their concentrations may be
sufficiently innocuous for discharge directly into the environment or for further biological
treatment.
89
SOLAR DETOXIFICATION
4. NO3- and CO2.
5. Increasing the number of trapped e- of the e-/h+ pairs, generating more •OH as well as
other oxidising species, increasing the oxidation rate of intermediate compounds and
avoiding problems caused by a low concentration of O2.
6. Hydrogen peroxide and persulphate.
7. Noble metals. After excitation, the electron migrates to the metal where it becomes
trapped and e-/h+ recombination is avoided.
8. Liquid chromatography (HPLC). This method permits direct injection of the aqueous
sample into the analytical column avoiding the necessity of extraction procedures.
9. The identification of unknown compounds produced during photocatalytic treatment of
contaminated water.
10. Bacterial screening tests.
90
SOLAR DETOXIFICATION
FIGURE CAPTIONS
Figure 5.1. Major general processes for the photo-oxidative or photo-reduction degradation of
organic compounds in aqueous solution sensitised by semiconductor particles. Examples of
photo-oxidation (PCP) and photo-reduction (CCl4) are shown.
Figure 5.2. HPLC-UV chromatograms of photodegradation of oxamyl before photocatalytic
treatment (A) and when the oxamyl has completely disappeared (B).
Figure 5.3. Evolution of H+ and Cl- during pentachlorophenol degradation. To more clearly
demonstrate that reaction 5.4 is completed, the concentration of TOC in mM is calculated
considering 1 mMol TOC = 6 mMol of C = 72 mg of C.
Figure 5.4. Chemical structures of pyrimethanil and its degradation products obtained during a
photocatalytic treatment with TiO2.
Figure 5.5. Electrons capture by a metal in contact with a semiconductor surface.
Figure 5.6. Concentrating solar reactor with platinum/titanium dioxide catalyst on ceramic
saddles. Tested on air contaminated by spray paint at Fort Carson Army Base in Colorado
(USA). Courtesy of National Renewable Energy Laboratory (USA).
Figure 5.7. The excitation process in a semiconductor-semiconductor photocatalyst.
Figure 5.8. Steps of excitation with a sensitizer in the presence of an adsorbed organic
electron acceptor (A).
Figure 5.9. Degradation pathway proposed for pirimiphos-methyl dissolved in water when
illuminated in the presence of TiO2.
91
SOLAR DETOXIFICATION
6. SOLAR DETOXIFICATION TECHNOLOGY
AIMS
This unit discusses the basic factors related to solar photocatalytic process technology and
applications. Main key issues related to the use of solar ultraviolet radiation and their
implications for appropriate materials and reactors for efficient light collection and use are
discussed here.
OBJECTIVES
When you have completed this unit, you will have a basic knowledge and understanding of
the following areas:
1. The key issues in collector technology related to solar water detoxification.
2. The specific peculiarities of the use of solar ultraviolet light.
3. Main characteristics of different types of collectors for solar water detoxification
applications.
4. Advantages and disadvantages of using non-concentrated sunlight.
5. Factors concerned with main technological components: reactor, reflectors and catalyst.
NOTATION AND UNITS
Symbol
Description
CPC
Compound Parabolic Concentrator
CPVC
Chlorinated polyvinyl chloride
CR
Concentration Ratio (solar collector)
C
Geometric concentration ratio
Effective concentration ratio
Cε
d
Reactor tube diameter
D
Aperture width (parabolic collector)
f
Focal length (parabolic trough collector)
ECTFE
Ethylenechloride tetrafluroethylene
ETFE
Ethylenetetrafluoroethylene
FEP
Fluorinated ethylenepropylene
IEP
Isoelectric Point
IFSH
Institut für Solarenergieforschung GmbH (Hannover, Germany)
IR
Infrared light (solar radiation)
NREL
National Renewable Energy Laboratories
PSA
Plataforma Solar de Almería
PTFE
Polytetrafluoroethylene
PTC
Parabolic Trough Collector
PVDF
Polyvinylidene fluoride
PZC
Point of Zero Charge
Re
Reynold number
UV
Ultraviolet light (solar radiation)
TFE
Tetrafluoroethylene
Semi-acceptance angle (parabolic collectors)
θa
Specular reflectance
ρs
Optical error (reflective surfaces)
σ
Units
mm
mm
mm
degrees
mrad
6.1 SOLAR COLLECTOR TECHNOLOGY GENERALITIES
Traditionally, different solar collector systems have been classified depending on the level of
concentration attained by them. The concentration ratio (CR) can be defined as the ratio of
92
SOLAR DETOXIFICATION
the collector aperture area to the absorber or reactor area. The aperture area is the area
intercepting radiation and the absorber area is the area of the component (either fully
illuminated or not) receiving concentrated solar radiation. This CR is directly related to the
working system temperature and, according to this criterion, there are three types of
collectors:
• Non concentrating or low-temperature, up to 150º C
• Medium concentrating or medium temperature, from 150º C to 400º C
• High concentrating or high temperature, over 400º C.
This traditional classification considers only the thermal efficiency of the solar collectors.
However, in photocatalytic applications, the thermal factor is irrelevant (as already explained
in Chapter 4) whereas the amount of useful radiation collected (in the case of the TiO2
catalyst, with a wavelength shorter then 385 nm) is very important.
Non-concentrating solar collectors (Figure 6.1) are static and non-solar-tracking. Usually,
they are flat plates, often aimed at the sun at a specific tilt, depending on the geographic
location. Their main advantage is their simplicity and low cost. An example is domestic hotwater technology.
Figure 6.1 Non-concentrating solar collectors for domestic heat water application
Medium concentrating solar collectors concentrate sunlight between 5 and 50 times, so
continuous tracking of the sun is required. Parabolic Trough Collectors (PTC) and
holographic collectors (Fresnel lenses) are in this group. The first have a parabolic reflecting
surface (Figure 6.2) which concentrates the radiation on a tubular receiver located in the focus
of the parabola. They may be one-axis tracking, either azimuth (east-west movement around a
north-south-oriented axis) or elevation (north-south movement around an east-west-oriented
axis), or two-axis tracking (azimuth + elevation). Fresnel lens collectors consist of refracting
surfaces (similar to convex lenses) which deviate the radiation at the same time they
concentrate it onto a focus.
Figure 6.2 Medium concentrating solar collector. Recirculating parabolic trough reactor for water purification
using titanium dioxide slurry at NREL. Courtesy of National Renewable Energy Laboratory (USA)
High concentrating collectors have a focal point instead of a linear focus and are based on a
93
SOLAR DETOXIFICATION
paraboloid with solar tracking. Typical concentration ratios are in the range of 100 to 10000
and precision optical elements are required. They include parabolic dishes and solar furnaces.
Figure 6.3 High concentration solar collector. Fix Focus solar reactor (PSA, Spain)
Up to now, the solar collectors used for photocatalysis have been in the two first categories. In
order to illustrate the variation in performance between different orientations, Figures 6.4 and
6.5 show, respectively, a comparative analysis of medium concentrating and non
concentrating solar collector efficiency with regard to direct incident radiation. Direct
radiation is the radiation which has no interference from the atmosphere and, consequently, a
known direction, and can therefore be concentrated. Global radiation is composed of direct
and diffuse radiation. The data represented in Figure 6.4, correspond to direct radiation in an
ideal cloudless year (based on average meteorological data on sunny days at the Plataforma
Solar de Almería) and show the energy available from direct radiation on the aperture plane of
a one-axis parabolic-trough collector with different orientations: elevation tracking, azimuth
tracking and azimuth tracking slightly tilted 8º with regard to the horizontal.
Figure 6.4 belongs here
Yearly efficiency of solar collectors: PTC-one axis with different orientations
The calculations performed are geometric and based on the cosine of the incident angle, this
angle being the one formed by the solar ray with the line normal to the aperture plane of the
collector. They allow to know the amount of direct radiation available at any given time for
each collector configuration. In Figure 6.4, it may be observed that the annual efficiency of
azimuth tracking (east-west movement around a north-south-oriented-axis) is about 10%
better than elevation tracking (north-south movement around an east-west-oriented axis). In
the first case, this efficiency increases notably in the summer and decreases in the winter
(identical in the Northern and Southern Hemispheres) whereas it is almost constant around the
year in the second case. A slight 8º tilt to the south in the northern hemisphere and the
opposite in the south increases yearly efficiency about 5% in the azimuth-tracking
configuration due to the reduction of the cosine factor over the year.
Figure 6.5 belongs here
Yearly efficiency of solar collectors: flat plate with different inclinations
In the case of non-concentrating flat-plate collectors, it may be observed that efficiencies are
lower than one-axis PTCs, attaining maximum efficiency with an inclination (to the south in
the Northern Hemisphere and to the north in the Southern Hemisphere) from the horizontal
equal to the local latitude. This configuration, that is, angle of tilt set at the angle of latitude of
the site, maximizes the annual energy collection in a flat-plate collector. Although the
calculations made here are for a specific location and latitude, the comparisons of solar
radiation collection and conclusions obtained are qualitatively valid for any other location.
94
SOLAR DETOXIFICATION
6.2 COLLECTORS FOR SOLAR WATER DETOXIFICATION. FEATURES
6.2.1.- Specific features of solar UV light utilization.
The specific hardware needed for solar photocatalytic applications have much in common
with those used for thermal applications. As a result, both photocatalytic systems and reactors
have followed conventional solar thermal collector designs, such as parabolic troughs and
non-concentrating collectors. At this point, their designs begin to diverge, since:
- the fluid must be exposed to ultraviolet solar radiation, and, therefore, the absorber
must be UV-transparent, and
- temperature does not play a significant role in the photocatalytic process, so no
insulation is required.
The first engineering-scale outdoor reactor for solar detoxification was developed by Sandia
National Laboratories (USA) at the end of the eighties (see Figure 6.6) where a parabolictrough solar thermal collector was modified simply by replacing the absorber/glazing-tube
combination with a Pyrex tube through which contaminated water could flow. Since then,
many different concepts with a wide variety of designs have been proposed and developed all
over the world, in a continuous effort to improve performance and reduce the cost of solar
detoxification systems. Among these different concepts, several of the most important with
regard to the definition of the overall system are those related to whether or not radiation must
be concentrated, the type of reflective surface to be used, the way the water circulates through
the reactor (tube, falling film or stirred vessel) and the way in which the catalyst is employed.
Figure 6.6 First engineering scale outdoor solar detoxification reactor using one-axis parabolic trough
collector. Part of the 465 m2 parabolic trough system at Sandia National Laboratory. Courtesy
of National Renewable Energy Laboratory (USA)
One of the most important reactor design issues is the decision between concentrating or nonconcentrating collector system. Concentrating systems have the advantage of a much smaller
reactor-tube area, which could mean a shorter circuit in which to confine, control and handle
the contaminated water to be treated. The alternative of using high-quality ultraviolet-lighttransmitting reactors and supported-catalyst devices also seems more logical, both
economically and from an engineering point of view, if concentrating collector systems are to
be used.
Nevertheless, concentrating reactors have two important disadvantages compared to nonconcentrating ones. The first is that they cannot concentrate (i.e., use) diffuse solar radiation,
which is unimportant for solar thermal applications, because diffuse radiation is a small
fraction of the total solar radiation. However, solar photocatalytic detoxification with TiO2 as
a catalyst uses only the UV fraction of the solar spectrum and, since this radiation is not
95
SOLAR DETOXIFICATION
absorbed by water vapour, as much as 50 percent of this, or more in very humid locations or
during cloudy or partly cloudy periods, can be diffuse. As non-concentrating solar collectors
can make use of both direct and diffuse UV radiation, their efficiency can be noticeably
higher. The second disadvantage of concentrating collectors is their complexity, cost and
maintenance requirements. The consequence of these disadvantages is that present state-ofthe-art favours the use of non-concentrating reactors for solar photocatalytic applications. An
additional disadvantage of concentrating reactors is that the quantum efficiency is low, due to
a square root rather than linear dependence of rate on light flux, as already explained in
chapter 4.
For many of the solar detoxification system components, the equipment is identical to that
used for other types of water treatment and construction materials are commercially available.
Most piping may be made of polyvinylidene fluoride (PVDF), chlorinated polyvinyl chloride
(CPVC), or simply polyethylene. In any case, piping, as well as the rest of the materials, must
be resistant to corrosion by the original contaminants and their possible by-products in the
destruction process. Neither must materials be reactive, interfering with the photocatalytic
process. All materials used must be inert to degradation by UV solar light in order to be
compatible with the minimum required lifetime of the system (10 years).
Optical material requirements are similar to other closed solar systems, but photocatalytic
reactors must transmit UV light efficiently because of the process requirements. In some
cases, when a steam pressure of contaminants in water is sufficiently low, a closed system
could not be required and then a transmissive UV containment material could be avoided.
All pipes, reactor and connection devices must be strong enough to withstand the necessary
water-flow pressure. Typical parameters are 2 to 4 bar for nominal system pressure drop and a
maximum of 5 to 7 bar. Concentrating system materials must also be able to withstand
possible high temperatures that could result from absorption of concentrated visible and
infrared light in the reactor.
With regard to the reflecting/concentrating materials, aluminium is the best option due to its
low cost and high reflectivity in the solar terrestrial UV spectrum. Commercially available
film products incorporate a thin aluminium foil with an acrylic coating. The last peculiarity of
solar photocatalytic systems is the requirement of a catalyst; in the case of TiO2 it can be
deployed in several ways, such as a slurry or as a fixed catalyst (like a fiberglass matrix
inserted in the reactor tube).
6.2.2 Parabolic Trough Collectors
Solar photoreactors for water detoxification were originally designed for use in line-focus
parabolic-trough concentrators. This was in part because of the historical emphasis on trough
units for solar thermal applications. Furthermore, PTC technology was relatively mature and
existing hardware could be easily modified for solar photocatalytic processes. PTCs,
considered medium concentrating collectors, are of two types:
a) One-axis parabolic trough
b) Two-axis parabolic trough
As explained previously, the first engineering-scale facility was developed from one-axis
PTCs (Sandia National Labs, USA, 1989) and the second from two-axis PTCs (Plataforma
Solar de Almería, Spain, 1990, Figure 6.7). Both facilities are considerably large pilot plants
96
SOLAR DETOXIFICATION
(hundreds of square meters of collecting surface) and can be considered the first steps in
industrialisation of the photocatalytic process.
Figure 6.7 CIEMAT 384 m2 solar detoxification facility using two-axis parabolic trough
collectors at Plataforma Solar de Almería (PSA, Spain)
Although one-axis tracking has been demonstrated to be the most economically suitable for
solar thermal applications, certain particularities of photocatalytic research make two-axistracking PTCs efficient for finding out exactly how much radiation reaches the photoreactor
at any given time and also, therefore, permitting accurate evaluation of all the other
parameters related to the photocatalytic process. This accuracy allows comparison of
experiments carried out in such large photoreactors with lab-scale photoreactors, where the
calculation of incident radiation is much easier. This also makes it possible to reduce the
number of variables during testing, using the knowledge acquired by other authors.
The equation of the parabola is:
y=
x2
4f
(6.1)
where f is the focal length. If D is the aperture width and d, the reactor tube diameter, the
geometric concentration of the collector C is:
C=
D
ðd
(6.2)
The basic components of a parabolic-trough collector for photocatalytic applications are: the
reflecting concentrator, the absorber tube (photoreactor), the drive-tracking system and the
overall structure. Of these, the last two do not differ in photocatalysis from the applications
for which they were originally designed and are identical to those existing for thermal
applications. Reflective surfaces and photoreactor technology are specifically discussed under
point 6.4.1, as it can be considered independently of the solar collector used.
The collector structure supports the reflecting concentrator system, which reflects direct
insolation onto the receiver tubes. Two-axis PTCs consist of a turret on which there is a
platform supporting several parallel parabolic trough collectors with the absorber in the focus.
The platform has two motors controlled by a two-axis (azimuth and elevation) tracking system.
Thus the collector aperture plane is always perpendicular to the solar rays, which are reflected by
the parabola onto the reactor tube at the focus through which the contaminated water to be
detoxified circulates. One-axis PTCs have only one motor and a one-axis solar-tracking system;
the reactor tube (linear focus of the parabola) is then positioned in the same plane containing the
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SOLAR DETOXIFICATION
normal vector of the collector aperture plane and the solar vector (See Figure 6.8). The angle
formed by these two vectors is called the incident angle of solar radiation.
Figure 6.8 belongs here
Solar ray reflection on a one-axis parabolic trough collector
After all optical losses have been considered, the effective concentrating ratio of PTCs is
usually between 5 and 20. Typical overall optical efficiencies in a PTC are in the range of 50
to 75 percent, with the following breakdown:
− Tracking system: 90%-95%
− Reflector/Concentrator (reflectivity): 80%-90%
− Absorber/Reactor (transmittance): 80%-90%
− Mechanical collector errors: 90%-95%
Parabolic-trough collectors make efficient use of direct solar radiation and, as an additional
advantage, the thermal energy collected from the concentrated radiation could be used in
parallel for other applications. The size and length of the reactor is smaller, receiving a large
amount of energy per unit of volume, so handling and control of the liquid to be treated is
simpler and cheaper, and the risk of leaks, which in many cases can be dangerous, is lower. In
general, this can also be translated into a reactor able to withstand higher pressures and able to
employ potentially costly supported-catalyst configurations.
6.2.3 One-Sun (Non-Concentrating) Collectors
One-sun non-concentrating collectors (CR = 1) are, in principle, cheaper than PTCs as they
have no moving parts or solar tracking devices. They do not concentrate radiation, so the
efficiency is not reduced by factors associated with reflection, concentration and solar
tracking. Manufacturing costs are cheaper because their components are simpler, which also
means easy and low-cost maintenance. Also, the non-concentrating collector support
structures are easier and cheaper to install and the surface required for their installation is
smaller, because since they are static they do not project shadows on the others.
Based on extensive effort in the designing of small non-tracking collectors, a wide number of
non-concentrating solar reactors have been developed for solar photocatalytic applications,
which can be classified as follows:
- Trickle-down flat plate, based on a tilted plate facing the sun over which the water to be
treated falls slowly; the catalyst is fixed on plate surface.
- Free-falling film, similar to the trickle-down flat plate, but with a higher flow rate and
normally with a catalyst attached to the surface on which the liquid to be treated
circulates. It is usually open to the atmosphere, so it can be used only when volatile
compounds are not present.
- Pressurized flat plate, consisting of two plates between which water circulates using a
separating wall which can be filled in with fibre to which the catalyst is attached.
- Tubular: this kind of collector usually consists of many small tubes connected in parallel
to make the flow circulate faster than a flat plate, but functioning basically the same.
- Shallow solar ponds. This is a very interesting variety, as pond reactors are easily built onsite, especially for industrial wastewater treatment. Since manufacturing industries already
use ponds for microbiological treatment of wastewater, shallow solar ponds can be used
for the front or back end of a combined solar/microbiological treatment scheme (see also
chapter 8.1).
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SOLAR DETOXIFICATION
Figure 6.9 Experimental set up of a thin film fixed bed reactor tested by ISFH at PSA
installations. The housing is made of plexiglas and the catalyst is fixed on a flat
glass plate. Courtesy of Institut für Solarenergieforschung GmbH (Hannover,
Germany)
Any falling film or flat reactor must be covered to avoid direct contact with the atmosphere.
The use of an uncovered reactor is not recommended due to many factors: loss of volatile
contaminants, dust and dirt inside the reaction mixture, etc.
Although one-sun designs possess important advantages, the design of a robust one-sun
photoreactor is not trivial, due to the need for a large area of weather-resistant, and chemically
inert ultraviolet-transmitting reactors. The amount of materials required makes it necessary
for them to be relatively inexpensive. Those that best meet these requirements are certain
types of plastics, e.g., fluoropolymer films, but these, although highly versatile, possess lower
tensile strength than the rigid glass pipe and reduces the pressure capacity of the photoreactor
system. This combination of low pressure capacity and large volume, coupled with the need
to either keep a catalyst slurry suspended or ensure good mass transfer to a supported catalyst,
requires carefully designed fluid control..
Non-concentrating systems require significantly more photoreactor area than concentrating
photoreactors and, as a consequence, full-scale detoxification systems (hundred of square
meters of collectors) must be designed to withstand the operating pressures anticipated for
fluid circulation through a large field. As a consequence, the use of tubular photoreactors has
a decided advantage because of the inherent structural efficiency of tubing; tubing is also
available in a large variety of materials and sizes and is a natural choice for a pressurized fluid
system. Finally, its construction must be economical and should to be efficient with low
pressure drop.
If a supported catalyst is used, the photoreactor has to be much larger than in a concentrating
system. If the catalyst is circulated in a slurry, the design would have to avoid low-flow
regions where the catalyst could settle out of suspension, which means that turbulent flow
must be assured throughout the hydraulic circuit. Containment of volatile organic
contaminants to prevent their escape into the atmosphere is also of concern in a large reactor.
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SOLAR DETOXIFICATION
Figure 6.10 (a) and 6.10 (b)
One-sun water treatment reactors with PTFE tubes at NREL: early reactor (a) and reactor
with titanium dioxide immobilized on glass fiber bundles (b).
Courtesy of National Renewable Energy Laboratory (USA)
6.2.4 Compound Parabolic Concentrator (CPC)
CPC collectors are a very interesting cross between trough concentrators and one-sun systems
and are one of the best options for solar photocatalytic applications. CPCs have been found to
provide the best optics for low concentration systems and these non-imaging concentrators
were extensively employed for evacuated tubes. CPCs are static collectors with a reflective
surface following an involute around a cylindrical reactor tube; it can be designed with a
CR=1 (or near one), then having the advantages of both PTCs and one sun collectors.
Figure 6.11 belongs here
Solar reflection on a CPC collector
Thanks to the reflector design, almost all the UV radiation arriving at the CPC aperture area
(not only direct, but also diffuse) can be collected and is available for the process in the
reactor. The UV light reflected by the CPC is more or less distributed around the back of the
tubular photoreactor and as a result most of the reactor tube circumference is illuminated, but
due to the ratio of CPC aperture to tube diameter, no one point on the tube receives much
more than one sun of UV light. As a result, the UV light incident on the reactor is very similar
to that of a one-sun photoreactor and, as in the case of flat-plate collectors, maximum yearly
efficiency is obtained at the same collector angle inclination as the local latitude.
Performance is very close to that of the simple tubular photoreactor, but only about l/3 of the
reactor tube material is required. As in a parabolic trough, the water is more easily piped and
distributed than in many one-sun designs. All these factors contribute to excellent CPC
collector performance in solar photocatalytic applications.
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SOLAR DETOXIFICATION
The explicit equation for a CPC reflector with a tubular reactor can be obtained from Figure
6.11; a generic reflector point S can be described in terms of two parameters, angle θ ,
subtended by lines originating at O (centre of the reactor tube) to A and R, and distance ρ ,
given by segment RS:
θ = OA OR
(6.3)
ρ = RS
(6.4)
RS being tangent to the reactor tube at R. One important parameter for CPC definition is the
angle of acceptance 2θ a , which is the angular range over which all or almost all rays are
accepted (i.e., reflected into the reactor tube) without moving the collector.
Figure 6.12 belongs here
Obtention of CPC involute
The solution is given in two separate portions, an ordinary involute for A to B and an outer
portion from B to C:
ρ = rθ
ρ=r
for
θ ≤θa + π 2
θ + θ a + π 2 − cos (θ − θ a )
1 + sin(θ − θ a )
part AB of the curve
for θ a +
π
3π
≤θ ≤
−θ a
2
2
part BC of the curve
(6.5)
(6.6)
The CPC concentration ratio (CR) is given by:
C=
1
sinθ a
(6.7)
In the special case of θ a =90º, CR=1 and every CPC curve is an ordinary involute (points B
and C are coincident). So, optimum CPC acceptance half-angles ( θ a ) for photocatalytic
applications are obtained from 60 to 90 degrees either side of the normal. This wide
acceptance angle allows the reflector to direct both direct-normal and diffuse sunlight onto the
reactor, as UV light collection is not highly sensitive to these acceptance angles. An
additional advantage is that these wide acceptance reflectors forgive the reflector-tube
alignment errors, which is an important virtue for a low-cost photoreactor array.
101
y
Fig. 6.12
θa
C
R
r
O
x
θa
θ
S
A
102
B
SOLAR DETOXIFICATION
Figures 6.13(a) and 6.13(b) View of CPC shape (a) and CPC photoreactor array (b). PSA
(Spain)
CPC reflectors are usually made of polished aluminium and the structure can be a simple
photoreactor support frame with connecting tubing. Since this type of reflector is considerably
less expensive than tubing, their use is very cost-effective compared to deploying nonconcentrating tubular photoreactors without use of any reflectors, but preserving the
advantages of using tubing for the active photoreactor area.
6.2.5 Holographic Collectors
Another innovative idea is the holographic concentrator. This concept has been extensively
explored with regard to solar thermal applications as well as in the area of concentrators for
photovoltaic systems. Holographic surfaces are highly wavelength-selective and their
development for solar thermal applications, which require the full solar spectrum, has proved
to be a very difficult task. However, the holographic technologies could be very appropriate
for narrow-wavelength-band processes such as photovoltaics and photochemistry.
Holography is basically a diffractive technology. It records the interference pattern between a
reference beam of highly coherent monochromatic light and an object beam using the same
light source. In the case of solar holographic concentrators, the object beam is the one in the
focal region (point-focus or line-focus concentrator) and the reference beam is the virtual
image of the sun as a source. Once created, sunlight incident on the holographic optic element
will focus back to the focal region by either transmission or reflection depending on whether
the reference beam used to create the hologram strikes the diffractive material from the same
or opposite direction as the object beam. As a hologram is a passive optical device, it is not
possible to track the sun without moving.
Figure 6.14 belongs here
Holographic concentration of solar light
Normally, holographic elements are made with highly coherent monochromatic laser light in
order to obtain the most efficient hologram at that wavelength. Efforts carried out with the sun
as the source of light have resulted in a maximum usable bandwidth of about 100 nm,
obviously insufficient for thermal applications. However, as the photocatalytic process with
TiO2 uses 300 to 385 nm photons, the holographic concentrator could very well be a good
way not only to supply these photons while filtering out those that are unnecessary, but also
minimising thermal heating of the photoreactor.
103
104
Focus
A
Holographic Optic Element
Focus
Fig. 6.14
B
Holographic Optic Element
SOLAR DETOXIFICATION
Furthermore, solar holographic concentrators can theoretically avoid the technological
complexities associated with the curved support of conventional reflecting concentrators.
However, many significant issues remain unresolved. One of the most important is whether
the devices developed in research programs can be feasibly translated to realistic costeffective sizes for the collection of solar energy. Also, there are not many holographic
materials in the UV wavelength region and their ability to survive in an outdoor environment
is unknown.
6.3 CONCENTRATED VERSUS NON-CONCENTRATED SUNLIGHT.
The use of concentrating or non-concentrating collectors is based on laboratory and
engineering-scale experiments carried out by many different research groups. As already
mentioned, during these experiments it was found that non-concentrating collectors have the
advantage of collecting diffuse light. UV light is more susceptible to Rayleigh scattering by
atmospheric gases, mainly water vapour, than visible light. The same mechanism scatters blue
light more than red light, which is what causes the sky to appear blue. Because of this
scattering, as much as half of the UV radiation arrives at the earth’s surface as diffuse light,
even on a clear day. Near-UV wavelengths (from 285 to 385 nm) comprise only 2-3% of the
energy in direct sunlight, but they make up 4-6% of combined diffuse and direct sunlight.
Concentrating collectors focus only the direct sunlight and cannot collect the diffuse light.
Thin clouds, dust, and haze reduce the direct-beam component of sunlight more than the
diffuse component. As a result, non-concentrating collectors can use a resource that is not
only larger but also less variable than that available to concentrating collectors, permitting, in
many locations, continual operation of the non-concentrating detoxification system. Under
cloudy conditions, non-concentrating devices can continue operating (although at lower
rates), while a trough unit would have to shut down. This fact has been successfully
demonstrated even in northern European locations with small solar detoxification pilot plants
(Figure 6.15).
Figure 6.15 Solar detoxification pilot plant in Koln (Germany)
Courtesy of Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR)
An additional benefit of a non-concentrating design is that the efficiency of the photocatalytic
process frequently decreases as light intensity increases (as explained in Chapter 4). This
behaviour means that catalysts are unable to process all of the UV energy available to the
desired pollutant destruction reactions when subjected to high UV fluxes. Lower light
intensities also slow down the rate of recombination, thus yielding higher efficiency (see
figure 4.8). Nevertheless, different researchers have obtained different results when testing
with low radiation intensities, so presumably, they are significantly affected by experimental
conditions.
105
SOLAR DETOXIFICATION
Some authors attribute the transition of r = f (I 1.0 ) → r = f (I 0.5 ) , to the excess of
photogenerated species (e-, h+ and •OH). At higher radiation intensities, another transition
from r = f (I 0.5 ) → r = f (I 0 ) is produced. At this moment, the photocatalytic reaction departs
from its dependence on radiation received, depending only on mass transfer within the
reaction, so the rate remains constant although radiation increases. This effect may be due to
several causes, such as the lack of electron scavengers (i.e. O2), or organic molecules in the
proximity of TiO2 surface and/or excess of products occupying active catalyst centres, etc.
Actually, this phenomenon appears to be more frequent with supported catalysts and/or at
slow mixing speeds. This implies small catalyst surface in contact with the liquid and less
turbulence, which does not favour contact of reactants with the catalyst and dispersion of
products.
Finally, one-sun photoreactors have additional advantages, such as decreasing optical losses
from reduced or non-existing reflective surface. On the other hand, among possible
advantages of concentrating systems is a much smaller reactor-tube area requirement (smaller
circuit, better control and handling of contaminated water to be treated). Nevertheless, stateof-the-art technology cannot match the higher efficiency of non-concentrating systems. This
has been demonstrated by many comparative efficiency studies, such as the one performed by
NREL (National Renewable Energy Laboratories, USA) with seven different small collectors
(from 18 to 157 litres total capacity and from 0.4 m2 to 53 m2 reflecting surface). Results
showed that one-sun collectors were significantly more efficient than concentrating collectors.
Due to all the above, and, furthermore, although it was first thought that PTCs were the ideal
technological alternative, their high cost and the fact that they can only be operated with
direct solar radiation (implying location only in highly insolated areas), have decided the
question in favour of the static non-concentrating collector alternative. Their intrinsic
simplicity, low maintenance and operating cost and potential for reducing the manufacturing
cost, make one-sun systems the natural selection for solar water detoxification.
Among the different technologies previously described, CPCs seem to be one of the best
developed options for system design and implementation. In this specific case, and moreover,
since the absorber is illuminated differently during the morning and the afternoon, if the
radiation distribution is integrated over the solar day, an almost regular distribution of light is
obtained along the reactor perimeter (Figure 6.16). This agrees with the optical characteristic
of low-concentrating CPC-type collectors, which can collect, within their acceptance angle,
the radiation coming from the hemisphere and place it on the absorber perimeter.
Figure 6.16 belongs here
Flux distribution on absorber along the solar day (6:00 to 18:00). Simulation of CPC
behaviour with the following data: collector orientation: East-West; semi-acceptance angle:
60º; truncation angle: 80º; absorber radius: 13.6 mm; optical gap: 2 mm; concentration
ratio: 1.17. Courtesy of Renewable Energy Dept. INETI (Portugal)
6.4 TECHNICAL ISSUES
In addition to the solar collector type, the most important technical issues related with solar
detoxification hardware are the reflective surface and the reactor tube, which are addressed
here in more detail.
6.4.1
106
Reflective Surfaces
Fig. 6.16
107
SOLAR DETOXIFICATION
The optical quality requirements of reflective surfaces for solar applications are usually
related to the concentration required by the particular application under consideration. The
higher the concentration desired, the stricter the requirements for quality of parameters. Light
reflected off a polished or mirrored surface obeys the law of reflection: the angle between the
incident ray and the normal to the surface is equal to the angle between the reflected ray and
the normal. When light reflects off a rear surface mirror, the light first passes through the
glass substrate, resulting in reflection losses, secondary reflections, refraction, absorption, and
scattering of light passing through the transparent substrate (second-surface mirrors).
Precision optical systems use first-surface mirrors that are aluminized on the outer surface to
avoid these phenomena.
When light obeys the law of reflection, it is termed a specular reflection. Most hard polished
(shiny) surfaces are primarily specular in nature. Even transparent glass specularly reflects a
portion of incoming light. Diffuse reflection is typical of particulate substances like powders.
If you shine a light on baking flour, for example, you will not see a directionally shiny
component. The powder will appear uniformly bright from every direction. Many reflections
are a combination of both diffuse and specular components. One manifestation of this is a
spread reflection, which has a dominant directional component that is partially diffused by
surface irregularities (Figure 6.17).
Figure 6.17 belongs here
Specular, diffuse and spread reflection from a surface
In the case of solar detoxification applications, the strictest requirements are those of PTCs,
for example, UV-mirror materials need to have a specular reflectance between 300-400 nm in
order to achieve concentration ratios of from 1 to 20. For this configuration, the effective
concentration ratio (Cε) can be related to the optical performance parameters as:
 σ 
C ε = C  ρ s sum 
σ 

(6.8)
Where:
C = concentration in the absence of surface and tracking errors, as defined in eq. 6.2
ρ s = specular reflectance
σ sum = half angular extent of sun (Gaussian distribution) = 2.73 mrad
σ = total optical error, which is function of the slope, specularity and tracking errors
The greater the errors are, and particularly the reflective surface errors, the lower the effective
concentration ratio is. So, the reverse is also true: the lower the effective concentration ratio
is, the higher the optical errors may be and therefore, the lower the quality of reflective
surface required. This is an important additional factor in favour of low or non-concentrating
systems, since these lower quality requirements (lower specular reflectance) are directly
translated into lower manufacturing cost, since the reflector element can represent a
considerable fraction of collector cost.
Another important factor is the reflective base material. For solar photocatalytic applications,
the reflective surface must clearly be made of a highly reflective material for ultraviolet
radiation. The reflectivity between 300 and 400 nm of traditional silver-coated mirrors is very
low (reflected radiation/incident radiation) and aluminium-coated mirrors is the best option in
this case (Figure 6.18).
108
Specular
Fig. 6.17
Diffuse
Spread
109
SOLAR DETOXIFICATION
Figure 6.18 belongs here
Reflectivity of fresh metal coatings for mirrors
Aluminium is the only metal surface that is highly reflective throughout the ultraviolet
spectrum. Reflectivities range from 92.3 percent at 280 nm. to 92.5 percent at 385 nm.
Comparable values for silver are 25.2 percent and 92.8 percent, respectively. A new deposited
aluminium surface is fragile and needs to be protected from weathering and abrasion, but the
conventional glass cover used for silver-backed mirrors has the drawback of significantly
filtering UV light (an effect that is duplicated due to the light path through the glass). The thin
oxide layer that forms naturally on aluminium is not sufficient to protect it in outdoor
environments. Under such exposure conditions, the oxide layer continues to grow and UV
reflectance drops off dramatically.
Various optical technologies require UV-reflective elements, such as UV mirrors for medical
imaging, astronomical telescopes, microscopy, UV curing, indoor lighting, microlithography,
industrial micro machining and UV laser reflection. Nevertheless the specific requirements of
these applications are very different from those of solar, mainly with regard to outdoor
durability. The ideal reflective surface for solar photocatalytic applications must be:
- highly reflective in the UV range,
- acceptable durability under outdoor conditions for extended service lifetimes and
- reasonable price to permit the technology to be competitive against alternative
technologies
The surfaces currently available that best fit these requirements are:
− electropolished anodized aluminium (electrolytically formed aluminium oxide outer layer)
− organic plastic films with an aluminium coating (three-part “sandwich”-type plasticaluminium-plastic composition)
Anodized coatings with tin oxide can provide good protection against some chemicals and
good resistance to abrasion. Typically, thin (2-3 µm) oxide layers are used to provide some
measure of resistance to abrasion but little protection against moisture or pollutants is
provided. Thicker oxide layers (up to 50 µm) are usually specified when anodized aluminium
is intended for engineering/marine applications but, unfortunately, such a coating results in
considerably lower reflectance in the UV range.
An interesting alternative approach is to cover the aluminium with a protective acrylic
lacquer. Acrylic lacquering provides impressive outdoor resistance (more than 1000 hours in
salt-spray fog chamber without significant degradation), but also reduces UV reflectance. A
compromise between outdoor resistance and UV reflectance could be an optimum solution.
Another possible solution is an aluminium-coated plastic film. Several commercial coated
plastic film products have been used successfully in parabolic trough applications.
− ECP-244 (3M film no longer manufactured) consisting of a 10-µm-thick aluminium
surface covered by a 76-µm-thick acrylic surface. Average reflectivity of new film
between 280 and 385 nm thick is about 63 percent.
− SA-85P (manufactured also by 3M); this film has a 50-µm-thick polyester backing, an 10µm-thick aluminium surface, and a very thin 2.5-µm-thick acrylic covering. Average
reflectivity in the same range is 87 percent for new films. The superior reflectivity is due
to the much thinner protective acrylic coating.
110
SOLAR DETOXIFICATION
− ECP-305 (3M); a silver reflective film similar in composition to SA-85.
− Other companies and even some research institutions have developed similar coated films,
usually a combination of over-100-µm-thick polyester backing on 10-µm-aluminium foil
and a thin outer film of acrylic or some copolymer highly resistant to outdoor exposure
with good UV transmittance.
Normally, due to their lack of rigidity, these films must be bonded over a stiff substrate and
about two percent specular reflectivity is lost in this process. Also, the reflectivity of each
film at the end of its lifetime (from 5 to 10 years) would be only 88 percent of the new bonded
value.
Figure 6.19 belongs here
Reflectivity of different aluminium and plastic film surfaces.
Curve (d) by courtesy of Alanold Aluminium-Veredlung GmbH & Co (Germany)
6.4.2 Photocatalytic Reactor
The requirements for the photocatalytic reactor are similar to other advanced water or air
oxidation processes, with the additional necessity of an illuminated photocatalyst. The
photocatalytic reactor must contain the catalyst and be transparent to UV radiation providing
good mass transfer of the contaminant from the fluid stream to an illuminated photocatalyst
surface with minimal pressure drop across the system.
As mentioned before, the square-root dependence on light intensity provides better photoefficiencies for one-sun designs, which leads to a flat-plate geometry. This geometry is widely
used for solar-powered domestic hot water heater systems in large part because of its simple
design. Nevertheless, for water treatment, the reactor must be hard enough to work under
usable water pressure and tube configurations clearly seem the most appropriate for fluid
containment and pumping. Adequate flow distribution inside the reactor must be assured, as
non-uniform distribution leads to non-uniform residence times inside the reactor, resulting in
decreased performance compared to an ideal-flow situation. If the catalyst is used in
suspension (slurry in the case of TiO2), the Reynold number (Re) must always be over 4000
in order to guarantee turbulent flow. This is critical in avoiding catalyst settlement. Another
important design issue is that internal reactor materials must not react with either the catalyst
or the pollutants to be treated or their by-products.
The choice of materials that are both transmissive to UV light and resistant to its destructive
effects is limited. Also, temperatures inside a one-sun solar photocatalytic reactor can easily
exceed 40°C due to the absorption of the visible portion of the solar spectrum. Therefore, a
one-sun reactor must be able to withstand summer temperatures of around 60 to 70°C in order
to insure that there will be no damage which could reduce the flow. Finally, low pH resistance
is needed since the production of inorganic acids as reaction by-products is quite normal (i.e.
the destruction of chlorinated hydrocarbons leads to the production of HCl).
Common materials that meet these requirements are fluoropolymers, acrylic polymers and
several types of glass. Quartz has excellent UV transmission and temperature and chemical
resistance, but the slight advantage in transmission in the terrestrial solar spectrum over other
materials does not justify its high cost, which makes it completely unfeasible for
photocatalytic applications.
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SOLAR DETOXIFICATION
Figure 6.20 belongs here
Transmittance of different materials suitable for the manufacture of photoreactor tubes
Plastics work well as long as they fulfil transmittance, pressure and thermal resistance
specifications as well as maintaining their properties during outdoor operation.
Fluoropolymers are a good choice of plastic for photoreactors due to their good UV
transmittance, excellent ultraviolet stability and chemical inertness. Several different types,
such as ETFE (ethylenetetrafluoroethylene), PTFE (polytetrafluoroethylene), ECTFE
(ethylenechloridetetrafluroethylene), PVDF (polyvinylidene fluoride), FEP (fluorinated
ethylenepropylene), PFA and TFE (tetrafluoroethylene), can be extruded into tubing and used
as a photoreactor. Tubular fluoropolymers are very strong, possess excellent tear resistance,
and are flexible and lighter than glass. One of their greatest disadvantages is that, in order to
achieve a desired minimum pressure rating, the wall thickness of a fluoropolymer tube may
have to be increased, which in turn will lower its UV transmittance. In addition, due to the
lack of rigidity, tube connections can withstand much lower pressures than glass tubes. ETFE
and FEP are among the best candidates; ETFP has higher tensile strength (extrude-like) than
FEP; this could mean thinner-walled tubes resulting in cost savings (since less material is
used) and higher UV transmittance and therefore higher photoreactor performance. The
problem is that ETFE tubing is not as readily available as FEP tubing. 50-mm-outer-diameter,
0.6-mm-wall FEP tubing has a UV hemispherical transmittance (300 to 400 nm), of 61.6%.
This light is transmitted as diffuse, as fluoropolymer materials are poor IR-diffusers, but make
an excellent visible / UV diffusers (diffusion usually varies with wavelength).
Acrylics could also potentially be used to enclose the photoreactor. However, acrylics are
very brittle and would have to be employed in sheets, which increases their cost. On the
positive side, acrylic polymer sheets can be shaped with channels and flow patterns that could
themselves be used as solar reactors. Other lower-cost polymers are available in tube form,
but none possess the necessary UV and chemical stability for detoxification of water that may
be contaminated with a variety of solvents or other pollutants. Also, low cost polymeric
materials are significantly more susceptible to attack by the pollutant molecules and the
dissolution of organic contaminants in polymer materials could be a way of avoiding the
degradation process.
Glass is another alternative for photoreactors. Standard glass, used as protective surface, is not
satisfactory because it absorbs part of the UV radiation that reaches it, due to its iron content.
Borosilicate glass has good transmissive properties in the solar range with a cut-off of about
285 nm. Therefore, such a low-iron-content glass would seem to be the most adequate.
Figure 6.21 Glass tubes manufacturing. Different compositions mean that the glass can be
used for a wide variety of applications. Courtesy of Schott-Rohrglas GmbH
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SOLAR DETOXIFICATION
Two undesirable effects reduce the performance of a glass reactor for solar detoxification:
increased absorption in the solar UV-range between 300 and 400 nm and a further decrease of
UV-transmittance during operation due to the damaging impact of solar radiation in the same
wavelength region (UV-solarisation). Both effects are caused to a large extent by polyvalent
ions that change charge. The effect of the Fe-ions in the glass, which change charge from Fe2+
to Fe3+ due to photo-oxidation by photons having a wavelength below 400 nm, is especially
harmful. Furthermore, the oxidised Fe3+ ion absorbs in the UV. As a result, enhancement of
transmittance in the 300-400 nm region could only be accomplished by strong reduction in
iron content down to 50 ppm (Figure 6.22), but penalised by a corresponding increase in cost.
Figure 6.22 belongs here
Influence of Iron on transmittance of Borosilicate Glass Light Transmission (oxidative
conditions). Samples: flat glass; 3 mm thickness. Courtesy of Schott-Rohrglas GmbH
Therefore, as both fluoropolymers and glass are valid photoreactor materials, cost becomes an
important issue. In large volumes, glass piping could be more expensive than fluoropolymer
tubing, but from the perspective of performance, the choice is the material that has the best
combination of tensile strength and UV transmittance. On this basis, if a large field is being
designed, large collector area means also a considerable number of reactors and, as
consequence, high system pressure rating. Thus, fluoropolymer tubes are not the best choice
of material since high-pressure is linearly related to thickness and could result in higher cost.
A detailed analysis is recommended for any specific design.
One of the most important parameters in a tubular photoreactor design is the diameter, as in
both homogeneous or heterogeneous photocatalysis it must be guaranteed that all arriving
useful photons are kept inside the reactor and do not go through it without intercepting a
catalyst particle. The intensity of illumination affects the relationship between reaction rate
and catalyst concentration. The dispersion and absorption of light causes photon density to
diminish almost exponentially over the length of the optical path within a catalyst suspension.
At higher light intensity, the catalyst concentration can be higher.
In the case of TiO2 heterogeneous photocatalysis, when catalyst concentration is very high, a
“screening” effect produces excessive opacity of the solution, preventing the catalyst particles
farthest in from being illuminated and reducing system efficiency. The lower the catalyst
concentration, the less opaque the suspension. 1 g L-1 of TiO2 catalyst reduces transmittance
to zero in a 10-mm-inner-diameter cylinder with concentrated light in a parabolic trough
collector (Figure 6.23). Therefore, in a wider diameter tube, only an outer layer is illuminated.
This means that larger inner reactor diameter permits use of lower optimum catalyst
concentrations. Practical inner diameters for tubular photoreactor would be in the range of 25
to 50 mm. Diameters that are very small do not make sense because of the associated high
pressure-drop and very large diameters imply a considerable dark volume, thus reducing
overall system efficiency.
Figure 6.23 belongs here
Zone of tubular reactor where light penetrates if the catalyst concentration is 1 g L-1
(TiO2 heterogeneous photocatalysis)
6.5 CATALYST ISSUES
The catalyst plays a major role, not only because of its importance to the process, but also
from a technological point of view. This is especially relevant in heterogeneous
113
114
Fig. 6.22
1
0,9
T ransm itance
0,8
A
0,7
0,6
B
0,5
C
A: 0ppm F e , 91% ( 300- 400 nm )
B: 50ppm F e , 88%
C : 100ppm F e , 84.5%
D : 150ppm F e , 83.5%
E : 200ppm F e , 81%
F : 250ppm F e , 80%
D
0,4
E
0,3
F
0,2
0,1
0
250
275
300
325
W avelength [nm ]
350
375
400
Fig. 6.23
1 cm
R = 29 mm
ID = 58 mm
Illuminated area
[TiO2] = 1 g L-1
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SOLAR DETOXIFICATION
photocatalysis, where Degussa P-25 titanium dioxide is often the standard particulate material
against which other catalysts have been and continue to be measured. TiO2 in aqueous phase
applications can be used in suspensions (slurry) or supported. In the first case, the catalyst
must be recovered. The technological implications of this are discussed below.
6.5.1 Slurry versus Supported Catalyst
One of the major solar photoreactor system design issues is whether to use a suspended or
supported catalyst. The majority of experiments to date have used small TiO2 particles
suspended in the contaminated water, which makes it necessary to recover them after
treatment. This process is addressed under the next point.
Supported catalyst configurations eliminate the need for catalyst filtration, but with the main
objection of an important reduction in system efficiency. The idea is to attach the catalyst to a
support inside the reactor as is done for gas-phase stream treatment, which requires the
catalyst to be anchored onto some type of inert support. Desirable characteristics of such a
system would include being very active (comparable to slurries), have a low pressure-drop,
long lifetime, and reasonable cost but, in the case of water treatments, to present this has not
been possible.
Fixed-catalyst designs must solve several problems. As the catalyst must be exposed to
sunlight and in contact with the pollutant, the support must be configured to efficiently route
the pollutant to the illuminated zone and, at the same time, maintain a high flow rate in the
water to ensure good mixing without significantly increasing system pressure, which means
more power for pumping, and thereby higher operating costs. Also, the same criteria
discussed for photoreactor materials must be kept in mind and applied when choosing a
support. Supports tested so far have included fibreglass beads, metal fibres, steel mesh,
aluminium, many types of plastic (Figure 6.24) and ceramics such as alumina, silicon carbide
and silica, in the most diverse shapes. TiO2 coatings on tiny hollow glass beads called
microballoons for catalyst removal, by screening rather than filtering, have also been tested.
Figure 6.24(a) and 6.24(b) (a) Experimental concentrating solar reactor using titanium
dioxide immobilised on glass wool for treating contaminated air streams. (b) Parabolic trough
reactor for water purification with immobilised titanium dioxide. Courtesy of National
Renewable Energy Laboratory (USA)
Support of TiO2 on a stiff surface by adherence can be done using several different
techniques, such as dip-coating with solvents, deposits from precursors, vapour deposition,
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SOLAR DETOXIFICATION
and sol-gel formation. Several important performance requirements are directly related to the
process used for catalyst fixation, such as the durability of the coating, catalyst activity,
lifetime, etc. Surface area of the catalyst coating must also be considered because good
contact of photons and target molecules with the catalyst is required for efficient performance.
Studies performed to date have not yet identified a fixed-catalyst system that performs as
efficiently as slurry systems. Several exceptions, such as a test performed by NREL (USA)
using silica beads as the catalyst support, have approached the efficiency of the slurry, but the
high pressure-drop across the bed made the system impractical. In aqueous systems,
compared to an unsupported catalyst, immobilisation of TiO2 results in a reduction in
performance around 60 to 70 percent. An important direct consequence of this fact is the
necessity of multiply by a factor of about 3 the size of necessary solar collector field if similar
efficiencies want to be obtained, making the overall system clearly less cost efficient and
competitive than slurry systems. In addition to the above, a key question is how long a
supported catalysts will last in a real stream of water; a short period of activity would mean
frequent replacement and, consequently, an important rise in the overall system cost.
To the contrary of fixed catalyst configurations, slurry configurations have the advantage of
higher throughputs (in the range of 200 to 400 percent) a low pressure-drop through the
reactor and excellent fluid-to-catalyst mass transfer.
6.5.2 Catalyst Recuperation and reuse
The need to remove the catalyst from the clean water after treatment was initially considered a
major disadvantage of a slurry system because ultrafiltration, which was believed difficult and
costly, would be required for efficient separation of the submicron-sized TiO2 particles from
the slurry. Although it is true that titanium dioxide powder particles are about 0.03 µm (30
nm), or even smaller in some cases (specially manufactured TiO2 can be up to 0.01 µm), once
in water, the particles always agglomerate into larger ones (from 0.3 to 0.6 µm), facilitating
the problem considerably, since microfiltration, which is much less expensive than
ultrafiltration, can be used. 0.2-µm carbon-graphite membranes can effectively concentrate
titanium dioxide slurry from less than 1 g L-1 to up to 100 g L-1 with about 2 to 3 bar transmembrane pressure and permeate flow rates of about 2500 l h-1 per square meter of
membrane. Recovery rates of over 99% have been obtained with 0.5 µm membranes and no
titanium dioxide has been detected in water filtered with 0.2 µm membranes.
However, best performance can be obtained when microfiltration is combined with previous
titanium dioxide sedimentation. About 90% of the catalyst can easily be recovered by
sedimentation and the rest by microfiltration. This means a significant reduction in time and
electricity in the typical recovery process for TiO2 concentrations of about 200 mg L-1. The
lifetime of membranes and the time between cleanings is also increased considerably. This
could be particularly important with high volumes of water.
Titanium dioxide sedimentation is closely related to colloidal stability and TiO2 aggregation
conditions. The suspension can easily be destabilized by adding an electrolyte (such as NaCl)
and/or adjusting the pH to point zero charge (PZC) and the isoelectric point (IEP) on the
surface of the catalyst particles, as both factors modify the surface charge. Progressive particle
agglomeration (sizes from 1 to 10 µm) and settlement is then obtained. In the case of TiO2
(Degussa P-25), at concentrations of 200 mg L-1, the PZC is obtained at about pH 7 (6.8±0.2,
in experiments at the Plataforma Solar de Almería) when NaCl concentration is about 10-6
molar.
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SOLAR DETOXIFICATION
Therefore, more than 200 hours are needed for 75% of titanium dioxide to settle at pH 4.5, but
at pH 7, only 5 hours of storage are needed to recover 90 to 95%. This can be directly
recovered in a conic-bottom tank and 5 to 10% of the remaining catalyst can be recovered by
microfiltration (see also chapter 9).
Figures 6.25 belongs here
Sedimentation experiments at different pH. [TiO2]=0,2 g/L; [NaCl]=0 M. The Y axis shows
the absorbance of the solution at 800 nm; no absorbance means absence of TiO2 (see also
Figure 4.2)
Catalyst recovered, usually in highly concentrated slurry, can be reused but not indefinitely.
Slurry lifetime has been tested with satisfactory results under laboratory conditions (i.e.,
deionized water and only one contaminant), but with real water treatments catalyst lifetime
would be diminished due to poisoning by contaminants. However, in specific applications,
tests have demonstrated that the catalyst can be reused up to 10 times, or even more, without
any problem.
If part of the catalyst is lost in the drainage water, the percentage of catalyst that remains
useful after each run is a relevant parameter. In field tests conducted by NREL in Florida, it
was reported that approximately 10% of the catalyst was washed away in the discharge water.
In tests performed at the Plataforma Solar de Almería, 2.5% of catalyst was lost. From this
point of view, about a 5 to 10% addition of catalyst could be an interesting option to
compensate possible loss and periodic replacement in case of catalyst poisoning.
SUMMARY OF THE CHAPTER
Solar collectors are traditionally divided in three categories: non-concentrating (or low
temperature, up to 150º C), medium concentrating (or medium temperature, from 150º C to
400º C) and high concentrating (or high temperature, over 400º C). Concentrating solar
systems make use of direct radiation and need solar tracking mechanisms. Non-concentrating
systems are much simpler as they do not need solar tracking and can collect direct and diffuse
solar radiation with slightly lower yearly efficiencies. The specific hardware needed for solar
photocatalytic applications is very similar to that used for conventional thermal applications
with the following main differences: the fluid must be exposed to the ultraviolet solar
radiation, so the absorber must be transparent to this radiation and no thermal insulation is
required as the temperature does not play a significant role in the photocatalytic process.
Besides, solar photoreactors for water detoxification were originally designed to use linefocus parabolic trough concentrators, non-concentrating collectors are the choice for solar
photocatalytic applications. They are more efficient than concentrator-based systems due to
the use of both direct and diffuse UV light, the square root dependence between reaction rate
and light intensity and their intrinsic simplicity. The CPC (static collectors with a reflection
surface following an involute around a cylindrical reactor tube) are a very interesting cross
between trough concentrators and one-sun systems and have been found to provide the best
optics for low concentration systems. CPC’s designed with a CR=1, or near one, are one of
the best options for solar photocatalytic applications. Aluminium is the only metal surface that
offers high reflectivity values in the UV spectrum. Electropolished anodized aluminium and
organic plastic films with an aluminium coating film are the most appropriate reflective
surfaces to be used for solar detoxification applications. Photocatalytic reactors must be both
transmissive and resistant to UV light. Common materials that meet these requirements are
fluoropolymers, acrylic polymers and borosilicate glass and tubular photoreactors designs are
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SOLAR DETOXIFICATION
the best option. In TiO2 heterogeneous photocatalysis, suspended catalyst systems gives
efficiencies higher than supported catalysts. After their use, TiO2 can be agglomerated and
sedimented. Best recovery performances are obtained with a two step process: sedimentation
and microfiltration.
BIBLIOGRAPHY AND REFERENCES
1. Rabl, A. “Active Solar Collectors and Their Applications”. Oxford University Press.
1985.
2. Blake, D. M.; Link, H. F.; Eber, K. “Solar Photocatalytic Detoxification of Water”.
Advances in Solar Energy, ed. Karl W. Boer, 167-210, 7. Boulder, CO: American Solar
Energy Society, Inc., 1992.
3. Blake, D. M.; Magrini, K.; Wolfrum, E.; May E.K. “Material Issues in Solar
Detoxification of Air and Water.” Optical Materials Technology for Energy Efficiency
and Solar Energy Conversion XV, eds. Carl M. Lampert, Claus G. Granqvist, Michael
Gratzel, and Satyen K. Deb, 154-62, Proceedings of SPIE, Bellingham, WA: SPIE–The
International Society for Optical Engineering, 1997.
4. Blanco, J.; Malato, S. et al. “Final Configuration of PSA Solar Detoxification Loop”.
Plataforma Solar de Almería. Technical Report: TR 06/91. 1991.
5. Fernández, P.; De las Nieves, F.J.; Malato, S. “TiO2 Sedimentation Procedure”. Proc. 2nd
Users European Workshop Training and Mobility of Researchers Programme at
Plataforma Solar de Almería, Serie Ponencias, CIEMAT (Ed.), Madrid, 1999.
6. Jorgensen, G.; Rangaprasad, G. Ultraviolet Reflector Materials for Solar Detoxification of
Hazardous Waste, SERI/TP-257-4418. Solar Energy Research Institute, Golden, CO,
1991. DE91002196.
7. Kreider, J. F. “Medium and High Temperature Solar Processes”. Academic Press. 1979.
8. May, E. K.; Gee, R.; Wickham, D.T.; Lafloon, L.A.; Wright, J.D. Design and Fabrication
of Prototype Solar Receiver/Reactors for the Solar Detoxification of Contaminated Water,
Industrial Solar Technology Corporation, Golden, Colorado, 1991. Final report for an
NREL subcontract.
9. Pacheco, K.; Watt, A.S.; Turchi, C.S. “Solar Detoxification of Water: Outdoor Testing of
Prototype Photoreactors.” ASME/ASES Joint Solar Energy Conference, eds. Allan
Kirkpatrick, and William Worek, 43-49, New York, NY: American Society of Mechanical
Engineers, 1993.
10. Wendelin, T. “A Survey of Potential Low-Cost Concentrator Concepts for Use in LowTemperature Water Detoxification.” ASME International Solar Energy Conference, eds.
William B. Stine, Jan Kreider, and Koichi Watanabe, 15-20, New York, NY: American
Society of Mechanical Engineers, 1992.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. The best yearly efficiency of a static non-concentrating solar collector is obtained when
the inclination from the horizontal is equal to the local latitude.
2. Higher inclination than the local latitude gives higher efficiencies in winter than in
summer (non-concentrating solar collector).
3. Temperature plays an important role in solar photocatalytic degradation of water
contaminants.
4. Diffuse UV solar light is a small portion of total UV radiation, especially in the case of
clouds.
5. Non-concentrating solar collectors can use both direct and diffuse solar light.
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SOLAR DETOXIFICATION
6. Concentrating solar collectors obtain better efficiencies than non-concentrating ones in
solar water detoxification applications.
7. Silver is not used to produce UV solar reflector due to its price.
8. Quartz is the best material for photocatalytic reactors.
9. Supported catalyst systems have similar efficiencies when compared with slurry systems.
10. Besides the particles of titanium dioxide are very small, once in water, they agglomerate
into bigger ones, 10 to 20 times bigger.
PART B.
1. Why are concentrating solar systems more efficient than non-concentrating ones, when
thermal applications are considered?
2. Why are non-concentrating solar systems more efficient than concentrating ones, when
UV photocatalytic applications are considered?
3. Which must be the diameter of a PTC reactor tube if the aperture width is 2.5 m and a
geometric concentration ratio of 10 is desired to be achieved?
4. What is the geometric concentration ration of a CPC if their semi-aceptance angle is 70º?
5. A PTC with a geometric concentration ratio of 6 has an effective concentration ratio of 4.
What is the total optical error of the reflective surface if their specular reflectance is 85%?
6. What is the main factor that limits the UV transmittance of standard glass?
7. Why reactor tubes of 200 mm inner diameter are not practical at TiO2 slurry systems?
8. Cite at least three of the main problems of supported catalyst systems.
9. How can titanium dioxide slurry particles be aggregated and sedimented?
10. Why the Reynold number must be higher than 4000, at TiO2 slurry systems?
ANSWERS
PART A
1. True; 2. True; 3. False; 4. False; 5. True; 6. False; 7. False; 8. True; 9. False; 10. True
PART B
1. Because they need a solar tracking system and the collectors are always looking at the
sun.
2. Because, besides non-concentrating systems have not solar tracking device, they can
collect the UV solar light, which is an important contribution to the yearly efficiency.
3. Using equation (6.2); d = 2500 / (10 x 3.1416) = 79.6 mm.
4. Using equation (6.7); C = 1 / sin (70) = 1.06.
5. Using equation (6.8); σ = 6 x 0.85 x 2.73 / 4 = 3.48 mrad.
6. The presence of Fe ions, specially Fe+3, which absorbs the UV light.
7. Because of most of the inner volume of the reactor would be dark due to the opacity of the
catalyst suspension.
8. Less efficiency, much higher pressure-drop and the necessity of periodic replacement.
9. TiO2 slurry suspensions can be destabilized by adding a small quantity of electrolyte, such
as NaCl, and/or adjusting the pH to 7 in order to get the point of zero charge and the
isoelectric point of the colloidal solution.
10. Because turbulent flow regime must be assured to guaranty adequate fluid mixture and to
avoid possible catalyst settlement.
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SOLAR DETOXIFICATION
7 SOLAR DETOXIFICATION APPLICATIONS
AIMS
This unit describes main proven and potential applications of solar detoxification processes
with demonstrated technical feasibility. Advantages and limitations of these applications are
also discussed.
OBJECTIVES
When you have completed this unit, you will have an appreciation of the following subjects:
1. Limiting factors and necessary conditions to feasible solar detoxification applications.
2. Main applications related with contaminated water.
3. Technical considerations about gas phase treatment processes.
NOTATION AND UNITS
Symbol
Description
AOT
Advanced Oxidation Technologies
BTEX
Bencene, Toluene, Ethyl benzene, and Xilenes
BWTP
Biological Wastewater Treatment Plant
CVOC
Chlorinated Volatile Organic Compounds
EDTA
Ethylene Diamine Tetraacetic Acid
EPA
Environmental Protection Agency (USA)
HMX
1,3,5,7-tetranitro-1,3,5,7-tetrazacycloctane
IR
Infrared light
LLNL
Lawrence Livermore National Laboratory (USA)
NREL
National Renewable Energy Laboratory (USA)
PAH
Polynuclear Aromatic Hydrocarbons
PCP
Pentachlorophenol
PCO
Photocatalytic Oxidation Processes
PSA
Plataforma Solar de Almería (Spain)
PTFE
Polytetrafluoroethylene
PVC
Polyvinyl Chloride
RDX
Hexahydro-1,3,5-trinitro-1,3,5-triazine
SNL
Sandia National Laboratories (USA)
TCE
Trichloroethylene
TNT
Trinitrotoluene
VOC
Volatile Organic Compounds
Units
7.1 INTRODUCTION
Detoxification is today the most successful photochemical application of solar photons, with
several relevant installations and projects already in operation. This is due not only to the fact
that solar detoxification is an outstanding demonstration of how well suited solar energy is to
environmental conservation, but also because, contrary to most photochemical processes, it is
non-selective and can be employed with complex mixtures of contaminants. During the last
decade, the number of references and related patents on heterogeneous photocatalytic removal
of toxic and hazardous compounds from water and air can be counted by thousands and the
number of applications and target compounds are numerous.
However, as indicated by Dr. M. Romero (CIEMAT, Spain), an analysis of the historical
evolution of solar photocatalysis clearly identifies three different stages of development.
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SOLAR DETOXIFICATION
Initially, the efforts of solar-conscious researchers focused on transferring laboratory research
carried out by photochemical groups to solar-engineered testing with existing technology.
These first results produced excitement in the photochemical research community. Their
extrapolation to practical situations assumed an ability to degrade almost any organic
contaminant as well as some metals. Later, more appropriate collectors and designs were
developed, but the need to know the fundamentals of certain aspects of the reaction led to an
increasing number of studies on kinetics, mechanisms, performance of mixtures and operating
parameters, with dissimilar results. It is a period of both promising and discouraging results.
At present, a third scenario seems to be underway, in which the boundary conditions of
applications are determined, and the technology is focusing on a few specific initial
applications, with the peculiarity that early development and still unsolved questions coexist
with near-commercial and industrial applications of the technology. As a result, the
environmental market, although very receptive to clean energy sources, is still reluctant to
"risk assuming" initiatives with regard to solar detoxification processes.
From the point of view of practical applications, heterogeneous photocatalysis with a
semiconductor (TiO2) is the process for which the solar technologies are most extensively
employed today, although in some specific applications, a well-known homogeneous phase
reaction, Photo-Fenton, shows higher water-phase degradation rates. In other solar
detoxification options, such as sensitized-photochemical oxidation by singlet oxygen, there is
still very limited experience to date.
Photocatalytic oxidation processes (PCO) currently under development are included in the
same group of Advanced Oxidation Technologies (AOT) with other radical-promoting
processes like plasma, electron-beam, etc. The main advantage of PCO over other AOTs is its
potential for incorporating solar energy in the form of solar photons, whereby the degradation
process acquires significant additional environmental value. If solar photons can be collected
and used efficiently at low cost, the number of opportunities for PCO may increase
dramatically. Although solar driven PCO was at first considered a universal method of
degrading organic pollutants, a profusion of contradictory results during recent years (positive
results in almost real problems together with other experimental results pointing out
uncertainties and negative performance) has lead to confused public perception. Solar PCO
technology is currently viewed as very sensitive to many things and scientific opinion is
evolving toward a more conservative period of specific applications.
Within this context, treatment of industrial wastewater, though difficult to develop, seems one
of the most promising fields of application of solar detoxification. The only really general rule
is that there is no general rule at all, each real case being completely different from any other.
In some cases, the Photo-Fenton process has demonstrated higher degradation efficiencies
than heterogeneous TiO2 photocatalysis, but in others, the Fe cycle is affected by the
contaminants and Photo-Fenton does not work at all. As consequence, preliminary research is
always required for assessing potential pollutant treatment and optimising the best option for
any specific problem, nearly on a case-by-case basis.
To find out if a specific water-contamination problem can be treated with solar detoxification
technology is not always easy, since low efficiencies produce hydroxyl radicals and slow
kinetics may limit economic feasibility. As mentioned above, preliminary tests are normally
needed for assessment of process viability. In attempt to provide some guidelines for the
reader, the following general affirmations may be made:
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SOLAR DETOXIFICATION
-
-
-
Maximum organic concentration of several hundred mg L-1. Photodegradation processes
work well in low or medium concentrations up to a few hundred ppm of organics. The
limit always depends on the nature of the contaminants, but concentrations over 1 gr L-1
normally are not suitable for solar photocatalytic processes unless previously diluted.
Non-biodegradable contaminants. When possible, biological treatments are always the
most cost-effective processes. Only when the contaminants are persistent (nonbiodegradable) do photocatalytic processes make sense.
Hazardous contaminants present within complex mixtures of organics. One of the main
advantages of solar photocatalysis is that it is non-selective, so non-biodegradable
contaminants can be treated within a complex mixture of other organic compounds. Solar
detoxification also works very well with individual contaminants, but a mixture could be
an indication of its utility. Hazardous contaminants also usually appear in concentrations
susceptible to photocatalytic treatment.
Contaminants with no easy conventional treatment. An additional hint of whether solar
photocatalysis will be useful can be provided by the fact that contaminants are present in
concentrations that make conventional treatment difficult.
The above recommendations provide an indication of the type of industrial wastewater for
which solar detoxification can potentially be employed. Nevertheless, several additional
conditions are needed before a complete solar detoxification feasibility study can be
undertaken.
-
-
-
-
-
Throughputs should be reasonable. Spatial velocities or surface throughputs in the solar
collector and solar-photon consumption related to the total volume to be treated should be
acceptable. The treatment capacity must be high enough to make photocatalytic
degradation practical. Many aqueous organic oxidation processes are too slow to be
economically viable.
Solar photons must be used efficiently. The technology to be applied must optimise the
collection of solar photons to be used. The overall energy needed per molecule destroyed
must also be low enough to make the process feasible and the use of external oxidants,
such as electron scavengers, like S2O8=, H2O2 or O3, must be possible to increase the
quantum yield.
The photocatalytic process should be reliable (no catalyst deactivation). The degradation
process must work continuously without problems, such as catalyst deactivation.
Collectors' components, catalyst and overall system must also be durable, guaranteeing
long periods of operation without incident.
Operation and maintenance processes must be simple. The implementation of any real
solar detoxification technology application requires minimum operation and system
supervision. As treatment processes are non-productive, personnel cost associated must be
reduced to a minimum. These considerations also apply to maintenance.
Batch system treatment. It is clear that water treatment with solar detoxification should be
run in recirculation mode with batch loads of contaminants to guarantee complete
destruction. This means that the treatment must be independent of the process generating
the wastewater and that on-line treatments normally are not feasible.
Applications that fulfil both groups of requirements may be considered serious candidates for
solar detoxification and a detailed feasibility test study would be worth to be considered.
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7.2 INDUSTRIAL WASTE WATER TREATMENT
As indicated above, solar detoxification technology may be considered feasible for industrial
effluents containing highly toxic compounds for which biological waste treatment plants are
unfeasible at medium or low pollutant concentrations. “Solardetox” is a bidimensional
technology that is linearly dependent on the energy flux. It is therefore also linearly dependent
on the collector surface, and investment in turn depends on the collector surface. Land
limitations must also be considered. Normally, several hundred or even 1000 to 2000 m2 are
not a problem for a medium-sized factory. As a conclusion, reasonable order of magnitude of
inflows for a Solar Detoxification plant, with the actual state-of-the-art of the technology,
would be in the range from several dozens up to a few hundreds of m3 per day.
At the moment, and from the experience accumulated by many scientists and researchers in
the last 10 years, solar detoxification seems to be a good solution for destroying, among
others, the following contaminants found in industrial waste-water:
− Phenols
− Agrochemical waste
− Halogenated hydrocarbons
− Antibiotics, antineoplastics and other chemical biocide compounds from the
pharmaceutical industry
− Wood preservative waste (PCP, fungicides)
− Hazardous metal ions
7.2.1 Phenols
These substances include phenol, cresol and other phenol substitutes. All the phenols are very
toxic. Their maximum concentration in a biological wastewater treatment plant inlet (BWTP)
is 1-2 mg/L. Even very low concentrations of phenols (1-10 mg/L) in fresh water produce
unpleasant odour/taste during the chlorinating process, so any discharge of phenols must be
avoided. The solar detoxification technology would therefore be useful for treatment of water
containing this type of contaminants.
Phenols can easily be degraded by solar photocatalysis. A few hundred square meters of solar
CPC collectors could crack or completely destroy the phenols contained in small volumes of
industrial effluents prior to discharging to a BWTP. Figure 7.1 shows two degradation tests
carried out on chemical industry waste water containing a large number of compounds,
mainly phenols, and including formol, phthalic acid, fumaric acid, maleic acid, glycol
compounds, xylene, toluene, methanol, butanol and phenylethylene, among others.
Figure 7.1 belongs here
Complete mineralization of a complex mixture of organic contaminants containing phenols
using persulphate as electron scavenger
As observed in Figure 7.1, complete degradation of all initial substances and their
intermediates is possible at reasonable degradation rates. Table 7.1 shows examples of
industrial processes that generate wastewater-containing phenols that could be good targets
for solar photocatalytic processes.
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SOLAR DETOXIFICATION
Chemicals
1.Phenols &
formaldehyde
Process
Phenol-formaldehyde
resins
2.Phenols
Storage tanks, ships,
lorries, piping
Waste description
Condensation water containing less
than 2000 mg/L of phenols and
formaldehyde
Rinsing water containing less than
15000 mg/L
3.Phenols
Phenols production
Spills, cleaning water
Quantity
500 L/ton of phenolic
resin
0.1% of the total
production diluted in 60
volumes of water
0.1% of the total
production diluted in 60
volumes of water
Table 7.1 Examples of processes producing phenol residues in waste water
One such application is the treatment of condensation water from the manufacture of phenolformaldehyde, one of the oldest synthetic resins used in industry, for which phenols are the
raw materials. Phenolic resins are obtained from the reaction of phenol or a phenol substitute
with formaldehyde. The reaction may be portrayed as indicated in Figure 7.2.
Figure 7.2 belongs here
Formation of phenolic resins. Courtesy of Ecosystem S.A. (Spain)
The ingredients are boiled in a reflux condenser (Figure 7.3) and condensation water is
usually removed by vacuum distillation. This condensation water contains several mg/L of
reagents, being of which are very toxic.
Figure 7.3 belongs here
Reactor condensation waste-water from manufacture of phenolic resins.
Courtesy of Ecosystem S.A.(Spain)
Phenol-formaldehyde resins are usually prepared with 40% formaldehyde by volume, then,
approximately 750 L of water is removed for each ton of final product. When solid
formaldehyde is used, only 160 L of condensation water are removed from the final mixture.
These phenolic residues contain between 600–2000 mg/L of phenol and between 5001300 mg/L of formaldehyde.
7.2.2 Agrochemical compounds
This family includes a broad range of chemicals, extensively used in agriculture. Some of
these compounds are soluble in water, others are used as suspensions, some are oil-based
pesticides and some are used as dusts. However, the majority of them are dissolved,
suspended or emulsified in water prior to spraying and the amount of wastewater generated
could vary greatly depending on handling. Water-borne contamination originates in many
different processes, such as cleaning and rinsing of spraying equipment, dumping of leftover
spray solutions, plastic bottle recycling, etc.
Pesticides are one of the best fields of application of solar detoxification technology for which
solar detoxification appears to be an ‘omnivorous’ technique. This is because they are
employed in very diluted solutions or suspensions of mullet-component formulas, in small
volumes and in batches. Very good results have been obtained with organohalogenated and
organophosphorous pesticides, carbamats, thiocarbamats, 2,4-D (2,4 dichlorophenoxyacetic
acid), triazines, etc. Besides the large amount of pesticide waste generated in agriculture,
there is also a huge amount of industrial waste from factories producing the active ingredients
125
126
Fig. 7.2
SOLAR DETOXIFICATION
and, especially, in factories where the active components and other additives are stored,
weighed according to the product formula, mixed and packed.
Chemicals
Pesticides
Process
Packaging
Pesticides
Manufacturing /
Packaging
Pesticides
Land application
Pesticides
Pesticides
Land application
Plastic container
recycling
Waste description
Wet scrubbing alkaline solution (NaOH)
containing dissolved, suspended and
partially degraded pesticides (the whole
family at high/medium concentration)
Floor cleaning water, mixer & reactor
cleaning and rinsing water
Quantity
500 m3/year for a medium size
company
2000 m3/year for a medium size
company, with 0.05% of the
active components dissolved or
suspended in 100 parts of water
------
Cleaning and rinsing water of application
equipment. It is formed by diluted spray
solution (< 100 mg/L of the whole family)
Spraying solution prepared in excess
-----0.2 to 0.5 g of pesticide per each
Cleaning and rinsing water of plastic
bottles prior to crushing and pelletisation. empty plastic bottle of 2L
average volume, dissolved in 500
(< 100 mg/L of the whole family)
volumes of water can be
estimated
Table 7.2 Examples of processes potentially producer of wastewater
containing agrochemical residues
A complete description of an example of use of the photocatalytic degradation technology for
these compounds may be found in chapter 9. Another potential example is the treatment of
rinse water from cleaning sprayers in agrochemical applications. Correct management of this
rinse water containing pesticides is necessary to avoid contaminating the soil, groundwater
and surface water that can occur when this material is improperly discharged. In many areas,
farmers prepare the spraying solution at the spring used by the whole town. Such places are
suitable for the installation of a small-medium sized solar treatment plant. It could also be of
interest to large farm owners or pesticide application companies.
7.2.3 Halogenated hydrocarbons
Solar detoxification has also been demonstrated to be efficient in the degradation of
halogenated solvents. Halogenated compounds are found in much of pharmaceutical industry
waste and increasing concern about volatile organic compound (VOC) emissions and
environmental regulations and directives are pushing industrial managers to control VOC
emissions. One of the VOC emission control methods is wet scrubbing, and the contaminated
water from the scrubbers can be treated by solar detoxification. Other sources of halogenated
wastes are factories of halocompounds manufacturing. The waste can be estimated as a low
percentage of the total production, dissolved at 100 – 200 mg/L. One example could be the
factories of PVC production, where each ton of produced PVC gives 2.5 m3 of wastewater
effluent contaminated with short-chain polymers or monomer of PVC.
Figure 7.4 belongs here
Photocatalytic degradation of dichloromethane, chloroform, trichloroethylene and
tetrachloroethylene using a TiO2 catalyst manufactured by CISE. Courtesy of University of
Torino (Italy)
Example: estimation of the required field. A feasibility study has determined that a 6 m2
photoreactor can completely mineralize 250 L of wastewater containing 100 mg/L of
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SOLAR DETOXIFICATION
chloroform in 8 hours of sunlight and 25000 L are to be treated. As all the parameters are
linear, assuming unchanged weather and waste-water conditions, the direct result is 600 m2 of
collector field in 8 hours, or 300 m2 in 16 hours to treat the 25000 L, while for a 100-m2 field
48 sunny hours would be necessary to complete the treatment.
Another potential halocompound application of solar detoxification is treatment of the
effluents from the manufacture of PVC (polyvinyl chloride), which is produced, in huge
quantities worldwide. The volume of these effluents is 2.5 times greater than the final
production of PVC (1 ton of PVC produces approximately 2.5 m3 of effluent) and may be
contaminated with short-chain polymers or vinyl chloride, the PVC monomer.
7.2.4 Antibiotics, antineoplastics and other pharmaceutical biocide compounds
There are two main types of pharmaceutical industries: factories that produce fine chemicals
and those that produce and mix raw materials and components to obtain a final product. Since
antibiotics are intrinsically biocides, they cannot be treated in BWTP. Antineoplastics are
citotoxic (some of them inhibit cellular division) and are commonly used in chemotherapy of
leukemia and tumors. Both of them are ecotoxic and should be degraded before dumping into
streams or sewage systems. Manufacturing equipment (hoppers, conveyors/ belt feeders,
weighing and mixing equipment, pill conforming or vial filling machines) is normally cleaned
thoroughly with a huge amount of water which can be stored and treated in batches, having
then the ideal conditions for solar detoxification treatment.
Chemicals
1.Antibiotics
2. Antibiotics
3. Antineoplastics
4. Antineoplastics
Process
Pill conforming & raw
material mixing process at
manufacturing of
pharmaceutical specialities
Antibiotic production.
Waste description
Cleaning water coming from
pill and vials filling equipment,
containing less than 1000 mg/L
of organic compounds
Fermentators and liophilisators
cleaning
Quantity
3-5000 L/d at a medium
sized factory
0.05% of total production
dissolved in 500 volumes
of water
Idem 1
Idem 2
Table 7.3 Examples of processes potentially producer of antibiotic and antineoplastic waste
The chemical industry also produces a great variety of biocides used as preservatives,
especially in paint. Common examples of these compounds are: phenyl mercuric acetate,
dithiocarbamates and other sulfuric compounds, halogenated phenols, halogenated phenolformaldehyde condensates, quaternary ammonium salts, etc. A medium sized chemical
company could typically produce 500 t/yr of this type of product. As equipment must be
cleaned between each batch, approximately 500–1000 m3 of rinse water could be generated
per year, with a normal concentration of 200–500 mg/L of the biocide, for which solar
treatment is feasible.
7.2.5 Wood preservative waste
The most toxic and persistent of the wood preservatives is pentachlorophenol (PCP). This
insecticide is very well know to solar detoxification researchers because it was one of the first
compounds studied exhaustively in photocatalytic degradation. Although it is now banned,
PCP is still produced and used in wood protecting varnishes because of its effectiveness
against woodworms and termites.
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SOLAR DETOXIFICATION
Other compounds in this group are creosote and organic insecticides and fungicides. Although
precise information on the amount and chemical composition of these residues is scant, the
general impression is that they produce a huge amount of waste. In one Spanish province
alone there are about 100 wood-preserving firms and the wood (timber and sawed pieces) are
treated in concentrated pesticide baths. When the baths become weakened, their content is
replaced with fresh product. A conservative assumption is that 5% of the chemicals in the
bath are discharged as waste.
Figure 7.5: Degradation of PCP at PSA Solar Detoxification Facility
(Helioman’s collectors loop) See also Figure 5.3 (solar photocatalytic mineralization of PCP)
7.2.6 Removal of hazardous metal ions from water
One additional advantage of the heterogeneous photocatalytic process is the presence of a
reductive chemistry pathway that can be exploited to remove reducible species, such as
hazardous heavy metal ions. It is important to remember that both oxidation and reduction
must occur simultaneously to maintain process activity, so it is possible to use the presence of
organics as reductants for the reduction of metals and, at the same time, the metals as oxidants
for the oxidation of organics. In general, the higher the concentrations of organics, the faster
the metal reduction rates. Similarly, an increase in the concentration of metals increases the
organics oxidation rate.
Oxygen is usually the oxidant when there are no other oxidants or metals present and water
acts as the reductant element. Dissolved oxygen can compete with dissolved metals for
conduction-band electrons and therefore can inhibit the rate of metal removal. However,
dissolved oxygen can be helpful for the degradation of the organics if the concentration of
metals is insufficient. The ability to remove metals depends on the standard reduction
potential for the reduction reaction. For example, Ag2+, Cr6+, Hg2+, and Pt2+ can be treated by
photocatalytic reduction, but Cd2+, Cu+2, and Ni+2 cannot.
Figure 7.6 Cr+6 to Cr+3 solar photocatalytic reduction at PSA Solar Detoxification Facility
(CPC’s collectors loop). See also Figure 7.7
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SOLAR DETOXIFICATION
Specific applications described in the literature are elimination of specific organometallic
compounds (organic agents forming complexes with metal ions) like phenylmercury or the
removal of silver from black-and-white photoprocessing waste. The presence of
organometallic compounds usually complicates wastewater treatment, as the metal becomes
more inert and the effectiveness of traditional processes for aqueous metal removal, such as
alkaline precipitation and ion exchange, is reduced. Although the rate of metal removal is
generally slow, it remains attractive to solar detoxification technology, as these applications
are normally widely distributed and can be processed in batches. When there are only metals,
inexpensive and non-hazardous sacrificial reductants (such as EDTA or citric acid) can be
added to enable photocatalytic treatment.
Figure 7.7 belongs here
Simultaneous oxidation of phenol and reduction of Cr+6 to Cr+3 using solar detoxification
technology (PSA, Spain)
Another application of interest is the reduction of Cr+6 to Cr+3. Photocatalytic reduction of
Cr(VI) is very sensitive to pH, and is more efficient below pH 2 and leaves the reduction
product, Cr(III), in solution in this range of pH. Around pH 5, Cr(III) forms a stable
precipitate. The Cr(VI) reduction rate is very sensitive to the nature of the organic that is
simultaneously destroyed. Generally, the more easily oxidized the organic compounds, the
higher the photocatalytic reduction rate; this means that different waste streams may exhibit
dramatically different treatment rates depending on their specific chemical composition.
7.2.7 Other applications
Many other potential applications of solar photocatalysis may be found in the literature.
Among them, the destruction of organics in recyclable water from the microelectronics
industry, the removal of explosives (such as TNT, RDX and HMX) from aqueous munitions
waste and the oxidative degradation of cyanides.
Because they are highly toxic, photocatalytic degradation of cyanides is another potential
industrial application having the advantage that it neither produces highly toxic substances
such as cyanogen chloride nor sludges and avoiding the use of additional chemical reactants
such as chlorine.
+
CN − + 2 OH − + 2 hVB → OCN − + H 2 O
(7.1)
Furthermore, photocatalytic oxidation can transform CN − into less toxic substances, such as
OCN − , and by carefully selecting reaction conditions, complete oxidation to CO2 and N2 can
be obtained:
+
OCN − + 2 H 2 O + 3 hVB → CO 3
−2
+
1
2
N2 + 4 H +
(7.2)
In all the applications mentioned in this chapter, the presence of inorganic ions (such as
chloride, phosphate, nitrate, sulphate, etc) in water can have a negative effect on the
destruction rates of the target compounds. High concentrations of inorganic ions have been
found to reduce the performance of the titania catalyst and, as a consequence, reduce
significantly the feasibility of the overall treatment process.
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SOLAR DETOXIFICATION
7.3 SEAPORT TANK TERMINALS
A large percentage of the international chemical trade is shipped by sea and there are many
tank terminals in harbours all over the world for the main purpose of receiving, storing and
distributing raw chemicals and fuels to industry. Final distribution of the stored product is in
bulk by tank trucks, except for a small amount in drums in the same terminal enclosure.
Seaport tank terminals must carry out cleaning operations and also have to control emissions
from the stored products. These cleaning operations usually produce huge amounts of water
contaminated with low concentrations of the chemical from tanks, pipelines, jetty pipelines
and hoses. Among the compounds normally transported and distributed this way, many
chemicals, e.g., phenol, metham sodium, perchloroethylene, trichloroetylene, methylen
chloride, sodium dichromate, etc., and other compounds, such as styrene, toluene, nonil
phenol, benzene chloride, etc., may be successfully treated by solar detoxification.
Example: Cleaning of phenol tanks. At the end of a contract for the rental of a 600-m3 tank
that had contained phenol, terminal personnel proceed to clean it for refill with styrene. This
is done in a two-step operation. First saturated steam is injected to dissolve any phenol
crystals remaining in the top and valves. 7400 L of condensed water containing 6.4% phenol
is then discharged into a storage waste tank for later recovery of the phenol by distillation or
its destruction by thermal treatment. Then the inside of the tank is rinsed out using a hose with
a special spray nozzle. This rinse water contains 450 mg/L of phenol in a total volume of
16000 L. The conditions (volume, concentration, transparency) of the waste generated in this
second step are ideally suited to solar detoxification.
Figure 7.8 shows an example of another potential application: treatment of wastewater from
cleaning a tank that had contained metham sodium. Metham sodium or Vapam is a
dithiocarbamate used as soil fumigant to control weeds, nematodes, fungi and insects (soil
steriliser). This compound is a direct competitor of methyl bromide, another soil disinfectant
and one of the ozone-depleting chemicals.
Figure 7.8 belongs here
Degradation of metham sodium wastewater from tank cleaning. A stop in the process is
observed after partial degradation of initial TOC content (PSA, Spain)
In a medium-sized seaport tank terminal (typically about 70 tanks with 28000 m3 average
storage capacity) at least 15 cleaning operations are completed each year, with huge amounts
of residues generated annually in the pipeline connection pit (snake pit), hoses and cleaning of
ships. There is also a potential application in any wet scrubber these plants may use for VOC
emission-control equipment. All these residues can easily be managed by solar detoxification
because they can be stored individually in small tanks or containers until the treatment
process can be run.
7.4 GROUNDWATER DECONTAMINATION
Remediation of contaminated groundwater is a problem that can be found in many parts of the
world and especially in developed countries. Typical substances found in contaminated
groundwater are chlorinated hydrocarbons (including PCB’s), aromatics from a variety of
sources including fuels (e.g., benzene and toluene), and a wide variety of other chemicals that
include pesticides, solvents, phenols (PCP, creosote), TNT & DNT, dyes, polynuclear
aromatic hydrocarbons (PAHs) and even dioxins.
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SOLAR DETOXIFICATION
Solar detoxification can be considered a good solution for ‘in-situ’ treatment and
decontamination of groundwater containing substances where conventional biological
treatment is difficult due to dilution of pollutants. Also, groundwater is clear and transparent
and there is always good storage capacity (the aquifer is in itself a holding ‘tank’ for
contaminated water). Tests have shown that the majority of pollutants found in contaminated
groundwater are easily destroyed by photocatalysis. Groundwater remediation also has the
advantage of a processing time-scale of months or years.
The difficulty is that standard discharge must normally be drinking-water quality, which is
difficult and costly (for any treatment technology to be used). Groundwater also tends to have
a high mineral and salt content, resulting in the need for pre- and post-treatment systems.
Typically contaminated sites are old chemical plants, oil recycling plants (engine lubricant
oil), refineries, chemical weapon and explosive factories, pesticide plants, coke and gas
factories, airforce bases, harbours, railway stations, power plants and substations. The
contaminated groundwater must be pumped out from a series of extraction wells located
downstream of the contamination plume. After treatment, the effluent is injected again into
the water table through upstream injection wells.
Figure 7.9 belongs here
Generic concept of contaminated groundwater treatment
Courtesy of ECOSYSTEM S.A. (Spain)
The first known treatment of contaminated groundwater by solar photocatalysis was carried
out on the grounds of the Lawrence Livermore National Laboratory (LLNL) in Livermore,
California. This was also the first on-site application of solar photocatalysis technology.
During World War II, a part of the grounds now occupied by LLNL was a Naval Air Station
training and maintenance facility. Trichloroethylene (TCE) and other toxic chemicals were
used extensively in normal operations to clean engine parts and other machinery. Over the
years, unconfined TCE and other volatile organic compounds entered local ground water,
where they are now slowly migrating off site. Today, TCE is present in the groundwater at
concentrations ranging up to 500 ppb, which means 100 times the acceptable EPA limit for
drinking water. The field experiment, developed by three US government laboratories
(NREL, SNL and LLNL), was conducted at LLNL in 1991 using available trough technology
and demonstrated the technical feasibility of this application.
The system consisted of two solar troughs, each 36.5 m long, with effective concentration
ratio of approximately 20 and total solar collector area of 158 m2 using a TiO2 slurry and
yielded outlet concentrations below 5 ppb, which meets the limits for drinking-water process
feasibility demonstration. One of the main problems reported was the required acid
pretreatment which had an important negative effect on the effectiveness of the process.
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SOLAR DETOXIFICATION
Figure 7.10 Part of the 156 m2 parabolic trough water treatment system tested on
contaminated ground water at a site at LLNL (USA) in 1992. Courtesy of National
Renewable Energy Laboratory (USA)
Another application in the literature is the one-sun solar detoxification facility installed in
1993 on a groundwater site contaminated by a former jet-fuel handling area at Tyndall Air
Force Base in Florida (USA). Contaminants of interest that were present in the groundwater
included benzene, toluene, ethyl benzene, and xylenes (BTEX). 30 one-sun collector modules
(1.22 m x 2.4 m) connected in series were used, each collector module made up of 66 UV
transparent parallel tubes with a 0.64-cm inner diameter and 2.4-m length (Figure 7.11).
Treatment of initial concentrations of 1 to 2 mg/L of BTEX was performed in a batch system
obtaining typical destruction levels of 50% to 75% in 3 hours. The total volume treated during
each run was 530 L.
Figure 7.11 One-sun reactor built by American Energy Technology, Inc. for treating
contaminated groundwater in Florida (USA) in 1992. Courtesy of National Renewable Energy
Laboratory (USA)
One last example is the photocatalytic cleaning of groundwater contaminated by gasoline
from underground storage tanks (Figures 7.12 a & b). In this case the groundwater is
contaminated with BTEX (benzene, toluene, ethyl benzene and xylenes) at a concentration of
about 1 ppm. The test site is a gasoline station and the system consists of six nonconcentrating solar reactors of 3 m2 each using TiO2 slurry and a 1900 litre storage tank.
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SOLAR DETOXIFICATION
Figures 7.12(a) and 7.12(b)
Non-concentrating solar detoxification system for BTEX-contaminated groundwater at a
commercial site in Gainesville, Florida (USA), 1996. Courtesy of the Solar Energy and
Energy Conversion Laboratory, University of Florida
7.5 CONTAMINATED LANDFILL CLEANING
Soil decontamination is another potential application of solar detoxification. There are
contaminated landfill sites all over the world for which, depending on the nature of the
contaminants, water or gas-phase photocatalytic treatments can be used. If the contaminants
are soluble in water, water can be used to extract them, and this water can then be treated later
by solar detoxification. If the contaminants are volatile organic compounds, they can be
desorbed from the solid as a gaseous product using either vacuum extraction or heat and
pumped into a gas-phase solar detoxification system (See also chapter 7.7).
One example of this application is the mineralization of lindane suspended in landfill
leachate. The reaction between benzene and chlorine under UV light produces 6 stable
isomers of hexachlorocyclohexane which are carcinogens. Only one of them, Lindane, which
makes up 14% of the final mixture, is actually an insecticide. The other isomers, very toxic
and also very stable in the environment, accumulate in factories, are dumped in very badly
managed landfills or have been spread throughout the environment for decades. Lindane is
one of the oldest chlorinated insecticides, which, due to its stability in the environment, is
banned for agrochemical and veterinary use in the majority of countries. However, it is still
used to eliminate lice in children (ectoparasiticide) and other pharmaceutical and medicinal
uses with the approval of sanitary authorities.
Figure 7.13 belongs here
Simulation of contaminated landfill treatment using solar detoxification. Mineralization of
Lindane (PSA Solar Detox Facility, Spain)
Many sites are contaminated by lindane, some of them very well known in Europe. Figure
7.13 shows the photocatalytic degradation of Lindane (technical grade) suspension in water,
simulating the final step of contaminated landfill treatment and demonstrating the technical
feasibility of the solar treatment process (maximum solubility of lindane: 10 mg/L).
7.6 WATER DISINFECTION
Chlorine is the most commonly used chemical for water disinfection because of its ability to
inactivate bacteria and viruses. Nevertheless, the presence of organic impurities in the water
can generate unwanted by-products of disinfection, such as trihalomethanes and other
carcinogens. As a result, other water disinfection technologies are becoming increasingly
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SOLAR DETOXIFICATION
important. Among them, ultraviolet irradiation with lamps is widely used to destroy biological
contaminants, primarily at a 254-nanometer wavelength. Solar ultraviolet, which is primarily
at 290 to 400 nm wavelengths, is much less active as a germicide (see Figure 7.14).
Figure 7.14 belongs here
Common ultraviolet band designations,based on biological effects.
Courtesy of International Light, Inc.(USA)
Despite the broad spectrum of research, the potential use of solar detoxification technology
for water disinfection is still essentially unexplored. However, the antibacterial effect of TiO2
has been demonstrated on several microorganisms, including Escherichia Coli, Lactobacillus,
Streptococcus, and others. In all cases, the photocatalytic oxidation effect of TiO2 particles
was able to effectively sensitize bacteria to photo-killing by solar exposure. This photo-killing
action is associated with the disruption of the cell wall and membrane through
photocatalytically induced surface oxidation resulting in the disintegration of the cell.
Disinfection of viruses, such as Phage MS2 and poliovirus 1, can also be found in the
literature.
Homogeneous methylene-blue-sensitized photochemical disinfection has also been found to
be highly effective with E. Faecalis bacillus spores through singlet oxygen photooxigenation.
Singlet oxygen is generated by absorption of a photon by a photosensitizer (dye) which is
excited and transfers its energy to a dissolved oxygen molecule. Photo-killing occurs when
this singlet oxygen penetrates into the microorganism resulting in oxidation and inactivation
of cellular components. Successful pilot plant experiments using this technique have been
carried out in Israel and Germany.
Two major disadvantages of sensitized photodisinfection in comparison with conventional
techniques are the generally slow kinetics and the lack of residual disinfection capacity (after
exposure to light). On the positive side, the presence of biological and organic contamination
in surface waters is usually highest in summer, when the greatest amount of solar radiation is
available for the process. One possible realistic approach might be preliminary solar
detoxification to partially disinfect water and reduce the level of organic contaminants,
followed by limited chlorination to maintain disinfection in distribution pipelines and avoid
formation of undesired by-products.
7.7 GAS-PHASE TREATMENTS
Gas-phase photocatalytic oxidation is one of the Advanced Oxidation Technologies (AOTs)
with promising applications for end-of-pipe treatment of gaseous emissions and air
purification. The first pilot experiments have only been carried out in the last few years. In the
USA, development of the photocatalytic oxidation technology was motivated by the
ambitious DOE Solar Industrial Program. Some examples of applications are cleaning of
exhaust streams from the microelectronics industry and treatment of off-gases from extraction
of contaminated soil vapour. Indoor air quality in houses, vehicles and spacecraft has also
been a matter of study both in the USA and Japan. In Europe several initiatives also address
the development of gas-phase solar PCO as an end-of-pipe technology for the treatment of
VOC emissions but, unlike water detoxification, all the above mentioned experiments have
been conducted with lamps. No solar installations can be found in the literature and there
continues to be a serious lack of solar-driven gas-phase PCO.
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SOLAR DETOXIFICATION
The fundamental background of radical attack on organic contaminants photocatalyzed with
the assistance of smog have been addressed by several tropospheric chemistry specialists but,
unfortunately, some crucial questions related to the basic fundamentals of gas-phase PCO still
persist. A critical screening of the related literature reveals conflicting results among the
authors regarding the influence of different process parameters. The reaction mechanism in
gas-phase, based on hydroxyl radicals, has also recently been revised by other possible
options, resulting in a presently confusing stage for gas-phase photocatalytic treatment
processes.
Figure 7.15 belongs here
Scheme of TCE gas-phase mineralization with PCO and a monolithic catalyst based on
sepiolite/TiO2/Pt
Figure 7.15 shows the possible trichloroethylene (TCE) PCO degradation pathways. Some
intermediates detected are formed in the presence of chloride, so it has been proposed that °Cl
could be the driving force instead of °OH. Other authors have proposed that both mechanistic
approaches via °Cl and °OH are essentially correct and the eventual pathway depends on the
characteristics of the catalyst (e.g., internal surface or porosity). TCE is one of the most
studied compounds because it is a major gas-phase air pollutant. Their photocatalytic
oxidation with titanium dioxide has received good reports in the literature.
Figure 7.16 belongs here
Destruction efficiencies of TCE 400 ppm. Thermocatalytic and photocatalytic processes at
different temperatures by using a MgSiO4/TiO2/Pt monolithic catalyst (CIEMAT, Spain)
Figure 7.16 shows a lab experiment performed on TCE using a monolithic TiO2-based
catalyst at different temperatures. Photoeffect was clearly differentiated by using appropriate
filters. As it can be observed, temperature played a negative role due to adsorption-desorption
phenomena. Nevertheless, apart from high photonic efficiencies (up to 95%) reported with
TCE, it is not easy to find applications for pure photocatalytic gas-phase treatment processes,
as not many compounds have been found to be significantly affected by photocatalysis, being
necessary in most cases a combination of photocatalytic degradation and destructive
thermocatalysis.
In addition to that, a major problem for solar gas-phase PCO is the difficulty, or even
impossibility, of working in batch systems, implying an important additional difficulty for
solar-driven processes due to the natural uncontrollability of the sun as the energy source. As
a result, it is not trivial to find a niche for solar driven PCO applied to gas-phase and air
purification. Hybrid solar-electric or pure electric devices using lamps are therefore envisaged
for initial practical applications.
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SOLAR DETOXIFICATION
Compound
Rate
Compound
Rate
m
methanol
m
acetaldehyde
s
methylene chloride
s
acetonitrile
s
methylethyl ketone
m
acetone
s
methylisobutyl ketone
m
benzene
m
perchloroethylene
f
benzaldehyde
m
propyl amine
m
butyl acetate
m
propylnitrile
s
butyl amine
m
tetraethylethylene
m
decane
m
toluene
s
dichloroacetyl chloride
s
trichloroethylene
f
dichlorobenzene
m
xylenes
m
diethyl amine
m
sec-butyl amine
m
diisopropyl amine
s
t-butyl amine
s
ethane
m
1-butanol
m
ethanol
m
1-propanol
m
ethyl acetate
m
1,1-dichloroethylene
f
ethylbenzene
m
2-propanol
m
ethylene
fuel oil
m
1,2-dichloroethylene
f
Table 7.4 Degradation rates at different compounds screened for photocatalytic activity: slow
(s), medium (m) and fast (f). Courtesy of National Renewable Energy Laboratory (USA)
Table 7.4 summarises NREL experience with gas-phase photocatalitic degradation of
different contaminants. Aromatic compounds like BTEX have slow or medium reaction rates
and TCE is revealed as the fastest. BTEX tests demonstrate deactivation of the TiO2 catalyst
and the need to add ozone to enhance and make PCO degradation possible.
This and other possible technology combinations could be another way of promoting gasphase applications. Possible targets are the majority of hazardous air pollutants requiring
abatement technologies: halogenated, aliphatic and aromatic hydrocarbons, alcohols, glycols,
ethers, epoxides and phenols normally present in air streams at concentrations of less than
5000 ppm. Some applications for air purification or degradation of emissions in the
semiconductor industry are also under development.
Figures 7.17(a) and 7.17(b)
Flat plate reactor for treating contaminated air exiting from air stripping units. McClelland Air
Force Base. California, 1997. Courtesy of National Renewable Energy Laboratory (USA)
137
SOLAR DETOXIFICATION
Figure 7.17 shows a recent one-sun reactor with PTFE window material and titanium dioxide
immobilized on polypropylene supports for gas-phase solar detoxification successfully tested
by NREL (USA). For additional information on gas-phase experimental treatment systems,
see also Figure 6.24 and Figure 5.6.
Contrary to water phase, in which catalyst may be used in the more efficient slurry
configurations, in gas phase this is not possible and all experimental systems developed to
date use an immobilised catalyst. This is apparent in the figures above. As discussed in
Section 6.5, the catalyst must be anchored onto some type of inert support inside the gasphase stream treatment reactor. Characteristics of such a system must be being very active
(high degradation efficiency), have a low pressure-drop, long lifetime, and reasonable cost.
Very diverse types of catalysts have been used in gas-phase systems, from MgSiO4/TiO2/Pt
monoliths to TiO2 immobilised on glass fibers, ceramic substrates, inert plastic materials, etc.
As in water phase, residence times must be as long as possible to increase degradation
efficiency.
One interesting potential application of gas-phase solar detoxification technology is the gas
treatment from contaminated landfill recovering (see chapter 7.5). Chlorinated volatile
organic compounds (CVOC) are among the most pervasive subsurface ground contaminants
and the remediation technologies most used for CVOC ultimately produce a gas-phase stream
containing CVOC, either from soil venting from the unsaturated zone or ground water
recovery from the saturated zone and subsequent air stripping. In the past, direct discharge of
gaseous CVOC to the atmosphere was normal, but early in the 90s, many regulations began to
require the abatement of these emissions using the best available technologies.
Other possible applications are the small air pollution sources. These applications are not
uncommon and some examples are dry cleaners, auto repair shops, bakeries, or coffee
roasters, etc. Additional potential solar applications include remote sites, storage tank vents,
or potentially explosive waste streams. Finally, the regeneration of a carbon bed or other
adsorbent materials could be another promising application for a solar system. In this
application, the adsorbent can perform 24-hour VOC removal, and the solar system can be
used to purge and destroy the contaminants during daylight hours.
With regard to air disinfection, most of the work carried out has been focused on the use of
TiO2 particles as sensitizers for the destruction of bacteria and viruses in air. In this field, a
very interesting application is the fabrication of self-disinfecting surfaces. Several studies
have reported the antibacterial effect of titanium dioxide on indoor air in combination with
UV fluorescent lamps. The use of thin TiO2 films as a method of keeping surfaces free of
biological material would be of particular value where sterile surfaces are essential, such as
operating rooms in hospitals. Although very few demonstrations have been reported to date, it
is expected that more will take place in the future.
138
SOLAR DETOXIFICATION
Finally, solar photocatalysis is the process that makes self-cleaning glasses work efficiently.
Fouling of glasses is mainly due to dust and/or atmospheric particles stuck on the surface on
greasy stains, mainly of fatty acids. Self-cleaning glasses are coated with an invisible thin
layer of titania, which under the simultaneous presence of oxygen (air), atmospheric water
vapour and solar UV-light, is able to decompose fatty acids by successive
photodecarboxylation (photo-Kolbe) reactions and allow coated glasses to recover their initial
clearness (Figure 7.18).
Figure 7.18 Self-cleaning windows. Evolution of photocatalytic treatment process on glasses
coated with titanium dioxide. Courtesy of Laboratoire de Photocatalyse, Catalyse et
Environnement (E.R. au CNRS), Ecole Central de Lyon (France).
SUMMARY OF THE CHAPTER
Solar detoxification technologies can provide the environmental waste management industry
with a powerful new tool to destroy waste with clean energy from the sun. Nevertheless, in
spite of the large number of patents and publications, industry is still reluctant to adopt the
process due to some discouraging and contradictory results that have led research to focus on
specific potential applications. Solar detoxification is a useful technology for addressing nonbiodegradable hazardous contaminants where no easy treatment by conventional technologies
is available at concentrations up to several hundred mg L-1. Phenols, agrochemical wastes,
halogenated hydrocarbons, biocide compounds from the pharmaceutical industry, wood
preserving waste, hazardous metal ions, cyanides, aqueous munitions waste, etc, are among
the industrial waste-water applications. Other interesting applications are the treatment of
groundwater contamination, seaport tank terminals, cleaning of contaminated landfills and
water disinfection. Gas-phase applications are in an even more confusing situation as some
crucial questions related to the basic fundamentals still persist. In gas gas-phase PCO, catalyst
must be used immobilised, being used diverse types supporting substrates (glass fibers,
ceramic substrates, inert plastic materials, monoliths, etc). Some potential gas-phase
applications are gas treatment from contaminated landfill recovering, degradation of
emissions in the semiconductor industry, air purification, air disinfection, self-cleaning
windows, etc.
BIBLIOGRAPHY AND REFERENCES
1. Acher, A.; Fischer, E.; Zellingher, R.; Manor, Y. “Photochemical disinfection of effluents.
Pilot plant studies”. Wat. Res. Vol. 24, No. 7, pp. 837-843. 1990.
2. Anon. “Development of a Homogeneous Aqueous Phase Photocatalyst for the Solar
Detoxification of Water”. Final Report to NREL. Solarchem Environmental Systems,
Markham, Ontario, 1995.
3. Goswami, D. Yogi, J. Klausner, G. D. Mathur, A. Martin, K. Schanze, P. Wyness, Craig
T. Turchi, and E. Marchand. “Solar Photocatalytic Treatment of Groundwater at Tyndall
139
SOLAR DETOXIFICATION
AFB: Field Test Results.” Solar 1993. Proceedings of the American Solar Energy Society
Annual Conference; pp. 235-239. ASES, 1993.
4. Malato, S. “Solar photocatalytic decomposition of pentachlorophenol dissolved in water”.
Doctoral thesis. Editorial Ciemat. 1999.
5. Mehos, M.S.; Turchi, C.S. “Field Testing Solar Photocatalytic Detoxification on TCEContaminated Groundwater”. Environmental Prog. 12(3); pp. 194-199. 1993.
6. Nimlos, R; Wolfrum, E.J.; Gratson, D.A.; Watt, A.S.; Jacoby, W.A.; Turchi, C. “Review
of Research Results for the Photocatalytic Oxidation of Hazardous Wastes in Air”.
NREL/TP-433-7043, 1995.
7. Prairie, M. ; Evans, L.R.; Martinez, S.L. “Destruction of Organics and Removal of Heavy
Metals in Water Via TiO2 Photocatalysis”. Chemical Oxidation: Technology for the
Nineties, Second International Symposium. Lancaster, PA: Technonomic Publishing
Company, 1994.
8. Vincent, M. “Solar Detox Market”. Training and Mobility of Researchers Summer
School: Industrial Applications of Solar Chemistry. September 1998. Editorial Ciemat.
9. Watts, R.J.; Kong, S.; Orr, M.P.; Miller, G.C; Henry, B.E. “Photocatalytic inactivation of
coliform bacteria and viruses in secondary wastewater effluent”. Wat. Res. Vol. 29, No. 1
pp. 95-100, 1995.
10. Yves, P.; Blake, D.; Magrini-Bair, K.; Lyons, C.; Turchi, C.; Watt, A.; Wolfrum, E.;
Prairie. M. “Solar Photocatalytic Processes for the Purification of Water: State of
Development and Barriers to Commercialization.” Solar Energy 56, No. 5, pp. 429-437,
1996.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. Preliminary research is always required for assessing potential treatment of pollutants in
any specific problem on a case-by-case basis.
2. Solar Photocatalytic processes work well with organic concentrations of over 1 gr/L.
3. Non biodegradable organic contaminants are the most logical target for solar
photocatalytic processes.
4. There is no sense in addressing complex mixtures of hazardous contaminants with solar
photocatalysis.
5. A hint of solar photocatalytic technology being of possible application could be lack of an
easy conventional treatment.
6. External oxidants must always be used as electron scavengers, when feasible, to increase
the quantum yield of degradation processes.
7. There is usually no difference between treatment of contaminated water in a batch or online process.
8. Typical solar detoxification plants process from several dozen up to a few hundred m3 per
day.
9. The presence of high concentrations of inorganic ions negatively affects the destruction
rate and the overall feasibility of solar detoxification processes.
10. It is difficult to find applications for pure photocatalytic gas-phase treatment processes as
not many compounds have been found to be affected by a significant photocatalytic effect.
PART B.
1. Why is the environmental market still reluctant to accept solar detoxification processes?
2. Indicate the three existing solar photocatalytic processes.
140
SOLAR DETOXIFICATION
3. What is the main advantage of Photocatalytic Oxidation Processes over the rest of the
Advanced Oxidation Technologies?
4. In addition to the use of solar energy, indicate one main advantage of solar photocatalysis.
5. Indicate at least three conditions indicative of a viable solar photocatalytic process.
6. Indicate why water treatment with solar photocatalysis must normally be in a batch
system.
7. As solar photocalysis is an oxidation-reduction process, what is the normal reductant
agent when only organic compounds are present?; and when metals are present?
8. From testing it has been found that a 4-m2 collector system can treat a 50-L sample of
wastewater containing 200 mg/L of TOC from organic compounds in 4 hours of sunlight.
How long would it take to treat 100 m3 of the same wastewater with a 500-m2 treatment
system?
9. What are the proposed mechanisms for gas-phase photocatalytic degradation of
chlorinated organic compounds?
10. What is the worst situation for water treatment process based on TiO2 photocatalysis?
ANSWERS
PART A
1. True; 2. False; 3. True; 4. False; 5. True; 6. True; 7. False; 8. True; 9. True; 10. True.
PART B
1. Because initially successful experimental results presumed a wide range of practical
applications, but later confusing and discouraging results led to current concentration on
applications with specific required boundary conditions.
2. Heterogeneous TiO2 photocatalysis, homogeneous photo-Fenton and homogeneous
sensitized-photochemical oxidation by singlet oxygen.
3. Their potential solarization, introducing an important additional environmental value in
the pollutant degradation process.
4. The fact that it is a non-selective process, making possible the treatment of complex
mixtures of non-biodegradable organic contaminants.
5. Reasonable throughputs, efficient use of solar photons and reliable photocatalytic process
with no catalyst deactivation.
6. Because the degradation process is driven by solar photons, a source which can not be
controlled, making it necessary to separate the treatment from the wastewater generating
process.
7. When only organic compounds are present, oxygen is the normal oxidant and water acts
as the reductant element. When metals are present, oxygen is also the oxidant and the
metals themselves are the reductant elements.
 100000



500
8. R = 4 
 = 64 hours of sunlight.
50

4


9. Via °Cl, °OH or °Cl + °OH radicals attack, depending the eventual pathway on the
catalyst characteristics.
10. The presence of high concentration of organic contaminants and inorganic ions
simultaneously.
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SOLAR DETOXIFICATION
8 ECONOMIC ASSESSMENT
AIMS
This unit discusses the main factors entering into the cost of solar detoxification systems and
describes how to go about the economic assessment of specific solar photocatalytic
applications. They are also qualitatively compared with other treatment technologies.
OBJECTIVES
After completing this unit, you will be able to:
1.
2.
3.
4.
Determine the feasibility of a two-step photocatalytic biological treatment process.
Estimate the cost of solar photocatalytic treatment.
Estimate the feasibility of solar detoxification as a function of the local solar resource.
Compare possible alternative or complementary treatment technologies.
NOTATIONS AND UNITS
Symbol
Description
AOPs
Advanced Oxidation Processes
ASTM
American Standards for Testing and Materials
Atmosphere attenuation coefficient at λ.
cλ
COD
Chemical Oxygen Demand
Cosine of the incident angle of solar radiation to collector
cos ϕ
surface
CPC
Compound parabolic concentrator
Extraterrestrial irradiation
E(λ)
EPFL
Ecole Politechnique Federale de Laussanne (Switzerland)
f
Cloud Factor
FCR
Fixed Charge Rate
GAC
Granular-activated carbon
hs
Solar time angle
H
Henry´s Law constant
Total yearly hours of operation of a lamp based system
HL
HS
Total yearly hours of operation of a solar detoxification system
Intensity of solar irradiation over terrestrial surface at
Ib,λ
specific wavelength, λ.
Intensity of extraterrestrial solar irradiation at the same
Io,λ
wavelength, λ.
Isc,λ
Solar Constant associated with a spectral interval (λ1,λ2)
Yearly
average global UV irradiation
I UVg
L
Local latitude
m
Air mass ratio
mo
Air mass ratio at sea level
M
Molar
mM
Mili Molar
MW
Molecular Weight
N
Daynumber
NREL
National Renewable Energy Laboratory (USA)
Number of electric lamps
NL
Total yearly collected UV photons (energy higher than 3.2 eV)
NUV
142
Units
dimensionless
moles of O2 L-1
W m-2 µm-1
%
%
degrees
dimensionless
hours
hours
W m-2
W m-2
W m-2
W m-2
degrees
dimensionless
dimensionless
Moles L-1
10-3 moles L-1
gr mol-1
dimensionless
photons
SOLAR DETOXIFICATION
NΦ (E>3.2 eV)
PCP
RDX
S
TCE
TNT
TOC
Tf
Tfm
Tfv
UV-A
z
α
δs
φPH
ΨAOS
Number of photons with energy higher than 3.2 eV per unit of photons m-2 h-1
time and surface
Pentachlorophenol
Hexahydro-1,3,5-trinitro-1,3,5-triazine
Surface of solar collector field
m2
Trichloroethylene
Trinitrotoluene
Total Organic Carbon
mg L-1 or
moles of C L-1
Treatment Factor
Mass Treatment Factor
g h-1 m-2
Volumetric Treatment Factor
L h-1 m-2
Ultraviolet Radiation A
Elevation over sea level
m
Solar altitude
degrees
Solar declination
degrees
Nominal average photonic flux electric lamp
photons s-1
Average Oxidation State
dimensionless
8.1 PHOTOCHEMICAL AND BIOLOGICAL REACTORS COUPLING
The Mass Treatment Factor (Tfm) of a specific solar detoxification system can be defined as
the amount of organic substances the system is able to treat per unit of time and solar
collector surface:
T fm =
∆ m (TOC )
tS
 g

2
h m



(8.1)
The Volumetric Treatment Factor (Tfv) of a specific solar detoxification system can be defined
as the volume of contaminated water the system is able to treat per unit of time and solar
collector surface:
T fv =
∆V
tS
 L

2
hm



(8.2)
Tfm and Tfv obviously depend on the specific solar system and the waste water to be treated.
The same system yields different treatment factors with different contaminants and it is also
different depending on the solar irradiation, higher on sunny days than cloudy days. The most
practical units are those indicated in Equations 8.1 and 8.2: the grams of TOC degraded per
hour and square meter of solar collector field, in the case of Tfm, and the litres of water treated
per hour and square meter of solar collector field in the case of Tfv.
Example: calculation of treatment factors. If a specific solar detoxification plant having
300 m2 of collectors can treat 5 m3 of waste water containing 50 mg L-1 of organic
compounds in 90 minutes, the treatment factors will be:
T fm =
50 mg L−1 x 5000 L
g
−3 g
= 0.55
x 10
2
mg
300 m x 1.5 h
h m2
(8.3)
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SOLAR DETOXIFICATION
T fv =
5000 L
L
= 11.11
2
300 m x 1.5 h
h m2
(8.4)
Treatment Factors are a powerful tool for comparison of different detoxification systems and
checking treatment feasibility for different waste-water problems. The efficiency of solar
detoxification systems is usually around 0.05 to 1 g of organic compounds degraded per hour
and square meter. This is a function of several parameters, such as system geometry and
materials, the contaminants and their concentration, UV-A irradiation, catalyst concentration,
etc. Nevertheless, this parameter does not necessarily imply better or worse performance of a
specific system, because this depends on the discharge objectives. Contaminant treatment, in
its strictest meaning, is the complete mineralization (TOC = 0) of the contaminants, but, as
indicated in Chapter 7, normally photocatalytic processes only make sense for hazardous nonbiodegradable pollutants (toxic organic pollutants not treatable in conventional biological
treatment plants). When feasible, biological treatment of biodegradable residual waters are the
cheapest treatment technologies and also the most compatible with the environment.
Therefore, biologically recalcitrant compounds could be treated with photocatalytic
technologies until biodegradability is achieved, later transferring the water to a conventional
biological plant. Such a combination of photocatalytic and biological treatments, reduces
treatment time and optimises the overall economics, since the solar detoxification system can
be significantly smaller. Due to the kinetic mechanism explained in chapter 4, the first part of
the photocatalytic process is the quickest, producing the main part of TOC degradation, and
the last phase is the longest, with a minimum contribution to complete degradation.
Figure 8.1 Conceptual scheme of photocatalytic + biological technologies
coupling Courtesy of EPFL (Switzerland)
An example might be agrochemical compounds, which are normally quickly cracked by
photocatalytic degradation processes. The active component of the pesticide disappears after
several minutes of irradiation, but TOC becomes negligible only after two or three hours of
irradiation. Design of a solar detoxification system to reduce the TOC (i.e., disappearance of a
specific compound or compounds) only until toxicity is low enough to be sent on to a
biological treatment plant might therefore be of interest.
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SOLAR DETOXIFICATION
Figure 8.2 (a) and 8.2 (b)
Elimination of tensioactive compounds from agrochemical industrial wastewater after a few
minutes of photocatalytic treatment. (PSA, Spain)
In the above example (equation 8.2), “treat” could mean totally degrade organic contaminants
or only to biodegradability. Therefore, the Treatment Factor may also depend on the system
objective and a very low Treatment Factor for complete mineralization could be transformed
into a feasible one for integration with biodegradation.
The feasibility of combining photocatalytic and biological treatments to achieve highly
efficient abatement of pollutants that cannot be treated by either biological or photocatalytic
methods alone has already been demonstrated at EFPL (Pulgarin et al., 1999). This strategy is
based on the Average Oxidation State (ΨAOS), which can be defined from the following
formula:
 TOC − COD 
Average Oxidation State = Ψ AOS = 4 x 

TOC


(8.5)
Where TOC (Total Organic Carbon) and COD (Chemical Oxygen Demand) are expressed in
moles of C L-1 and moles of O2 L-1, respectively. Average Oxidation State can be between +4
for CO2, the most oxidised state of carbon, and -4 for CH4, the most reduced state of carbon.
The latter is given by the reaction:
CH 4 + 2 O 2
→ CO 2 + 2 H 2 O
(8.6)
Where, TOC = 1 (moles of C L-1) and COD = 2 (moles O2 L-1) so, according to equation 8.5,
ΨAOS = –4. With CO2, as oxidation is impossible, COD = 0 and ΨAOS = +4. An example is
oxidation of formic acid:
H − COOH + O 2
→ CO 2 + 2 H 2 O
(8.7)
In this case, TOC = 1 (moles of C L-1) and COD = 1 (moles O2 L-1), so ΨAOS = 0. When
photodegradation is applied, the Average Oxidation State of intermediates is a function of
photo-treatment time and could be a good indication when to switch from the Advanced
Oxidation Process to Biological Treatment.
145
SOLAR DETOXIFICATION
Figure 8.3 belongs here
Photodegradation Process: usual relationship between the Average Oxidation State and
Time. Points A and B are also related to figure 8.4. Courtesy of EPFL (Switzerland)
Oxidised substances are typical of biological processes. Usually treatment performance is best
for low-molecular-weight organic compounds. Photodegradation treatment performance is
also better at the beginning, when organic molecules are broken up into simpler ones. As a
consequence, it makes sense to always check the feasibility of a two step process in which the
photocatalytic treatment is employed first to increase biodegradability and, afterward,
biological treatment. Figure 8.3 shows the typical evolution of ΨAOS when recalcitrant toxic
compounds are treated. Normally, a plateau is observed after a time, suggesting that the
chemical nature of intermediates formed will no longer vary significantly. As Figure 8.4
shows, there are two possible extreme situations (A and B) with regard to the evolution of
toxicity.
Figure 8.4 belongs here
Photodegradation Process: normal relationships between Toxicity and Time. Points A and B
are also related with figure 8.3. Courtesy of EPFL (Switzerland)
Sometimes, there is a direct correlation between ΨAOS and toxicity. This is the best situation,
as reflected in Curve A (Figures 8.3 and 8.4), because it means that the overall chemical
status of the degradation process does not progress, ΨAOS remains constant, and toxicity is
decreasing, thereby increasing biodegradability. Nevertheless, even at low concentrations,
toxic substances could be formed during the degradation process (Curve B, Situation B)
increasing overall toxicity. This does not necessarily impede connection to biological
treatment, but it does force the point of connection to be moved to the right (Point B).
As previously indicated, the feasibility of combining Photo-Fenton (see section 1.3.3), and a
biological process to treat certain non-biodegradable toxic compounds, has been demonstrated
at EPFL (Switzerland). A TiO2 photocatalytic process followed by aerobic biotreatment has
also been successfully employed by NREL (USA) for the treatment of waste water from
munitions manufacturing plants (pinkwater) containing low concentrations of high explosives,
such as RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine), TNT (trinitrotoluene), HMX and
PETN.
It must be pointed out that photocatalytic degradation of recalcitrant organic compounds quite
often initially increases the toxicity of the wastewater. The reason is the proliferation at the
beginning of the process of many reaction by-products as well as with the initial contaminants
themselves. Therefore, it is important that the photocatalytic treatment be long enough to
eliminate this increased toxicity. Also, with regard to the parameters of toxicity, it is very
important that no intermediates formed be more toxic than the initial compounds, because this
could disable the overall treatment process.
The feasibility of such a photocatalytic-biological process combination must always be
assessed, because it could provide an important cost reduction in the treatment of hazardous
compounds in wastewater, by reducing the size of the necessary solar collector field. It must
be taken into account that, as with most solar systems, economics of the water detoxification
systems are dominated by their capital cost. Thus, by reducing the capital cost of the system,
processing cost is decreased.
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SOLAR DETOXIFICATION
8.2 COST CALCULATIONS
Cost is always a key topic when innovative technologies are considered and standard
commercial procedures indicate that any new technology should provide important reductions
in processing costs over competing technologies or significant new technical contributions for
successful marketing. In the case of Solar Detoxification, its main contribution is the
environmental added value of using solar energy to solve contamination problems.
This section addresses the main factors influencing final installed cost, operating cost and
treatment cost of a solar detoxification system. The installed cost includes all costs associated
with the design, construction, and start-up of the facility, but it is assumed that normal
services and facilities such as water, electricity power, etc, are available on site and that land
is also available at no cost. The four main components of installed cost are:
− Facility Cost. The total field cost includes the direct cost of equipment and system
components, plus the direct cost of installation labour. The major components are:
concrete foundations, civil engineering, connection to power and water supply, solar
collectors, tanks, manifolds, piping, pumps and valves, structures and supports, electrical
equipment, control system, instrumentation, catalyst separation unit, collector, and piping
and connection assembly, etc.
− Project Contingency. Since the general conceptual design cannot include all the details of
the specific final design, at that point, total field costs are still necessarily incomplete, and
to properly account for any unidentified costs, a contingency item must be added to the
cost. This could be from 10% to 20% of the Facility Cost, depending on the degree to
which the design definition. The sum of Facility Cost and Contingencies is the Total
Facility Cost.
− System Engineering and Assembly. This includes system design and integration into
existing installations, specifications, procurement of system components, scheduling,
project management, system assembly, training of operators and engineering support.
Engineering costs are estimated to be 50% of the Total Facility Cost.
− Spare Parts. 0.5% of the total facility cost is normally included as a spare parts allowance.
The sum of the facility cost, contingencies, engineering and spare parts is the Total Installed
Cost. It is important to notice that the cost of a previous feasibility study is not considered
here, because the extension and depth of this study depends on many factors, specific to each
situation, which are difficult to estimate. Table 8.1 shows the cost for different typical facility
sizes considering a CPC solar field. These costs are based on real bids and plants which have
been built.
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SOLAR DETOXIFICATION
Collector Area (m2)
100
200
300
500
Control, pumps and instrumentation
20,221
23,228
25,190
27,900
32,048
50,793
Piping, valves, manifolds, tanks,
structures and supports
14,647
22,201
28,315
38,471
58,311
232,139
Catalyst separation and recovery
2,314
3,507
4,473
6,078
9,212
36,673
21,220
42,440
63,660
106,100
168,556
1,338,891
6,025
12,050
18,075
30,125
60,250
602,500
601
911
1,162
1,579
2,393
9,525
65,028
104,337
140,875
210,251
330,770
2,270,522
650
522
470
420
331
227
Collectors and reactor tubes
Civil engineering
Equipment transport
TOTAL INVESTMENT
2
Cost / m
1,000
10,000
Table 8.1 Estimated cost of typical sizes of a Solar Detoxification Facility. Cost
are indicated in 1999 Euros
In addition to the total installed cost, the second determining factor in the cost of treatment is
the operating cost. The Operating Cost must include all personnel and materials costs required
to operate and maintain the facility (supervision, procurement, maintenance, etc), chemical
supplies and electricity costs.
− Personnel. Personnel is linearly dependant on facility size. Although solar detoxification
facilities normally operate 7 days per week and 52 weeks per year, operating personnel
requirements for a 500-m2 solar plant are estimated to be only 0.1 man-year, since the
plant is completely automated. Also due to the simplicity of one-sun collectors, the total
annual maintenance requirement is estimated to be 0.25 man-year, mainly for periodic
cleaning.
− Cost of Maintenance Materials. Annual cost of maintenance materials are mainly due to
mechanical, electrical and instrumentation equipment which could amount to about 2% of
the Total Facility Cost yearly. It is assumed that reactor tubes and solar collectors last for
the facility lifetime.
− Electricity. The facility consumes electricity for pumps, control system, instrumentation
and lighting, the vast majority (around 90%) by the main system pump(s). The average
annual consumption of this auxiliary power consumption is difficult to estimate per m3 of
treated water as the ratio between total treated volume and plant size is directly dependent
on the nature of the contaminants. It is more easily estimated as a function of the solar
field size, as this determines total pressure drop and, consequently, pump specifications.
In view of this, yearly power consumption can be estimated as between 40 to 80 kWh/m2
of installed solar field.
− Chemical supplies. Although one of the main factors in the yearly system operating cost is
the chemicals purchased, an estimation cannot be given here because it strongly depends
on each specific treatment process. The chemicals used by Photo-Fenton processes are
completely different from TiO2 processes. They also depend on the nature and
composition of the wastewater, for which some pre or post-treatment might be needed,
with specific chemicals in each case. A case-by-case cost study must therefore be
performed. Two examples can be found on sections 8.2.1 and 8.2.2.
The sum of personnel, maintenance materials, electricity and chemical supplies, is the Total
Operating Cost of the solar detoxification system. To obtain the Annual Treatment Cost, per
m3 of treated wastewater, total installed costs are converted to an annual levelized cost
considering a fixed-charge rate (FCR), which is obtained by calculating all the fixed costs
148
SOLAR DETOXIFICATION
(excluding operation) for the life of the plant. The annual levelized cost and treatment cost
can be calculated as:
Annual Levelized Cost = Total Installed Cost x FCR + Operating Cost
Treatment Cost ( Euros / m 3 ) =
Annual Cost
Annual Treatment Capacity
(8.8)
(8.9)
The Fixed Charge Rate represents the equivalent revenue that must be generated annually to
meet all the charges on each Euro of facility investment. FCR is normally equal to the sum of
the return on debt, taxes, depreciation, insurance, etc. For solar detoxification treatment
plants, a plant life of 12 years with a depreciation period of 10 years may normally be
assumed. With this data, and considering, for example, that the plant is financed by a debt
with a 5% interest rate and that the total annual expenses due to taxes, insurance, etc, are 2%
of the total installed cost, an FCR of 17% is obtained.
8.2.1 Example A: TiO2-based Solar Detoxification Plant
A 300-m2 solar detoxification plant has been designed (see Chapter 9 on this point) for the
yearly treatment of 6,000 m3 of wastewater contaminated by pesticide, using TiO2-Persulfate
(see Section 5.2) photocatalytic system:
TiO 2 + hν
−
+
→ eCB
+ hVB
+
hVB
+ H 2O →
−
S 2 O82 − + eCB
•
→
SO4− + H 2 O →
•
(λ < 390 nm)
OH + H +
•
(8.10)
(8.11)
SO4− + SO42−
(8.12)
•
(8.13)
OH + SO42 − + H +
It is estimated that the plant would operate about 3,000 hours per year. An estimation of the
yearly treatment cost, in 1999 Euros, can be performed as follows:
149
A
Facility Cost
Data from table 8.1 (300 m2 of solar collector system
aperture area)
B
Project Contingency
15% of the Facility Cost is estimated
21,131 Euros
C
Engineering and Setup
50% of A+B (Total Facility Cost)
81,003 Euros
D
Spare Parts
0.5% of A+B (Total Facility Cost)
8,100 Euros
E
Total Installed Cost
A+B+C+D
F
Personnel cost
0.25 man-year is considered. A 20,000 Euros/man-year of
cost is considered
G
Maintenance material cost 2% of A+B (Total Facility Cost) is estimated
3,240 Euros
H
Electricity
5 kW of average electricity consumption so the 3,000
yearly operation hours would consume 15,000 kWh of
electricity. A cost of 0.1 Euro/kWh is considered
1,500 Euros
I
Chemical supplies
TiO2 catalyst used at 200 mg L-1 of concentration
and reusing up to 10 times. This means 120 kg
per year.
S2O8Na2 (MW = 238) used at concentrations from
5 to 10 mM with an overall consumption per
batch cycle of 15 mM. This means 21,420 kg per
year.
251,109 Euros
Sodium hydroxide (NaOH; 50% solution) for pH
adjustment and neutralization of the treated water. 4,800 kg
of yearly estimated consumption
J
150
Total Operating Cost
140,875 Euros
F+G+H+I
5,000 Euros
1,080 Euros
53,550 Euros
1,344 Euros
65,714 Euros
Table 8.2
SOLAR DETOXIFICATION
E
Total Installed Cost
A+B+C+D
251,109 Euros
F
Personnel cost
0.25 man-year is considered. A 20,000 Euros/man-year of
cost is considered
G
Maintenance material cost 2% of A+B (Total Facility Cost) is estimated
3,240 Euros
H
Electricity
5 kW of average electricity consumption so the 3,000
yearly operation hours would consume 15,000 kWh of
electricity. A cost of 0.1 Euro/kWh is considered
1,500 Euros
I
Chemical supplies
TiO2 catalyst used at 200 mg L-1 of concentration and
reusing up to 10 times. This means 120 kg per year.
S2O8Na2 (MW = 238) used at concentrations from 5 to 10
mM with an overall consumption per batch cycle of 15
mM. This means 21,420 kg per year.
Sodium hydroxide (NaOH; 50% solution) for pH
adjustment and neutralization of the treated water. 4,800 kg
of yearly estimated consumption
J
Total Operating Cost
F+G+H+I
5,000 Euros
1,080 Euros
53,550 Euros
1,344 Euros
65,714 Euros
Table 8.2 Estimated operating cost of a TiO2-Persulfate
a Solar Detoxification plant in 1999 Euros
To obtain the treatment cost, an FCR of 17%, with 10-year plant depreciation, is estimated.
K
Annual Levelized Cost
E x FCR + J (formula 8.8)
108,402 Euros
L
Annual Treatment Cost
K is divided by 6,000 (yearly treated volume)
18.0 Euros/m3
Table 8.3 Estimated annual treatment cost of a TiO2-Persulfate a Solar
Detoxification plant in 1999 Euros
It should be noted that a 4-percent reduction in the FCR (from 0.17 to 0.13) has a greater
effect on the Annual Treatment Cost than a 50% reduction in the Solar Collector Field. Also,
about 50% of the yearly operating cost are the cost of the persulfate itself, so a reduction in its
price very significantly affects the treatment cost. In this way, it must be taken into account
that addition of consumable reagents (such as persulfate) may not necessarily be required by
specific TiO2 photocatalytic applications.
8.2.2 Example B: Photo-Fenton based Solar Detoxification Plant
Similar to the previous case, a 200-m2 solar detoxification plant has been designed for the
yearly treatment of 6,000 m3 of wastewater contaminated by pesticides, using Fenton’s
reagent (H2O2 and Fe2+) irradiated in the UV-Vis range (Photo-Fenton method):
Fe 2 + + H 2 O 2
Fe
3+
→ Fe 3+ + OH − + • OH
+ H 2 O + hν
→ Fe
2+
+
•
+ H + OH
(8.14)
(λ < 580 nm)
(8.15)
As in the previous case, 3000 operating hours per year are estimated. The yearly treatment
cost would then be:
151
SOLAR DETOXIFICATION
A
Facility Cost
Data from table 8.1 (200 m2 of solar collector system
aperture area)
B
Project Contingency
12% of the Facility Cost is estimated
12,520 Euros
C
Engineering and Setup
50% of A+B (Total Facility Cost)
58,429 Euros
D
Spare Parts
0.5% of A+B (Total Facility Cost)
5,843 Euros
E
Total Installed Cost
A+B+C+D
F
Personnel cost
0.20 man-year is considered. A 20,000 Euros/man-year of
cost is considered
G
Maintenance material cost 2% of A+B (Total Facility Cost) is estimated
2,337 Euros
H
Electricity
4 kW of average electricity consumption so the 3000 yearly
operation hours would consume 12,000 kWh of electricity.
A cost of 0.1 Euro/kWh is considered
1,200 Euros
I
Chemical supplies
FeSO4±7 H2O (MW = 278) catalyst at 1mM concentration,
which means 1,668 kg per year.
549 Euros
Hydrogen peroxide (H2O2; 30% solution) with an overall
consumption per treatment cycle of 33.75 mM. As MW =
34, the total yearly needed amount is 22,950 kg.
17,442 Euros
J
Total Operating Cost
104,337 Euros
181,129 Euros
4,000 Euros
Sulphuric acid (SO4H2; 98%) at 1 mM concentration for
iron sedimentation by pH adjustment. As MW = 98, the
yearly consumption is 580 kg.
180 Euros
Sodium hydroxide (NaOH; 50% solution) for batch mode
neutralization of the treated water. MW = 40; 3,036 kg of
yearly estimated consumption.
849 Euros
F+G+H+I
26,557 Euros
Table 8.4 Estimated operating cost of a Photo-Fenton Solar Detoxification plant
in 1999 Euros
Also as in the previous example, an FCR of 17% with a 10-year plant depreciation period is
estimated.
K
Annual Levelized Cost
E x FCR + J (formula 8.8)
57,349 Euros
L
Annual Treatment Cost
K is divided by 6,000 (yearly treated volume)
9.5 Euros/m3
Table 8.5 Estimated annual treatment cost of a Photo-Fenton Solar Detoxification
plant in 1999 Euros
Two additional conclusions from this example may be added:
− For a depreciation period of 20 years (longer facility lifetime), the FCR would be 12% and
this would mean a very significant reduction in treatment cost.
− Operation is a significant portion of the cost of treated water. Reduction is likely to be
difficult due to the low capacity factor of the facilities and the costs of consumable
supplies, which are proportional to the volume of treated water.
Two additional examples of solar photocatalytic treatment cost found in the literature are the
following:
− Groundwater remediation at Livermore, California, by NREL (USA). Average treatment
capacity of 4.4 L/s of water, with a peak flow of 30 L/s, inlet concentration of 400 ppb
TCE and maximum outlet concentration of 5 ppb. Total treatment cost was reported as
$4.07/m3.
152
SOLAR DETOXIFICATION
− Study for a site at the Rocky Flats Plant near Boulder, Colorado (Bechtel Corp., 1991) to
treat an annual volume of 2246 m3 with a peak water flow of 6.3 L/s. The levelized cost of
treated water was estimated at $10.57/m3. This cost was heavily dominated by the
expensive system required to treat inorganic components of the water at Rocky Flats.
8.3 SOLAR OR ELECTRIC PHOTONS?
Another important aspect to be in mind in an economic assessment is the threshold at which
the collection of solar photons required to drive the photochemical reactions becomes cheaper
than photons generated by electricity. Obviously, when only a small amount of solar
irradiation is available, solar-driven processes make no sense, so the measurement of the solar
resource available at a specific location might permit a reasonable initial estimation of the
advisability of a more in-depth study on solar photocatalytic processes.
Electric ultraviolet lamps are currently available on the market for a variety of applications,
such as lighting, food processing, medical treatment, tanning, lacquer drying, photochemical
synthesis, photopolymerisation processes, attracting insect, etc. They can also be used for
photocatalytic degradation processes.
The most suitable of the different types of electric lamps available for UV-photon production
are the low-pressure mercury fluorescent lamps (the same common fluorescent light normally
used for illumination), which are based on the generation of an electric arc through a sealed
chamber containing mercury vapour. The result is 245-nm radiation which is absorbed by a
phosphorous wall coating, producing a fluorescent radiation which, depending on the coating,
can be adjusted to any spectrum. As for any arc lamp, ballast is required to provide the
appropriate current and voltage to start. Several studies have demonstrated that these lamps
are the easiest, simplest and cheapest way to produce UV photons and, in fact, are already being
used in commercially available TiO2 photocatalytic lamp systems.
Another type is the germicidal lamp, which is basically the same, but the 254-nm light passes
directly through transparent glass. As their peak light emission is centred at the mentioned
wavelength, they produce fewer valid photons and are less efficient for photocatalytic
applications. Medium and high-pressure mercury lamps are also similar to the low-pressure
lamps, but the higher pressure generates higher intensity radiation and also tends to shift the
spectrum towards the visible light. Finally, metal halide lamps, high-pressure sodium lamps
and incandescent lamps cannot be used for UV light production.
Figure 8.5 shows two typical low-pressure 40-W mercury fluorescent lamps with dominant
emission spectrum centred at 313 and 340 nm. According to manufacturer data, the initial
standard efficiency of these lamps is between 25% and 30% of UV-photon production and an
average efficiency (lifetime) of about 20%. Average lamp life is around 20,000 hours.
Figure 8.5 belongs here
Ultraviolet spectra: solar (standard and PSA measured spectrum) and two low-pressure 40 W
mercury lamps (QUV fluorescent lamps: UVB-313 and UVA-340)
With the above data, it is easy to obtain the equivalence between UV-lamp and solar systems. As
an example, we can calculate how many standard 40-W UV-fluorescent lamps are equivalent to
a 500-m2 TiO2-based solar plant with an average yearly global UV irradiation of 20 W m-2. The
parameter used to compare them is the number of useful UV photons generated or collected. In
153
154
Fig. 8.5
SOLAR DETOXIFICATION
the case of the solar plant, this can be obtained directly from the standard ASTM solar spectrum
(assuming constant spectral distribution) using the following equation (see also Section 8.4):
N Φ ( E > 3.2 eV ) = 5.8 10 21 I UVg
 photons 


2
 m h 
(8.16)
Where:
NΦ (E>3.2 eV) = Number of photons with energy over 3.2 eV (wavelength up to 387 nm) per unit of
time and surface.
I UVg = Yearly average global UV irradiation (W m-2).
The total amount of UV photons collected yearly by the solar plant would be:
N UV = N Φ ( E > 3.2 eV ) H S S = 5.8 x 10 21 x 20 x 3000 x 500 = 1.74 x 10 29 (8.17)
Where:
NUV = Total UV photons (with energy over 3.2 eV) collected yearly.
HS = Total yearly hours of operation of a solar detoxification system. This value depends on the
geographic location, but it may be estimated at 3500 (near the equator to parallel 20) to 2500
(40th to 50th parallels). In the example, 3000 is used.
S = Surface area of solar collector field (500 m2 in the example).
I UVg = Yearly average global UV irradiation during the 3000 estimated hours of operation. 20 W
m-2 in this example (see also Chapter 8.4).
So if the lamps must supply the same amount of UV energy:
N UV = 1.74 x 10 29 = Φ PH H L N L = 5.04 x 10 22 x 8760 x N L
(8.18)
Where:
φPH = Nominal average photonic flux of the lamp. Manufacturer’s information usually places this
value between 1.3 to 1.4 x 1019 photons per second (standard mercury UV lamps with emission
spectrum centred at 360 nm). In the example, this has been translated to photons per hour.
HL = Total yearly hours of operation of a lamp-based detoxification system = the full year (8760
hours).
NL = Number of lamps of the system.
So, according to equation 8.18, in the example considered, the equivalent number of lamps
would be 394.
From this particular example, the following general equation for estimating the equivalent
number of any type of electric lamps can be obtained:
 I UVg
N L = 5.8 x 10 21 
Φ
 PH
  HS

H
 L

 S

(8.19)
155
SOLAR DETOXIFICATION
Based on these equations, the cost of collecting UV photons with solar technology may be
compared to the generation of the same amount of UV photons using electric lamps. This is done
considering the following amounts of photons to drive the photochemical process:
- 1028
(1.E+28)
28
(5.E+28)
- 5 x 10
- 1029
(1.E+29)
29
- 5 x 10
(5.E+29)
- 1030
(1.E+30)
Using equations 8.16 and 8.17, the solar field necessary (with different possible yearly average
UV global irradiation) can be calculated to collect the targeted amounts of photons. Once the
solar fields have been calculated, their cost can be estimated from the data in Table 8.1 (using a
polynomial fitting). Finally, with the estimated cost of the solar field, the annual cost can be
calculated using equation 8.8, considering an FCR of 17% as in the previous examples. No
operating costs are considered. It should be noticed that the only cost included here is the cost of
the solar collector, including the reactor tubes, as it is the only hardware related to UV-photon
collection. The results are shown in Tables 8.6 to 8.10.
Hs
S Investment Cost Yearly Cost
Photon Cost
I UVg
2
(1999 Euros) (FRC = 17%) (Euros/1.E+25 photons)
2 (hours) (m )
(WUV/m )
40
3500
12
13345
2269
2.27
35
3500
14
13638
2319
2.32
30
3500
16
14029
2385
2.39
25
3000
23
15124
2571
2.57
20
3000
29
16082
2734
2.73
15
2500
46
18953
3222
3.22
10
2500
69
22779
3872
3.87
5
1500 230
49457
8408
8.41
Table 8.6 Estimated Yearly Cost of collecting 1.E+28 solar UV-photons at different yearly
average UV global irradiation. Costs are indicated in 1999 Euros.
Hs
S Investment Cost Yearly Cost
Photon Cost
I UVg
2
(hours)
(m
)
(1999
Euros)
(FRC
=
17%)
(Euros/1.E+25
photons)
(WUV/m2)
40
3500
62
21550
3663
0.73
35
3500
70
23013
3912
0.78
30
3500
82
24963
4244
0.85
25
3000 115
30419
5171
1.03
20
3000 144
35187
5982
1.20
15
2500 230
49457
8408
1.68
10
2500 345
68404
11629
2.33
5
1500 1.149
198524
33749
6.75
Table 8.7 Estimated Yearly Cost of collecting 5.E+28 solar UV photons at different yearly
average UV global irradiation. Costs are indicated in 1999 Euros.
I UVg
156
Hs
S
Investment Cost
Yearly Cost
Photon Cost
SOLAR DETOXIFICATION
(WUV/m2) (hours) (m2)
40
3500 123
35
3500 141
30
3500 164
25
3000 230
20
3000 287
15
2500 460
10
2500 690
5
1500 2.299
(1999 Euros) (FRC = 17%) (Euros/1.E+25 photons)
31782
5403
0.54
34701
5899
0.59
38589
6560
0.66
49457
8408
0.84
58942
10020
1.00
87262
14835
1.48
124709
21201
2.12
376772
64051
6.41
Table 8.8 Estimated Yearly Cost of collecting 1.E+29 solar UV photons at different yearly
average UV global irradiation. Costs are indicated in 1999 Euros.
Hs
S
Investment Cost Yearly Cost
Photon Cost
I UVg
2
(hours) (m )
(1999 Euros) (FRC = 17%) (Euros/1.E+25 photons)
(WUV/m2)
40
3500
616
112712
19161
0.38
35
3500
704
126990
21588
0.43
30
3500
821
145945
24811
0.50
25
3000 1.149
198524
33749
0.67
20
3000 1.437
243928
41468
0.83
15
2500 2.299
376772
64051
1.28
10
2500 3.448
546036
92826
1.86
5
1500 11.494
1479332
251486
5.03
Table 8.9 Estimated Yearly Cost of collecting 5.E+29 solar UV photons at different yearly
average UV global irradiation. Costs are indicated in 1999 Euros.
Hs
S
Investment Cost Yearly Cost
Photon Cost
I UVg
2
(hours) (m )
(1999 Euros) (FRC = 17%) (Euros/1.E+25 photons)
(WUV/m2)
40
3500 1.232
211554
35964
0.36
35
3500 1.407
239321
40684
0.41
30
3500 1.642
276016
46923
0.47
25
3000 2.299
376772
64051
0.64
20
3000 2.874
462527
78630
0.79
15
2500 4.598
706316
120074
1.20
10
2500 6.897
999924
169987
1.70
5
1500 22.989
2048971
348325
3.48
Table 8.10 Estimated Yearly Cost of collecting 1.E+30 solar UV photons at
different yearly average UV global irradiation.
Costs are indicated in 1999 Euros.
The comparison of electric and solar UV photon collection is only affected by the collector
and reactor system, since the rest of the system components are quite similar in design and
cost. The only exception is the electrical installation, which would logically be more
157
SOLAR DETOXIFICATION
expensive in the case of lamps, depending on the total installed power. Using equation 8.18
the equivalent number of lamps (NL) to generate the same amount of UV photons can be
obtained directly:
- 1.E+28 photons ⇒ 23 lamps (40 W low pressure mercury fluorescent tubes)
- 5.E+28 photons ⇒ 113 lamps (40 W low pressure mercury fluorescent tubes)
- 1.E+29 photons ⇒ 226 lamps (40 W low pressure mercury fluorescent tubes)
- 5.E+29 photons ⇒ 1132 lamps (40 W low pressure mercury fluorescent tubes)
- 1.E+30 photons ⇒ 2265 lamps (40 W low pressure mercury fluorescent tubes)
With these data, Table 8.11 shows the estimated cost of UV photon generation with electric
lamps. The main advantage of an electric system is their total availability (24 hours a day),
which is an average of 3 times higher than solar systems. Total yearly cost is obtained using
equation 8.8 with an FCR of 17% as calculated above for the solar system. The operating cost
includes only electricity, replacement of lamp and the cost of labour involved in replacement. To
obtain these values, a 20,000-hour lifetime has been considered for the lamps. The cost of the
lamp system may be considered linearly dependent on the number of lamps.
Number of photons
1.00E+28 5.00E+28 1.00E+29 5.00E+29 1.00E+30
Number of lamps
Unit
23
113
226
1132
2265
Lamp, ballast and accessories
6.80
154
770
1540
7701
15402
Reactor cost
2265
11325
22650
113249
226499
100.00
106.80
2419
12095
24190
120950
241900
A) INVESTMENT COST
Electricity cost (0.15 E/kWh)
52.56
1190
5952
11905
59524
119048
Lamp replacement
2.98
67
337
675
3373
6746
Labour cost to replacement
3.15
71
357
714
3571
7143
58.69
1329
6647
13294
66468
132937
B) OPERATION COST
TOTAL COST: A x FCR + B
76.85
1741
8703
17406
87030
174060
Cost per 1E+25 UV photons
1.74
1.74
1.74
1.74
1.74
Number of photons
1.00E+28 5.00E+28 1.00E+29 5.00E+29 1.00E+30
Number of lamps
Unit
23
113
226
1132
2265
Lamp, ballast and accessories
6.80
154
770
1540
7701
15402
Reactor cost
4530
22650
45300
226499
452997
200.00
206.80
4684
23420
46840
234199
468399
A) INVESTMENT COST
Electricity cost (0.15 E/kWh)
52.56
1190
5952
11905
59524
119048
Lamp replacement
2.98
67
337
675
3373
6746
Labour cost to replacement
3.15
71
357
714
3571
7143
58.69
1329
6647
13294
66468
132937
B) OPERATION COST
TOTAL COST
93.85
2126
10628
21256
106282
212564
Cost per 1E+25 UV photons
2.13
2.13
2.13
2.13
2.13
Table 8.11 Estimated Yearly Cost of UV photon generation with electric lamps considering
two different reactor costs: 100 and 200 Euros/lamp. Electricity cost = 0.15 Euros/kWh. FCR
= 17%. Costs are indicated in 1999 Euros.
158
SOLAR DETOXIFICATION
The main uncertainty in Table 8.11 is the cost of the reactor in the lamp-based photocatalytic
system. It is clear that this item would necessarily be an expensive component of the electric
system as it must include piping, external UV-reflectors to avoid loss of UV photons,
supports, wiring, and even, depending on how compactly the system assembly is, a cooling
system. As this cannot be estimated with any precision, Table 8.7 attempts to cover a
complete range of possible costs by presenting two conservative estimates, one in which the
per-lamp reactor cost is estimated low (100 Euros) and another in which it is estimated high
(200 Euros).
The main conclusions arrived at from a comparison of Table 8.11 and Tables 8.6 to 8.10, are
the following:
- The available solar UV-irradiation for a specific location is the main factor in determining
the cost of a solar-based system.
- The cost of collecting photons with solar technology decreases proportionally as the
yearly total increases because of the reduction in collector production cost for larger
numbers. This is not the estimated case of lamp-based systems as they are much more
compact.
- When fewer photons are needed (1.E+28), lamp-based systems are cheaper than solar.
- Solar-based systems with collector apertures of over 100 m2 are clearly cheaper than
lamp-based systems when the average yearly global UV irradiation is over 15 W/m2.
- The threshold UV radiation (below which lamp-based systems are cheaper than solar)
decreases as the yearly amount of photons increases.
All data included in Tables 8.6 to 8.11 are summarised in Figure 8.6, which shows the
comparative cost of solar and electric technologies for UV-photon collection and generation,
respectively. Three different amounts of photons are plotted (1.E+29, 5.E+29, 1.E+30) against
the average global solar UV-irradiation by adjusting the data in Tables 8.8, 8.9 and 8.10
(yearly cost).
Figure 8.6 belongs here
Comparative cost of UV photon collection/generation with solar technology (CPCs) and
electric lamps (electricity cost=0.15 Euros/kWh), respectively. Data from tables 8.6 to 8.11.
Costs are indicated in 1999 Euros.
Figure 8.6 enables easy calculation of the solar threshold which determines the advantage of
solar over lamp-based technology, as the yearly UV-photon requirement of the majority of
possible applications will be in the range of 1.E+29 to 1.E+30 This threshold is normally
between 10 and 12 WUV/m2, which corresponds to 0.015 Euros/kWh for electricity. If a different
cost is considered, Table 8.11 must be updated. This is the case of Figure 8.7, which corresponds
to a cost of 0.05 Euros/kWh. In this case, the threshold solar advantage over lamp-based systems
is logically higher than before.
Figure 8.7 belongs here
Comparative cost of UV photon collection/generation with solar technology (CPCs) and
electric lamps (electricity cost=0.05 Euros/kWh), respectively. Data from tables 8.6 to 8.11.
Costs are indicated in 1999 Euros.
8.4 SOLAR RESOURCES ASSESSMENT
Because the size of a solar photocatalytic system is directly related to the amount of UV light
available, accurate assessment of the solar resource is important. Solar UV-light (300 to 400
159
SOLAR DETOXIFICATION
nm) comprises roughly 2-3% of the direct beam energy and 4-6% of the combined direct
beam and diffuse irradiation (global). Non-concentrating systems have an advantage with
regard to resource assessment as there are extensive data on the total energy (direct and
diffuse) available from sunlight and the 4-6% conversion factor makes resource assessment a
straightforward process for non-concentrating systems.
The cost of a specific solar detoxification treatment plant may be easily estimated from the
indications in Sections 8.1, 8.2 and 8.3, once the degradation process has been assessed and
treatment factors have been and identified. Nevertheless, the yearly average UV irradiation at
the specific location of potential solar detoxification plants must be known. In the so-called
“solar belt”, this is usually between 15 and 30 W m-2. It would be best if the location had a
database of historical global UV irradiation available, such as the case in Figure 8.8.
Figure 8.8 belongs here
Average direct and global UV irradiance (sunrise to sunset) at PSA (Almería, Spain).
Meteorological data records from 1991 to 1995
According to Section 2.4, for reliable estimation of the annual average UV radiation, the socalled “cloud factor” (f) must be found. This is the average percentage of global UV radiation
that is expected to be lost due to clouds throughout the year. The cloud factor is also intended
to be a measurement of atmospheric transparency and is affected by all the atmospheric
components, which can absorb or scatter solar radiation. Figure 8.9 shows the results obtained
at PSA, following the procedure indicated in Section 2.4.
Figure 8.9 belongs here
Average “cloud factor” for global UV irradiance (sunrise to sunset) at PSA (Almería, Spain).
Meteorological data records from 1991 to 1995
Another interesting alternative for estimating the annual UV radiation availability is through
the Lambert-Beer Law (also called the Bourguer Law), which indicates that light attenuation
through a continuous medium is proportional to the flux radiation and the distance covered:
I b, λ = I o , λ e − cλ m
(8.20)
where:
Ib,λ = Intensity of solar irradiation on the earth’s surface at a specific wavelength, λ.
Io,λ = Intensity of extraterrestrial solar irradiation at the same wavelength, λ.
cλ = Atmosphere attenuation coefficient at λ.
m = Air mass ratio (see Chapter 2), defined as the oblique optical path described by a photon
in the atmosphere relative to the minimum vertical path to arrive at the same terrestrial
location. If the elevation of the sun is 90º then m = 1.
The air mass ratio can be obtained for any moment by the following equations:
m = m o e −0.0001184 Z
(8.21)
m o = 1229 + (614 sin α ) − 614 sin α
2
where:
mo = Air mass ratio at sea level
z = elevation over sea level in meters
160
(8.22)
SOLAR DETOXIFICATION
α = Solar altitude, defined as the angle formed by the solar vector to the horizontal surface of
the earth.
Solar altitude can be obtained as follows:
sin α = sin(L ) sin (δ S ) + cos (L ) cos (δ S ) cos (h S )
 2π (10,5 + N )
δ S = sin(23,45) cos

 365,25 
(8.23)
(8.24)
Where:
L = Local latitude
δs = Solar declination
hs = Solar time angle, which is zero at noon, positive before and negative after noon. It can be
calculated at any time by adding 15º of each hour of difference with respect to the noon
(Example: at 15’00 hours of solar time, hs = -45º).
N = Day number; it is 1 at January 1st and 365 (or 366) at December 31st.
23.45 is the angle formed by the earth’s axis and the plane formed by the sun and the elliptical
trajectory of the earth.
Equations 8.21 to 8.24 allow solar irradiation at any specific location on earth to be detailed;
Ib,λ must be measured at the specific location and Io,λ can easily be calculated. Due to the
earth’s slightly eccentric elliptical orbit, solar extraterrestrial radiation varies ±3.4% along the
year depending on the Day number:

 2πN 
I o, λ = I o ,λ (N ) = I SC ,λ 1 + 0.034 cos

 365 

(8.25)
λ2
I SC ,λ = ∫ E (λ ) dλ
(8.26)
λ1
Where:
Isc,λ = Solar Constant associated with the spectral interval (λ1,λ2) to which is intended to
obtain the atmospheric attenuation coefficient (cλ). This interval must also be coherent with
the measure of Ib,λ (i.e., if we are using a solar radiometer which measures within the interval
295 to 400 nm, Isc,λ must be calculated to the same interval).
E(λ) = Extraterrestrial irradiation, which can be found in Table 8.12.
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SOLAR DETOXIFICATION
λ
(µm)
Eλ
-2
-1
(W m µm )
λ
(µm)
Eλ
-2
-1
(W m µm )
λ
(µm)
Eλ
-2
-1
(W m µm )
0,115
0,14
0,16
0,18
0,20
0,22
0,23
0,24
0,25
0,26
0,27
0,28
0,29
0,30
0,31
0,32
0,33
0,34
0,35
0,36
0,37
0,38
0,39
0,40
0,41
0,42
0,007
0,03
0,23
1,25
10,7
57,5
66,7
63,0
70,9
130
232
222
482
514
689
830
1059
1074
1093
1068
1181
1120
1098
1429
1751
1747
0,43
0,44
0,45
0,46
0,47
0,48
0,49
0,50
0,51
0,52
0,53
0,54
0,55
0,56
0,57
0,58
0,59
0,60
0,62
0,64
0,66
0,68
0,70
0,72
0,75
0,80
1639
1810
2006
2066
2033
2074
1950
1942
1882
1833
1842
1783
1725
1695
1712
1715
1700
1666
1602
1544
1486
1427
1369
1314
1235
1109
0,90
1,00
1,2
1,4
1,6
1,8
2,0
2,2
2,4
2,6
2,8
3,0
3,2
3,4
3,6
3,8
4,0
4,3
5,0
6,0
7,0
8,0
10,0
15,0
20,0
50,0
891
748
485
337
245
159
103
79
62
48
39
31
22,6
16,6
13,5
11,1
9,5
5,9
3,8
1,8
1,0
0,59
0,24
0,0048
0,0015
0,0004
Table 8.12 Extraterrestrial solar irradiation
The Solar Constant for the whole solar spectrum (total solar radiation) is 1353 W m-2. In the
case of the UV spectrum (295 to 400 nm), using Table 8.12, it is:
400
I SC ,UV =
∫ E (λ ) dλ = 118.1 − 13.6 = 104.4 W m
−2
(8.27)
295
This value of 104.4 W m-2 represents 7.72% of total extraterrestrial irradiation. As the
percentage on the earth’s surface is lower (normally from 4 to 6 percent), this means that the
atmosphere filters UV radiation relatively more than the overall spectrum.
With this background, by measuring Ib,λ, the atmospheric attenuation coefficient (cλ) can be
calculated. This parameter include all the factors relevant to solar irradiation, such as Raleigh
dispersion, ozone absorption, atmospheric turbidity and spray, absorption by water molecules,
clouds, etc. The knowledge of cλ at a specific location makes it possible to forecast solar
irradiation during the year. Variations in cλ over the year also enable conclusions to be made
about changes in atmosphere performance (with regard to solar light) at the specific location.
8.5 COMPARISON WITH OTHER TECHNOLOGIES
A general qualitative approach to the range of application of different treatment technologies
(concentration of contaminant versus flow rate to be treated) is provided in Figure 8.10.
162
SOLAR DETOXIFICATION
Figure 8.10 belongs here
Technology fit map. Range of application of different water treatment technologies
The following technologies may be considered the main competitors of aqueous-phase solar
detoxification for the treatment of hazardous water contaminants. As only non-biodegradable
contaminants are considered here, biological treatment technologies are not discussed.
8.5.1 Thermal Oxidation
Most of the thermal oxidation technologies (also known as thermal incineration) are used to
destroy hazardous wastes at high temperatures (normally higher than 650ºC). At such
temperatures, chemical bonds are effectively broken and transformed into other more benign
substances. There are different technologies (rotary kilns, infrared furnaces, conventional
incinerators, etc), but almost all of them are variations of the same process. Incineration is
typically a two-step process in which a primary combustion chamber burns the waste material
at temperatures of about 1000ºC and the exhaust gases containing desorbed oxidized or
partially oxidized organic contaminants are transferred to the secondary combustion chamber
(afterburner) where these gases are burned to carbon dioxide, carbon monoxide, inorganic
acids and water. Afterburners generally operate at about 1200ºC and residence times in both
chambers depend on the contaminants to be destroyed. Natural gas or propane is the usual
energy source. Electricity can also be used in the primary chamber, as in infrared furnaces.
Incineration is the most frequently used treatment for highly concentrated wastes or
contaminated soil. The standard performance requirement is 99.9999% (required by law in
many countries).
Incineration is a source of debate with opposition to the construction of new incinerators from
the local populations. This is because of the increasing perception of incineration as a cause of
health and safety problems. Incineration is also an energy-intensive process subject to stream
size and concentration constraints with significant capital equipment and operating costs. As a
consequence, it is expensive compared to other treatment processes. Typical operating costs
are also high, in the range of 200 to 1000 Euros/ton depending on the nature of the
contaminants and the facility utilisation factor. Fuel is the primary factor affecting operating
cost.
The advantages of incineration are that the volume of waste is greatly reduced, often by as
much as 90%. Furthermore, the resulting ash is generally more stable and less likely to leach
into groundwater than the parent compounds. On the other hand, poor combustion during
incineration often produces by-products that are at least as toxic as the parent material. Metals
such as lead, mercury, and chromium could be released into the air. The burning of some
plastics can produce hydrogen chloride, which becomes hydrochloric acid rain if combined
with the moisture in air. Benzene, chloroform, and TCE have frequently been found in stack
emissions. Finally, generation of organic compounds and dioxins due to incomplete
destruction has always been a concern.
8.5.2 Catalytic Oxidation
Catalytic oxidation (also known as catalytic incineration) is very similar to thermal oxidation.
Contaminants are preheated, mixed, and combusted at high temperatures to form carbon
dioxide and water. The main difference is the presence of a catalyst inside the combustion
unit that reduces the activating energy necessary for combustion. Therefore, combustion
occurs at a lower temperature than for thermal oxidation and, as a result, fuel costs for
catalytic oxidation are usually lower than for an equally applicable thermal oxidation system.
163
164
2 000
WAO & Incineration Recovery
Incineration
1000
Wet Advanced Oxidation (WAO)
-1
TOC (mg L )
WAO & Biological
Redox Processes
Fenton
Biological
100
O3
H2O2
Biological
10
Biological
1
0
10
20
30
40
50
3
-1
Flow Rate (m h )
Fig. 8.10
60
70
80
SOLAR DETOXIFICATION
Catalyst materials, such as platinum, palladium, and metal oxides such as chrome-alumina,
cobalt oxide, etc, are introduced into the combustion unit in either a monolithic or beaded
configuration. The average catalyst lifetime is normally from two to five years, after which
deactivation by inhibitors, blinding by external particles and thermal ageing, render it
ineffective.
The most relevant characteristics of catalytic in comparison to thermal oxidation systems are
the following:
- Not every organic contaminant may be treated by catalytic oxidation systems, which are
not effective for streams containing lead, arsenic, phosphorus, bismuth, antimony,
mercury, iron oxide, tin, zinc or other catalyst deactivators.
- Catalytic oxidation systems are usually applied to low concentration streams, since high
concentrations may be associated with high heat content which can generate enough heat
from combustion to deactivate the catalyst.
- Associated with the above mentioned characteristics, air is usually not required for the
combustion process.
- Temperature and pressure through the catalyst bed should be monitored to preserve
catalyst activity. Catalytic oxidation systems normally operate at temperatures between
250ºC and 500ºC, since excessive heat can deactivate most catalysts.
- Catalyst poisoning from metals or halogens and / or binding from particulate matter over
time can decrease destruction efficiency. Periodic catalyst replacement or reactivation
would then be required.
Catalytic incinerators have a higher initial cost compared with thermal incinerators, but its
lower fuel costs and thereby lower operating cost offset this.
8.5.3 Air Stripping
Air-stripping is a liquid-to-gas transfer process whereby polluted water is sprayed into the air,
allowing volatile organics to escape. It is also widely used for contaminated soil containing
VOC’s. The basic principle of air-stripping technology is the process of mass transfer of
diffusion and volatilisation. The natural tendency of any organic compound to volatilize from
the aqueous to gaseous phase is given by the Henry´s Law, which states the partial pressure pA
produced by a compound of concentration cA dissolved in a liquid solvent.
pA = H cA
(8.28)
The Henry´s Law Constant H can then be expressed as a dimensionless ratio of a compound’s
concentration in air to its concentration in water. The application of Henry´s Law Constant
makes it possible to assess the maximum efficiency of an air-stripping system theoretically
possible under ideal conditions. A compound with high H values is more likely to be removed
from water and soil by air-stripping and, normally, compounds with an H over 0.001 can be
removed effectively.
H is over 0.01 for many organic compounds, which is the reason the air-stripping process is
so widely used. Conventional air-stripping treatments of hazardous organic compounds in
water are based on packed or tray columns in which water and air usually flow downward and
upward respectively, with a very high contact area between the two phases (spray chambers,
venturi scrubbers, plate or tray towers, packed towers, etc). The technology has been applied
to a wide range of organic compounds with varying degrees of effectiveness at contaminant
165
SOLAR DETOXIFICATION
concentrations between 250 and 10,000 mg L-1. The initial investment in air stripping may be
considered medium and operating costs are low.
The main environmental objection to air stripping is the undeniable fact that this technology
does not really destroy hazardous compounds, but simply transfers them to another medium,
as the organic compounds stripped from water are released directly into the atmosphere.
Tighter restrictions on emissions from air strippers are limiting their applicability and
increasing their cost, as secondary air-phase treatment systems (i.e., real treatment processes)
are required afterward.
8.5.4 Adsorption
Adsorption of organic compounds on a solid adsorbent is widely used for treating hazardous
organic compounds in water. Contaminated water is forced through tanks containing activated
carbon and contaminants are retained by weak intermolecular forces. Many organic and some
inorganic compounds are efficiently removed, including chlorinated hydrocarbons, organic
phosphorus, carbonate-based pesticides, polychlorinated biphenyls (PCB), and trace metals.
Carbon, derived from wood, coal, or other carbonaceous raw materials, is the most commonly
used adsorbent. Other adsorbents include silica gel, alumina, and zeolite. For carbon
adsorption, the organic compounds are transferred to a carbon canister that must be
transported for disposal or regeneration.
Three types of carbon adsorbents are common: activated granules, activated powders and
fibres. Granular activated carbon (GAC) is currently the most common type of carbon
adsorbent because of the significant surface area provided by the granules. Powdered
activated adsorbents are generally cheaper. They are of lower quality than granular activated
carbon and they cannot be regenerated. They are used in packed columns with high-pressure
drops. Powder coatings are used exclusively in batch operations. Carbon fibres are used in a
honeycomb structure to maximise surface area with adsorption directly on the fibre’s surface.
Carbon regeneration and recovery and disposal of contaminants are the primary factors in the
operating cost. Periodical regeneration in which the carbon is cleaned and reactivated,
restoring its capacity for adsorption and further use, is necessary. A thermal regeneration
process fueled by electricity, natural gas, or oil is the most commonly used. The adsorbed
materials are pyrolyzed, forced off, and finally oxidized at over 900ºC.
As with air stripping, the main objection of adsorption by GACs is that they only displace
contamination to a large sorbent phase, which remains a regeneration or disposal problem, the
contaminants not really having been treated. The use of active carbon adsorption is also
feasible only as long as current legislation continues to allow storage of this type of waste. In
the European Union, regulations on waste generation are becoming stricter. The common
practice of using carbon only once is expensive and polluting because the used carbon is
usually destroyed by open burning. Carbon regeneration by indirect firing is feasible only for
about four cycles, after which the carbon is destroyed by burning. Furthermore, each time it is
regenerated, only about 50% of the carbon is reused. The remainder is too fine and restricts
the flow in filtration units. Often, the regenerated carbon also loses some of its activity and is
not as efficient as new. Finally, increasing transportation and carbon regeneration costs are
reducing its cost-effectiveness. Consequently, GAC processes are expensive to operate and
generate undesirable secondary waste gases and changing environmental regulations indicate
that open burning of these materials may be prohibited within a few years.
166
SOLAR DETOXIFICATION
The initial investment is small and operating costs are medium. GAC is a mature technology
and significant cost reduction is not anticipated. Sensitivity studies have shown how costs are
likely to be affected by different contaminants, concentration and plant size. These studies
indicate that cost does not increase linearly with concentration, but much slower, because as
concentration increases, contaminant loads of carbon also increase. Therefore, GAC
processing costs are lower in large plants due to the economies of scale.
8.5.5 Membrane Technology
Membrane technology refers to the use of a semi-permeable membrane to separate
contaminants from a wastewater stream; this separation technique is often used to purify
materials or to pre-concentrate waste streams prior to treatment, so it is not a real treatment
technology. A waste stream (typically aqueous) is passed across a porous membrane, where
diffusion of specific species takes place, separating and concentrating species of interest. The
membrane is a composite of a polyamide barrier on a polysulfone support, which normally
rejects organic molecules with a high molecular weight.
The main membrane processes are reverse osmosis, ultrafiltration and microfiltration, all of
which are pressurised. Ionic exchange by resins can also be considered a membrane process.
Nevertheless, the most common process is reverse osmosis, which has been used for years to
treat drinking water. Polluted water is forced through semipermeable membranes (about 150
micrometers thick) under pressures as high as 100 bar, separating contaminants from the clean
water. Membranes (normally ceramic or polymeric) are manufactured in different
configurations such as plate-and-frame, spiral-wound, tubular, capillary tube, hollow fiber,
etc.
Many types of organic compounds can be separated by membranes, including salts and
chlorinated and organophosphoric pesticides. Reverse osmosis requires periodic cleaning and
rejuvenation of filters. Residues, which contain high concentrations of salts, heavy metals,
and toxins, must be disposed of either in landfills or by incineration. Additionally, reverse
osmosis is an especially intensive energy consumer, because of the high hydraulic pressures
needed to offset osmotic pressure, which require substantial pumping power. Depending on
membrane porosity and the nature of water contaminants, 2.5 to 5 kWh, are needed per cubic
meter of water treated. As a result, the major drawbacks of the membrane technology are its
relatively high cost for large-scale applications and the fact, again, that it is not a real
treatment process but only a separation technology.
8.5.6 Wet Oxidation
Wet oxidation, by which organic and inorganic compounds present in water (or any other
liquid) are oxidized with oxygen or air in the presence of water, has been commercially
available since the seventies (Zimpro Passavant Environmental Systems, Inc.). This process is
normally used with water containing 1% to 30% hazardous organic matter (10 to 300 g L-1)
and treatment conditions vary from 180ºC at 20 bar to 400ºC and 280 bar. During the process,
water favours the dissolution of oxygen and heat transfer to the compounds to be treated.
When treatment conditions are below the critical point of water, 374ºC and 225 bar, reactions
take place in the water and the process is denominated subcritical wet oxidation or low
pressure wet oxidation; otherwise, it is called supercritical wet oxidation. Low-pressure wet
oxidation achieves 99% efficiency in the destruction of contaminants with residence times of
around one hour; supercritical wet oxidation can achieve 100% efficiency with a residence
167
SOLAR DETOXIFICATION
time of 5 minutes, but higher temperatures and pressures are required. The main disadvantage
of wet oxidation systems is their very high installation and operating costs.
8.5.7 Ozone oxidation
Ozone (O3), used as an oxidizing agent since the beginning of the 20th century, is considered
an effective treatment process. Except for some controversial reports, ozone is not known to
produce toxic or mutagenic substances. Ozone can be generated by a high voltage discharge
in the presence of oxygen. After generation it must immediately be diluted in the water to be
treated, as it is a very unstable gas. This means that ozone must be generated at the point of
treatment.
Ozone treatment has become very common in recent years thanks to improvement in ozone
generators and the technology is quickly replacing many traditional oxidation processes based
on chlorine, hydrogen peroxide, permanganate, etc. Ozone is used to oxidise low
concentrations of organic contaminants and also inorganic compounds and water-soluble
metals converting them to their insoluble form to permit separation of e.g. Fe++ and Mn++.
Another important application of ozone is disinfection. Investment costs of ozone technology
are medium as are its operating cost of from 5 to 15 Euros/m3.
Ozone treatment is reported to be more effective for hazardous organic contaminants when
used in combination with UV oxidation, similar to the synergistic effect that can be observed
between UV radiation and an oxidiser such as hydrogen peroxide in wastewater treatment.
This treatment process can be used selectively and reportedly acts on chlorinated
hydrocarbons faster than on other organic compounds. Halogenated organic compounds are
oxidised to simpler organic forms and, in some cases, are oxidised to carbon dioxide, water
and innocuous salts. The combined UV-ozone system has also been observed to precipitate
heavy metals, such as oxides or metals, although UV oxidation is not usually used to remove
metals. The concentration of inorganic chemicals in wastewater must be low so as not to
absorb or shield the UV rays.
8.5.8 Advanced Oxidation processes
In addition to all the conventional technologies indicated above, a new type of treatment
process, commonly referred as Advanced Oxidation Processes (AOPs), is emerging. AOPs
are increasingly being considered as alternatives to more conventional technologies because
they destroy hazardous organic compounds rather than transferring them to other media, have
a potentially lower cost and greater effectiveness. AOPs are generally characterized by their
ability to generate hydroxyl radicals, some examples being UV/hydrogen peroxide, UV/ozone
and UV photocatalytic oxidation (Solar Detoxification).
Solar Detoxification has some unique advantages over other AOPs, such as:
- The use of sunlight as the photon source, which means it is a “green” technology.
- The process can be either heterogeneous (TiO2) or homogeneous (Photo-Fenton), with the
possibility of providing chemical pathways and surface interactions not available in other
treatment systems.
- A reductive chemical pathway is used to remove reducible species, such as heavy metal
ions and some organic compounds.
- It can be operated in the liquid or gas phase in contrast to processes using ozone or
hydrogen peroxide, which are generally applied only in the liquid phase.
168
SOLAR DETOXIFICATION
Solar Detoxification is a modular technology offering the advantage of system flexibility,
which is important when treating low to moderate flow rates. However, this same attribute
decreases the possibilities for the economies of scale that other pollution control technologies
benefit from. Thus, while the cost of Solar Detoxification remains relatively constant as flow
rate increases, the normalised costs for other competing technologies drop.
From the economic perspective, to date, Solar Detoxification cost estimates have been based
on limited field experience. However, this experience and projections of capital and operating
costs show that solar photocatalytic oxidation of water costs from a few Euros up to 20-30
Euros/m3. This cost is higher than for the air stripping, adsorption or membrane technologies,
but with the important advantage that while solar detoxification is a real treatment process,
the others are only contaminant separation technologies. There is also a possibility of
optimising the cost of a specific treatment process by combining two (or more) different
technologies; Figure 8.11 shows the combination of solar detoxification and GAC for removal
of pentachlorophenol (PCP).
Figure 8.11 belongs here
Optimisation of Solar Detoxification and GAC technologies to PCP degradation
The investment and operating costs of solar detoxification are lower than for technologies
such as incineration or wet oxidation while other AOP technologies have similar investment
and operating costs.
It is unclear at present how much of the hazardous-waste treatment market could be captured
by Solar Detoxification technology, but potentially it is very large. Environmental
remediation field studies have demonstrated that technology selection is clearly not based on
cost alone. Factors such as complete on-site treatment (which limits owner liability),
community acceptance and absence of undesirable by-products are commonly considered
carefully in addition to cost. It is expected that, because of it is so attractive in these areas, the
Solar Detoxification system should be able to build a significant market share in the near
future.
SUMMARY OF THE CHAPTER
As photocatalytic processes only make sense for hazardous non-biodegradable pollutants and,
when feasible, biological treatment of residual water is always the cheapest, it makes
economic sense to combine the two processes. Biologically recalcitrant compounds can be
treated with photocatalytic technologies until biodegradability is achieved, transferring the
water to a conventional biological plant later. Such a combination, based on the Average
Oxidation State parameter, reduces treatment time and optimizes the overall economics.
Operation and investment costs of Solar Detoxification plants are estimated to be lower than
alternative technologies such as incineration or wet oxidation, in the same range of ozone and
higher than air stripping, adsorption or membrane separation technologies. But the latter are
not real treatment technologies. Solar collecting of UV photons is logical where yearly
average solar UV irradiation is higher than 15 W m-2, which means almost the entire “sunbelt”.
BIBLIOGRAPHY AND REFERENCES
1. Bechtel Corporation. “Conceptual Design of a Photocatalytic Wastewater Treatment
Plant”. Sandia National Laboratory Report. SAND91-7005. 1991.
169
Costs
Photocatalysis
GAC
Photocatalysis + GAC
% Eliminated PCP
Fig. 8.11
170
SOLAR DETOXIFICATION
2. Blanco, J.; Malato, S. “Photocatalytyc Treatment of Hazardous waste Water; cost
comparison between solar and electrical technologies”. Int. Conf. on Comparative
Assessments of Solar Power Technologies. A. Roy, W. Grasse Eds. pp. 217-230.
Jerusalem, Israel, 1994.
3. Blanco, J.; Malato, S. “Tecnología de Fotocatálisis Solar”. Instituto de Estudios
Almerienses and CIEMAT Eds. ISBN 84-8108-106-X. Almeria, (1996).
4. Kenneth, M.E.; Gee, R.; Wickham, D.T.; Lafloon, L.A.; Wright, J.D. “Design and
Fabrication of Prototype Solar Receiver/Reactors for the Solar Detoxification of
Contaminated Water”. Industrial Solar Technology Corporation. NREL report. 1991.
5. Malato, S.; Blanco, J.; Richter, C.; Braun, B.; Maldonado, M.I. "Enhancement of the rate of
solar photocatalytic mineralization of organic pollutant by inorganic oxidizing species“.
Applied Catalysis B: Environ. Vol. 17, pp. 347-356, 1998.
6. Mehos, M., Williams, T.; Turchi, C.S. “Overview of Solar Detoxification Activities in the
United States”, NREL/TP-471-7262. National Renewable Energy Laboratory, Golden,
CO, 1994. DE95000264.
7. NEPCCO Environmental Systems. “Gas Phase Photocatalytic Oxidation Remediation and
Process Pollution Market Study”. Final Report for NREL. 1997.
8. O’Brien & Gere Engineers, Inc. “Innovative Engineering Technologies for Hazardous
Waste Remediation”. International Thomson Publishing Inc. 1995.
9. Pulgarin, C.; Invernizzi, M.; Parra, S.; Sarria, V.; Polania, R.; Péringer, P. “Strategy for
the Coupling of Photochemical and Biological Flow Reactors useful in Mineralization of
Biorecalcitrant Industrial Pollutants”. Catalysis Today, 1999, in press.
10. Ruppert, G.; Bauer, R. “UV-O3, UV-H2O2, UV-TiO2 and the Photo-Fenton reactioncomparison of adv. oxidation processes for wastewater treatment“. Chemosphere, Vol.28,
No 8, pp.1447-1454, 1994.
11. Schertz, P.; Kelly, D.; Lammert, L. “Analysis of Cost of Generating or Capturing
Ultraviolet Light for Photocatalytic Water Detoxification Systems”. Solar Kinetics, Inc.
Final Report for NREL. 1992.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1.
2.
3.
4.
5.
The Average Oxidation State of any organic compound is a number between 0 and +4.
When contaminated water is treated by oxidation, its Average Oxidation State increases.
If toxicity increases with oxidation, biodegradability will not be achieved.
If toxicity decreases during photocatalytic treatment, biodegradability increases.
The solar UV-irradiation available for a specific location is the main factor in determining
the cost of a solar system.
6. The threshold below which electrical systems are cheaper than solar decreases when the
total yearly amount of photons needed increases.
7. The air mass ratio is equal to 1 at solar noon at any latitude.
8. Standard required performance for thermal oxidation technologies is 99.9999%.
9. Catalytic oxidation processes usually require higher temperatures than non-catalytic
oxidation processes.
10. Ozone oxidation can normally be applied to high concentrations of organic contaminants.
PART B.
171
SOLAR DETOXIFICATION
1. Operating 2800 hours per year, a 300-m2 solar detoxification plant treats 5,000 m3 of
water containing 250 mg L-1 of hazardous contaminants yearly. If, once 50% of TOC
degradation has been attained, the wastewater is biodegradable and it is transferred to a
biological treatment plant, what would be the mass and volumetric treatment factors of the
photocatalytic facility?
2. Why may toxicity of wastewater increase at the beginning of photocatalytic treatment?
3. What are the main factors in the operating cost of a solar detoxification treatment plant?
4. What is the Fixed Charge Rate? How can it be calculated?
5. What is the standard average efficiency of UV photon production of mercury fluorescent
lamps?
6. A solar TiO2 detoxification facility can treat 3,500 m3 of contaminated water yearly with
an average global solar irradiation of 20 W m2. How much water could be treated at a
different location with an average solar irradiation of 25 W m2 ?
7. How many mercury fluorescent lamps (40W tube) would be equivalent to a 350-m2 solar
collector field working 3500 hours per year where average solar UV irradiation is 26 W
m2 ?
8. What are the main differences between catalytic and non-catalytic thermal oxidation ?
9. What is the main environmental objection to air-stripping, absorption and membrane
treatment technologies of hazardous wastewater?
10. When is an oxidation process defined as supercritical?
ANSWERS
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SOLAR DETOXIFICATION
Part A
1. False; 2. True; 3. False; 4. True; 5. True; 6. True; 7. False; 8. True; 9. False; 10. False
Part B
1. Using equations 8.1 and 8.2:
250 x 5000
∆ m (TOC )
2 = 0.74 g
T fm =
=
tS
2800 x 300
h m2
∆V
5000000
L
T fv =
=
= 5.9
t S 2800 x 300
h m2
The total mass of organic substances is divided in two as only 50% is really degraded, but
not the total volume which may be considered completely treated.
2. Due to the presence of not only the initial toxic compounds, but also the many byproducts of the degradation reaction at the beginning of the process.
3. Personnel, maintenance materials, electricity and chemicals cost.
4. FCR is the factor converts total installed cost (or total plant investment) into annual
treatment cost, considering the volume of wastewater to be treated. FCR is obtained by
calculating all the fixed costs (except operation) for the life of the plant.
5. 20%.
6. As solar irradiation is proportional to the amount of useful photons and this is linearly
dependent on the reaction rate:
25
V = 3500
= 4375 m 3
20
7. Using equation 8.19:
 I UVg
N L = 5.8 x 10 21 
Φ
 PH
  HS

H
 L

26
3500
 S = 5.8 x 10 21
350 = 434 lamps
19
1.35 x 10 x 3600 8760

8. Catalytic oxidation a) does not treat every organic contaminant; b) is typically applied to
low concentrations; c) does not usually require air for combustion; d) requires monitoring
of temperature and pressure to preserve catalyst activity; e) requires periodic replacement
or reactivation of the catalyst.
9. The air-stripping, absorption and membrane technologies only transfer the contaminants
to a different, more easily managed, medium, but they can not be considered real
hazardous-wastewater treatment technologies.
10. When it takes place at temperatures and pressures higher than the critical point of water
(225 bars and 374ºC). If a fluid other than water is used, these conditions are defined by
the specific critical point of the working fluid.
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SOLAR DETOXIFICATION
9 PROJECT ENGINEERING
AIMS
This unit describes the systematic process of feasibility study, preliminary design, final design
and construction of a solar detoxification plant, using the knowledge explained in the previous
chapters. Some important aspects of project management are also indicated.
OBJECTIVES
After completing this unit, you will have a basic knowledge of the following subjects:
1. Feasibility study for Solar Detoxification applications.
2. Initial pre-design of an engineering system.
3. Implementation and management of Solar Detoxification projects.
NOTATIONS AND UNITS
Symbol
Description
CIEMAT Centro de Investigaciones Energéticas, Medioambientales y
Tecnológicas (Spain)
COD
Chemical Oxygen Demand
CPC
Compound Parabolic Collector
DG-XII
Directorate General XII (of European Commission)
EC
European Commission
EUV,n
Accumulated UV energy incident on the reactor, since the
start of the degradation process up to tn, per unit of volume
GAC
Granulated Active Carbon
HS
Total yearly hours of operation of a solar detoxification system
LLNL
Lawrence Livermore National Laboratory (USA)
mM
Mili Molar
PLC
Programmable Logic Controller
ppm
Parts per million (equivalent to mg L-1)
PSA
Plataforma Solar de Almería
Re
Reynolds number
S
Solar collector field area
T
Experimentation time
Tfm
Mass Treatment Factor
Tfv
Volumetric Treatment Factor
TOC
Total Organic Carbon
tR
Residence Time
tT
Total elapsed time (since the beginning of photocatalytic
process)
Global ultraviolet solar irradiation
UVG
Average incident radiation on the collector surface for each ∆t
UV G,n
interval (∆tn = tn – tn-1)
VR
Illuminated reactor volume
VT
Total volume of hydraulic circuit
Total solar detoxification plant volume
VTOT
Units
mg L-1
kJ L-1
h
10-3 moles L-1
Dimensionless
m2
min
g h-1 m-2
L h-1 m-2
mg L-1
min / h
min
WUV m-2
WUV m-2
L
L
L
9.1 FEASIBILITY STUDY
Solar Detoxification projects, just as any other engineering project, must follow the logical
sequence of pre-design, preliminary design and final design with the final objective of
174
SOLAR DETOXIFICATION
designing and building facilities of feasible construction and satisfactory operation, for which
contractors can confidently make bids. Without competent design, implementation and
operation, no engineering project can succeed.
The pre-design phase must define the scientific and engineering data required before the final
design may proceed. In Solar Detoxification projects the pre-design phase might be more
properly called a Feasibility Study, as any new potential application of solar detoxification
must be tested before going ahead with the design and implementation of the treatment plant.
When complex mixtures of hazardous contaminants are present, degradation pathways and
reaction kinetics could be very different even from wastewater of a similar original. This
means that the design of a specific solar detoxification system is a complex procedure
requiring a prior feasibility study following the method defined below:
-
Identify targeted recalcitrant hazardous compounds.
Identify any pre-treatment that could enhance photocatalytic efficiency.
Identify the best photocatalytic process (TiO2 / TiO2 + persulfate / TiO2 + other oxidant /
Photo-Fenton).
Determine optimum process parameters (catalyst concentration, additional oxygen, etc).
Assess toxicity and biodegradability to determine the best moment to stop the
photocatalytic process.
Identify possible post-treatment processes.
Determine treatment factors.
All this can be done in the laboratory using indoor reactors with simulated solar radiation, as
described in Section 3.1. Nevertheless, it is always much better to carry out the feasibility
study using experimental solar devices, as the results obtained will necessarily be more
reliable when extrapolated to the engineering level. Section 3.2 describes several experimental
solar systems.
Figure 9.1 belongs here
Experimental mobile pilot plant for solar water detoxification. CIEMAT (Spain), 1996.
9.1.1 Identification of target recalcitrant hazardous compounds
The first step must be a complete analysis of the wastewater to be treated in order to identify
the target chemicals and their initial and desired final concentrations. This is very important to
avoid inadequate data in the following phases of the project. Also, depending on the specific
application, the nature of the contaminants and/or their concentration may change before the
system is actually erected, as the time between the study and its implementation could take
months or even years. As some differences normally arise when laboratory experiments are
compared to engineering scale pilot plant tests, at a later stage of the project, it is important to
know whether the differences originated in reactor and system configurations or from
variations in the wastewater.
9.1.2 Identification of possible pre-treatments
The objective of pre-treatment, if necessary, is to condition contaminated waters for organic
destruction. This can be achieved by addressing any constituents of the contaminated water
that interfere with the process. The presence of initial impurities in the wastewater to be
treated could produce a significant reduction in process efficiency because of:
- Unexpected chemical reactions due to the presence of specific compounds.
- Incomplete destruction of one or several components.
175
SOLAR DETOXIFICATION
-
Presence of substances that inactivate the catalyst.
Presence of substances that reduce the oxidation kinetics.
Presence of substances that block UV solar radiation.
These possible impurities, mainly inorganic ions, could even lead to the no viability of the
degradation process and the only way to avoid it is by designing some form of pre-treatment,
to adjust the chemistry of the water to conditions suitable for the detoxification process. It
should be noticed that such treatment increases both capital and system operating costs and
may also affect the control strategy of the treatment plant.
Examples of pre-treatment are the following:
- Filtering, in the case of turbidity or cloudy water that could block incoming solar UVirradiation.
- Acidification, to eliminate substances such as bicarbonate.
- Adjustment of pH to reduce the concentration of total dissolved solids by precipitation.
- Addition of hydrogen peroxide to precipitate substances such as dissolved iron ions,
which react forming a ferric oxide precipitate, which may then be removed by filtering.
- Cation bed exchange, in the case of impurities such as calcium ions that would adversely
affect the process.
In the end, the specific impurities for each case must be identified and, depending on their
effect on the overall photocatalytic process, design or not a particular pre-treatment.
Other pre-treatments are independent of the presence of impurities. This is the case of oxygen,
hydrogen peroxide, persulfate or any other oxidant injected into the process stream before the
start of the photocatalytic process to act as whole receptors on the titanium dioxide catalyst. In
many cases reduction of initial pH to between 5 and 6 is also recommended to enhance the
initial degradation rate.
9.1.3 Identification of most adequate photocatalytic process
As already indicated with regard to solar photocatalysis, when complex mixtures of organic
contaminants are present, no general indication can be given at all and each case is completely
different from every other. It is therefore recommended that different photocatalytic processes
be tested in the beginning to identify the one that is the most adequate. Until both
heterogeneous TiO2 photocatalysis and homogeneous Photo-Fenton processes have been
extensively analysed, no advantage can be previously attributed to any of them for any
particular application.
A good example is shown in Figure 9.2, where the Photo-Fenton photocatalytic process is
seen to be highly efficient in degrading about 6000 mg L-1 of initial TOC, compared to the
relative inefficiency of TiO2 heterogeneous photocatalysis. The reason for this is the waste
water, taken from a cataphoretic painting process at an automobile assembly plant, which
came from an ultrafiltration process, leaving no inorganic impurities at all present, and the
homogeneous process works quite well. By contrast, heterogeneous degradation does not
work due to the high presence of organic matter.
Figure 9.2 belongs here
Treatment wastewater from painting section at car assembly factory
(ultrafiltration from cataphoresis process). CIEMAT (Spain), 1999
176
SOLAR DETOXIFICATION
In other cases, the Photo-Fenton Fe cycle may be affected by the presence of impurities that
produce just the opposite result. In the case of Photo-Fenton, pH must also be around 2.5 to
avoid the formation of non-soluble iron hydroxides. It is thus clear that this first research step
is absolutely necessary in order to decide on the most appropriate process for further in depth
study.
9.1.4 Determination of the optimum process parameters
This phase seeks the definition of the essential scientific and engineering data required before
proceeding with system design. This can be done by defining, implementing and conducting
an experimental plant to determine the following parameters:
- catalyst load,
- pH required,
- oxygen
- additional oxidant load, if any,
- contaminant concentration, if this can be varied
and assessment of the photocatalytic degradation process over time:
- Total Organic Carbon of the waste water,
- individual concentration of key hazardous compounds identified during the first step
(Section 9.1.1),
- toxicity,
- Average Oxidation State
Figure 9.3 Catalyst mixing system for the LLNL water treatment system. Courtesy of
National Renewable Energy Laboratory (USA)
It might also be very advisable to individually test the photocatalytic degradation of the key
recalcitrant contaminants identified in order to check whether there are significant differences
from those obtained with the real wastewater problem. Reaction rates are normally higher
when individual compounds are tested, but if there are significant differences, it might be
worthwhile repeating the pre-treatment process again to try and identify possible additional
pre-treatments, which could speed up the detoxification process. Obviously, potential gains in
overall reaction rate must always be weighed against the cost of adding additional processes
and reagents.
The disappearance of hazardous contaminants normally must be parallel to the toxicity
reduction and the biodegradability increasing. These facts and the evolution of the Average
Oxidation State would determine the optimum point to stop the oxidative process and transfer
the wastewater to a conventional biological treatment plant (see section 8.1).
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SOLAR DETOXIFICATION
9.1.5 Post-treatment process identification
Post-treatment processes adjust the water chemistry to conditions suitable for discharge. In
the case of industrial wastewater for later biological treatment, the post-treatment may only be
reduced to:
- pH adjustment and
- catalyst separation/precipitation and recovery
Depending on the nature of the waste water treated and its later use, and whether solar
detoxification is used for water purification, such as in ground water decontamination, the
post-treatment system could then require additional processes, such as:
- carbon dioxide removal,
- elimination of possible residual hydrogen peroxide (if used in the photocatalytic process),
- cation-exchange bed to remove specific existing inorganic compounds,
- GAC filters to remove trace quantities of organic materials.
And any other possible treatment process necessary to meet the specific local regulatory
requirements before discharging water.
Another possible factor that could require post-treatment is the water temperature.
Temperature has a negligible influence on the photocatalytic process and the use of nonconcentrating solar technology considerably reduces the increase in water temperature during
the treatment process. However, the usual recirculation or batch system design may cause the
water to become too hot for the process in summer, volatilising compounds with low boiling
points, or for the system, because, e.g., a cation exchanger at the inlet of a post-treatment
system limits the inlet temperature. If so, a water-toair heat exchanger must be installed at the inlet of the post-treatment system, or inserted in the
water circuit in the solar collector field.
9.1.6 Determination of treatment factors
After all the above mentioned factors have been analysed and the steps indicated have been
followed systematically, a feasibility study to calculate the treatment factors for the best
system configuration proposed may be carried out. These treatment factors [Mass Treatment
Factor (Tfm) and Volumetric Treatment Factor (Tfv)], were already defined in Section 8.1
(equations 8.1 and 8.2) and must be associated with the average solar UV irradiation recorded
during the tests performed. These values will permit the necessary solar field to be sized.
All the information assembled during the feasibility study must be documented in a report,
which may also include any additional relevant data collected and recommendations for future
activities.
9.2 FEASIBILITY STUDY EXAMPLE
One example of feasibility study could be the treatment of pesticide contaminated water from
the recycling of agrochemical plastic containers.
9.2.1 Background
In the Mediterranean, intensive agriculture in greenhouses has become a very important part
of the economy, especially during recent years. In southeastern Spain, the province of
Almería alone has more than 40,000 hectares of such greenhouses, and this is now the most
important economic activity in the area. However, important environmental problems
178
SOLAR DETOXIFICATION
associated with this activity have also arisen, such as the extensive and intensive use of
pesticides (requirements are about 200 times greater than for conventional agricultural
methods).
One particular problem is caused by the enormous number of empty pesticide bottles disposed
of each year. According to 1995 data, 5,200 tons of pesticides were consumed in the region,
producing around 1.5 million empty bottles and containers, 99% of that are plastic, and having
an average volume of 1.9 litres. A small amount of pesticide residue always remains in the used
bottles and, therefore, they are hazardous waste, which cannot be handled in the same way as
conventional garbage. To date, there has been no way to dispose of these bottles and recover
them for reuse, and most of them are thrown away with the rest of the regular agriculture waste
or just dumped anywhere. In order to solve this environmental problem, a process was designed
to recycle the high-quality plastic in these bottles into a valuable raw material. The recycling
process shreds the plastic, which is then washed, leaving a relatively small amount of water
contaminated by a few hundred mg L-1 total organic carbon content of persistent toxic
compounds. As the water must be reused, those contaminants have to be treated and eliminated.
Solar Photocatalytic Detoxification was proposed to treat the wastewater and a feasibility
study was carried out.
Figure 9.4 belongs here
Tons of pesticides used in Almería region (1995 data). Source: AEPLA (Spain)
The experimental research was performed at the PSA Solar Detoxification Facility. Definition of
the work necessary started with research (Fig. 9.4) on the pesticide market in Almería that
provided the qualitative and quantitative distribution of pesticides consumed in the region. From
these, 10 pesticides, with all the main chemical families present, were selected as representative
from among those most used by the greenhouses, to carry out a complete, previously defined,
solar degradation test program. Table 9.1 shows the list of pesticides used.
Commercial name Producer
Active
Ingredient
Rhône-Poulenc Acrinatrin
Merck
Abamectin
AgrEvo
Endosulfan α-β
AgrEvo
Formetanate
Bayer
Imidacloprid
Ciba-Geigy
Lufenuron
Bayer
Methamidofos
DuPont
Oxamyl
AgrEvo
Pyrimethanil
AgrEvo
Propamocarb
Formula
Rufast
Vertimec
Thiodan
Dicorzol
Confidor
Match
Tamaron 50
Vydate
Scala
Previcur
C26H21F6NO5
C48H72O14
C9H6Cl6O3S
C11H16ClN3O2
C9H10ClN5O2
C17H8Cl2F8N2O3
C2H8NO2PS
C7H13N3O3S
C12H13N3
C9H20N2O2
Table 9.1 Selected pesticides for the feasibility study assessment. CIEMAT (Spain), 1996
9.2.2 Experimentation. TiO2-Persulphate tests
The Test Program included individual and combinations of tests of selected compounds, under
different conditions. In order to simplify the study, and the number of tests to be carried out, the
following hypothesis were assumed:
179
180
Nematocides (2314)
Molluskicides (23)
Insecticides (827)
Acaricides (52)
Fungicides (1,115)
Fig. 9.4
Herbicides (134)
Phytoregulators (698)
Others (55)
SOLAR DETOXIFICATION
- although pesticides, which are sprayed in concentrations between 200 ppm and 3000 ppm
depending on the product, are diluted in water before their application, pesticides in water
from the washing process are always more diluted than in normal use.
- the 10 pesticides selected are considered representative of the entire market (more than 300).
- the same number of empty plastic containers are generated by each of the 10 selected
products.
- the same amount of residue remains in all the containers.
Under these hypotheses, a mixture of the ten selected pesticides, each at the same concentration,
with a total TOC of 100 mg L-1, was considered representative of the water to be found after
washing, very similar what may be expected and was used in solar degradation experiments in
both Helioman (Fig. 7.5) and CPC (Fig. 7.6) systems, were performed.
Figures 9.5(a) and (b) belongs here
(a) Solar mineralization of TOC from the insecticide abamectin; test performed in a parabolic
trough system; average direct UV light: 38.1 watts m-2. (b) Samples of photocatalytic
degradation in CPC system; average global UV solar radiation were: 26.2 watts m-2
(acrinatrin test); 33.6 watts m-2 (methamidophos) and 33.7 watts m-2 (lufenuron test). TiO2
(Degussa P25):200 mg L-1. CIEMAT (Spain), 1996
Figure 9.5 (a) shows mineralization of total organic carbon from the insecticide abamectin in
the parabolic trough system. The TOC is observed to practically disappear after one hour of
exposition to sunlight (good weather conditions). Photocatalytic degradation tests performed
in a CPC system show a very similar pattern. Figure 9.5 (b) shows degradation of TOC from
the insecticides, acrinathrin, methamidophos and lufenuron (three different experiments on
sunny days). Each one (listed in Table 9.1) was individually tested and similar degradation
was observed: degradation of about 100 ppm of TOC in a residence time between 1 and 2
hours, with good solar irradiation. Residence times (tR) were calculated as:
tR =
VR
tT
VT
(9.1)
where tT is the total time elapsed since the beginning of the experiment, VR the (illuminated)
reactor volume and VT the total volume of the hydraulic loop.
However, the experiments of most practical interest are those in which the mixture of all 10 of
the selected compounds was used. These were performed in the CPC system as well as in
parabolic troughs assuming the four hypotheses considered. For these experiments 10 ppm of
each pesticide were added to 250 L of distilled water, and homogenised for 30 minutes.
Samples were taken periodically and the total organic carbon (TOC) in the suspension was
analysed and pesticide concentration monitored by liquid chromatography. Figure 9.6 shows
some of the results obtained, which included toxicity measurements throughout the
experiment. As the complete mineralization of all organics present in the water was not
intended, it should be observed that no highly toxic compound is generated. Microtox
(widely accepted toxicity measurement system), is usually expressed in terms of the EC50,
which is the effective concentration causing a 50% reduction in light from a luminescent
marine bacterium (photobacterium phosporeum), indicating that 50% of the bacteria present
have been killed.
181
Fig. 9.5 (a)
182
Fig. 9.5 (b)
183
SOLAR DETOXIFICATION
Figure 9.6 belongs here
TOC mineralization of a mixture of 10 selected pesticides (parabolic troughs); direct UV
light: 36.3 watts/m2; TiO2 (Degussa P25) concentration: 200 mg L-1; persulfate addition: 0.01
molar; treated volume: 250 l. EC50 toxicity measured by Microtox. CIEMAT (Spain), 1996
In the case displayed in the figure 9.6, a TOC reduction of 90% is followed by an important
toxicity reduction (to obtain the EC50, a 40% of concentration is just needed from initial
sample, while for the final one the needed concentration is 135%).
Figure 9.7 belongs here
TOC mineralization of a mixture of 10 selected pesticides (CPCs). TiO2 (Degussa P25)
concentration: 200 mg L-1, slurry. CIEMAT (Spain), 1998
All titanium dioxide tests were conducted in 200-mg L-1 slurry concentrations, with 5 to 10
mM persulfate, which has been found to be the optimum concentration in PSA experiments.
Na2S2O8 was added at the beginning of the tests and at regular intervals to assure continuous
presence throughout tests by measuring the S2O82- consumed. This addition of persulfate does
not imply any environmental problem since it only produces small concentrations of sulphates
which increase the salinity of the water treated (maximum sulphates permissible in drinking
water is 250 mg/L; there is no limit for waste water). In the experiments indicated in Figure 9.7,
80% of the TOC was removed in from 2 to 4 hours and 90% in 2,5 to 5 hours.
9.2.3 Photo-Fenton tests
Photo-Fenton experiments were conducted to determine the effect of the initial TOC and iron
concentration on the degradation rate. Initial concentrations were 100 to 500 ppm of TOC and
Fe of 0,25 mM to 2 mM. After homogenisation during 30 minutes, the pH was adjusted to 2.5
or 2.8 by addition of concentrated sulphuric acid. Ferrous sulphate was added immediately
afterwards and H2O2 was added in portions of 10 to 20 percent of the stoichiometry (from
previous COD measurements) until the experiment was completed. More iron increases the
degradation until a maximum rate is reached. This occurred when iron compounds absorbed
all the irradiated photons. Figure 9.8 shows the degradation curves for different experiments
with approximately 100 ppm of pesticides.
Figure 9.8 belongs here
Pesticide degradation by Photo-Fenton process. Comparison of different iron concentrations
for 100 ppm of pesticides in wastewater. CIEMAT (Spain), 1998
An increase in the concentration of iron did not improve the degradation rate as much as had
been expected from previous laboratory experiments. The poor performance of the 0.5-mM
iron test may have been caused by the high initial TOC and a slight deviation in pH
adjustment. Nevertheless, 80% of the initial TOC was removed in less than 3 hours. With an
excess of H2O2, degradation can be improved up to 90%.
One of the difficulties in both the Photo-Fenton and TiO2-Persulfate experiments giving rise
to possible error, was measurement of the initial amount of pesticide, since the highly viscous
liquids strongly adhered to the glass-measuring cylinder. Furthermore, the amount of distilled
water to be added could not be calculated accurately. Both of these factors caused fluctuation
in the concentration of total TOC. Since not all the pesticide could be dissolved at once in the
distilled water, TOC increased up to 60 minutes reaction time.
184
Fig. 9.6
185
SOLAR DETOXIFICATION
9.2.4 Conclusions and Treatment Factors
Total volume treated was the same for all the experiments (250 L), and the CPC collector
field (2 x 60° semi-aperture angle, 1 sun concentration and total irradiated volume of 110 L)
was formed by 3 stationary modules with an overall aperture area of 9 m2. Collectors were
orientated to the south and tilted 37º, which is similar to the local latitude, to obtain the
maximum yearly efficiency. With these data and the results in Figures 9.7 and 9.8, the Mass
Treatment Factor, Tfm (eq. 8.1), and Volumetric Treatment Factor, Tfv (eq. 8.2) for both the
TiO2-persulfate and Photo-Fenton processes can be obtained. Table 9.2 showed the obtained
results.
TiO2-Persulfate
(90% degradation)
Test 1
Test 2
Test 3
Test 4
Test 5
Test 6
Test 7
Average
Tfm
0.81
0.59
0.86
0.89
0.60
1.18
0.93
0.84
Tfv
6.44
4.69
7.41
7.65
6.06
13.89
11.26
8.20
Photo-Fenton
(80% degradation)
Tfm
2.28
3.88
3.11
2.43
3.07
--2.95
Tfv
18.52
31.45
26.88
22.83
28.74
--25.68
Table 9.2 Degradation of pesticide mixture. Mass Treatment Factor (Tfm) and Volumetric
Treatment Factor (Tfv) obtained for TiO2-persulfate and Photo-Fenton processes. CIEMAT
(Spain), 1998
Treatment factors are observed to be significantly higher with Photo-Fenton, and the last 10%
of degradation cannot be achieved with this process. This fact is an important issue in plant
design philosophy at the preliminary design stage.
9.3 PRELIMINARY DESIGN
The goal of the preliminary design is to develop specific parameters for the sitting, layout,
and size of the solar detoxification facility, based on the prior feasibility study. The
preliminary design is complete when provides all necessary basic elements for the final
design and construction and usually this is often considered equivalent to 30 to 40 percent of
the complete final design. No additional data should have to be developed after the
completion of the preliminary design because this should include sufficient engineering
details to proceed rapidly to the final design. This step typically formulates a schematic
process diagram and a site layout plan.
The final figures for volume of water to be treated, hazardous contaminant and its
concentration, necessary collector surface, pre and post-treatment processes, etc, must be
assessed at this point. A typical list of the data to required for the preliminary design might be
the following:
−
−
−
−
−
186
Amounts and characteristics of waste water.
Photocatalytic process.
Collector field surface and residence time.
Throughput capacity.
Flow rates (maximum, minimum and mean).
SOLAR DETOXIFICATION
−
−
−
−
−
−
−
−
−
−
−
−
−
−
Loading rates.
Cleanup targets.
Discharge rates.
Chemicals and dosages.
Horsepower.
Utility requirements.
Site security measures.
Spill-containment and leak-detection provisions.
Construction materials.
Instrumentation and control.
Monitoring and alarms.
Operating requirements.
Process diagram.
Plant layout.
These data are basically no different from any other hazardous-waste remediation engineering
process. The main difference here is the need to calculate the size of the collector field.
Determination of treatment factors gives a good idea of how the photocatalytic process works
with a specific waste water and there are valid figures for a feasibility study estimation, but
this cannot be used as the reference parameter for calculating the collector field area because
the solar radiation, which obviously is an essential parameter, is not included in the
calculation of the treatment factors, as Table 9.2 (obtained from Figures 9.7 and 9.8 in the
previous example) shows.
Treatment factors, Tfm and Tfv, are based on the use of residence time, tR , or the time the water
has been exposed to radiation, as the unit of calculation for analysis, and this could lead to
erroneous conclusions when there are important differences in the radiation incident in the
reactor due to clouds or time of day. One way to avoid this problem is to use a relationship
between experimental time, plant volume, collector surface and the radiant power density,
UVG, as measured by an UV solar radiometer. The amount of energy collected by the reactor
(per unit of volume) from the start of the experiment until each sample is collected may then
be found by equation 9.2:
EUV,n = EUV,n-1 + t n UV G,n
S CPC
;
VTOT
t n = t n - t n −1
(9.2)
Where:
UV G,n is the average UV radiation incident on the collector surface for each ∆t interval;
tn is the experimental time for each sample taken to monitor the degradation process;
SCPC is the solar collector surface (CPC or any other type of collector);
VTOT is the total plant volume;
EUV,n is the accumulated energy (per unit of volume, kJ L-1) incident on the reactor for each
sample taken during the degradation process.
Equation 9.2 permits any specific photocatalytic degradation experiment to refer to the useful
energy available to the process instead of the residence time, making direct comparison
between different experiments performed on different days with different weather conditions
possible, as well as calculation of the size of the solar field specific to the solar conditions at
the site. It must be remembered that only solar irradiation up to 390 nm is useful for the TiO2
process, but for Photo-Fenton, it is useful up to 580 nm.
187
SOLAR DETOXIFICATION
Equation 9.3, for calculation of the total solar collector field area, can be obtained from
equation 9.2. HS is the yearly total of hours of solar detoxification system operation. 20 to 25
percent larger collector area (than the theoretical figure) is always recommended.
S=
EUV VTOT
H S UVG
(9.3)
The feasibility study and preliminary design are basic components of the final design, as they
must supply all the data essential for the engineers and scientists to proceed confidently with
the final design. In addition, these steps must demonstrate that the final solar detoxification
plant will accomplish the treatment objectives before significant expenditures are made in
detailed design or implementation.
9.4 PRELIMINARY DESIGN EXAMPLE
As with the example of treatment of water contaminated by pesticides from the recycling of
agrochemical plastic containers (Section 9.2) the main item in the preliminary design is the
calculation of the collector field surface necessary, for which the following data have either
been previously collected or assumed:
− The yearly amount of empty pesticide bottles generated throughout the province of Almería
is 1.5 million.
− The plant is to be designed to treat an initial 50% of this yearly total of bottles (realistic
taking into account their geographical distribution): 750,000 plastic bottles.
− The CPC collector technology has been selected for the plant.
And the following hypotheses have previously been formulated:
− Average residual TOC in an empty pesticide bottle (from prior analysis) is about 0.7 g; if the
bottle has previously been rinsed, the quantity is 0.1 g.
− 0.5 g average residue content per bottle is used as a conservative estimate. This would mean
a total pesticide weight of 375 kg (750000 x 0.5 mg) to be treated yearly by the solar
detoxification facility.
− Average volume of plastic bottles is 1.9 L.
− Yearly total of operating hours in Almería for a solar detoxification facility is about 3000.
With equation 9.2 (ACPC = 9 m2; VTOT = 250 L), Figure 9.7 can be transformed into Figure
9.9, where TOC degradation paths are plotted against the useful UV energy collected, instead
of the residence time (see Section 9.3).
Figure 9.9 belongs here
TOC degradation of a mixture of 10 selected pesticides by TiO2 (Degussa P25)- persulfate
process. Degradation in function of UV collected energy (300-400 nm). CIEMAT (Spain),
1998
With the same procedure and the data from Figure 9.8, Figure 9.10 is obtained. In order to
make direct comparison of the Photo-Fenton and TiO2 processes possible, the amount of UV
light collected in Figure 9.10 was calculated the same way as for TiO2 (using data from the
same radiation sensor having a measuring range of 300-400 nm), even though iron
compounds absorb light up to wavelengths of 580 nm. From PSA measurements (Licor-1800
Spectroradiometer), an average of 7.11 times more solar energy up to 580 nm is available
than from the solar spectrum up to 390 nm.
188
SOLAR DETOXIFICATION
Figure 9.10 belongs here
TOC degradation of a mixture of 10 selected pesticides by Photo-Fenton process and with
different iron concentrations. Degradation in function of UV collected energy (300-400 nm).
CIEMAT (Spain), 1998
From the different various tests carried out, the best conditions found for the titanium dioxide
process are a TiO2 catalyst concentration of 200 mg L-1 with 10 mM of persulfate addition. In the
case of the Photo-Fenton process, iron concentration is 1 mM. When Figures 9.9 and 9.10 are
compared, it seems clear that the Photo-Fenton process, although unable to achieve full
mineralization of the contaminants in the water, in this case, is more energy-efficient. The TiO2persulfate process requires 27 kJUV L-1 for 80 percent of TOC degradation or 30 kJ L-1 for the 90
percent mineralization. The Photo-Fenton process needs only the equivalent of 11.5 kJUV L-1 for
80 percent disappearance of TOC.
The most appropriate conceptual design is, therefore, the one shown in Figure 9.11, where the
water contaminated with the pesticide is treated in a batch process until 80 percent mineralization
of TOC is achieved. At this point, water is transferred to the post-treatment process (iron
precipitation, sedimentation and recuperation), and either reused for bottle washing or discharged
through an activated carbon filter to guaranty discharge quality. The water to be reused is
pumped back to wash the shredded plastic until contamination reaches a TOC of 100 ppm. In
this closed cycle, water may be reused about 5 to 10 times before final discharge. With this
design, about 95 percent of the contaminants are mineralized by solar photocatalysis and the
remaining 5 percent would be removed with a GAC filter.
Figure 9.11 belongs here
Conceptual design of solar detoxification plant for pesticide bottles treatment and recycling.
CIEMAT (Spain)
The size of the solar field can be calculated with the following design parameters:
− Photo-Fenton is the photochemical degradation process selected.
− The initial TOC of water entering the solar detoxification facility will be 100 mg/L, which
includes not only the active ingredient, but also the rest of the components in the
commercial formulation.
− A TOC of 100 ppm is considered equivalent to about 200 mg L-1 of contaminant
concentration (from the ratio of average carbon weight against the average molecular
weight of the selected pesticides).
− The final TOC signifying water removal from the solar plant is 20 mg/L.
− The plant is designed to treat 375 kg of pesticides from the plastic-bottle washing process
yearly.
− The total volume of water to be treated yearly is 1,875 m3 (375,000 mg / 200 mg L-1).
− 3,000 hours of operation yearly.
− The average local global UV irradiation is 18.6 W m-2.
− The average solar energy necessary to degrade the contaminants is 12 kJUV L-1 (from
Figure 9.10).
So, using equation 9.3, the collector field area will be:
189
Chemical
oxidant
Catalyst
Oxygen
(Air)
Mixer
Sun
Pre-treatment
(pH adjustment,
filtering, etc)
Pump
Tank
Discharge
(irrigation
water)
Industrial
bottles
washing
process
TOC >
100 ppm
Pump
Contaminated
water
YES
NO
GAC filter
Reutilization ?
WASHING CYCLE
Filter
Chemical reactor
(CPC solar
collector field)
TOC <
20 ppm
Post-treatment
(catalyst recovering,
pH adjustment, etc.)
Solar UV
light
Batch process
DETOXIFICATION CYCLE
Fig. 9.11
190
Chemical
oxidant
TiO2
Oxygen
(Air)
Mixer
Sun
Pump
Pretreatments
(filtration, pH
adjustment, etc)
Tank
TOC >
100 ppm
Industrial
bottles
washing
process
OR
Pump
Contaminated
water
UV light
Filter
Chemical reactor
(solar collectors field)
TOC <
10 ppm
Catalyst separation
WASHING CYCLE
Recirculation
SOLAR DETOXIFICATION CYCLE
191
SOLAR DETOXIFICATION
 J L− 1  L
EUV VTOT 11.5 x 10 3 x 1875 x10 3 
 = 112 m 2
S=
=
−
3000 x 3600 x 18.6 s W m 2 
H S UVG


(9.4)
And the proposed size of the solar collector field (CPC) would be 140 m2 (including a 25
percent margin). With this and the data above, the cost of the treatment facility may be
estimated following the procedure indicated in chapter 8. Figure 9.12 shows a proposed
layout for the complete facility.
Figure 9.12 belongs here
Layout design of solar detoxification plant for pesticide bottles treatment and recycling.
CIEMAT (Spain)
Two important items in the layout in Figure 9.12 with regard to the pre and post-treatment
processes are sludge removal before treatment of the contaminated water in the solar field and
one-step iron sedimentation by neutralisation (in batch-mode), after the photocatalytic
treatment.
9.5 FINAL DESIGN AND PROJECT IMPLEMENTATION.
The fundamentals of the final engineering design process are no different from any other
treatment facility project. The objective is to develop the documentation necessary for
selected contractors and vendors to proceed with construction of the facility. This phase of the
project is usually associated with the construction of the treatment plant through a contract
between the owner or responsible party and the engineering firm responsible for the project.
Final design and construction are normally on a turnkey basis.
After the feasibility study and the preliminary design phase, the final design process might be
affected by different factors, such as:
− Uncertainty of key assumptions.
− Availability of design data.
− Availability of competent contractors.
− Project cost or size.
− Possible restrictions imposed by local regulations.
− Owner’s acceptance of risk, due to the implementation of a new technology based on
energy from the sun, input which obviously cannot be controlled.
The last factor, project risk, must not be dismissed, as it could be one of the key factors in
implementing the solar detoxification technology. The possibility of clouds blocking sunlight
for a period of several days (or even weeks) could force a similar period of plant inactivity,
which could well be incompatible with 24-hour-a-day operation. To avoid this risk, an
adequate buffer system must be designed and, even so, there is always a certain risk for the
owner.
One way to mitigate this problem, should it arise, might be to design a hybrid system
combining a solar collector and electric UV lamp systems. Such a concept would benefit from
continuous operation, even during prolonged periods of bad weather, because the lamp system
can be turned on when sunlight is insufficient. It can also increase the treatment rate to handle
occasional peak demand. However, it requires the additional installation of the UV-lamp
system and the energy to operate the lamps when sunlight is unavailable. Depending on the
192
Lighterage quay
(empty bottles)
Inspection
area
Shredding
area
Washing
area
Hot air dried &
bag packed area
Total plant required area:
2000 to 3000 square meters
Store
Area
Office
Solar CPC field (about
300 m2 of collectors)
Lab
Chemical
treatment
unit
15 m3
Microfiltration
unit
15 m3
Loading berth
(final product)
S
Fig. 6.12
193
SOLAR DETOXIFICATION
local solar resources (see Section 8.3 for a comparison between solar and electric photon
costs), solar-electric hybrid systems may be more expensive than solar-only systems, but they
could reduce the above-mentioned risk of solar detoxification installations at specific
locations.
Another possible problem could be the uncertainty of important assumptions used during the
feasibility study or the preliminary design phase. This could be the case of the example of
pesticide treatment shown in Sections 9.2 and 9.4, where the real wastewater to be treated
may contain more than 300 possible different compounds. As it is impossible to carry out a
feasibility study with all these contaminants (it was performed with 10), a real uncertainty
exists with regard to the final design of the plant.
A possible solution for this problem is to approach the final design and construction of the
facility in a two-step process, designing and installing a pilot plant first, with a reduced
number of solar collectors to check and validate the figures obtained in the feasibility and
preliminary phases. After some tests have been performed in this pilot plant the necessary
area of solar collectors is confirmed or modified and the plant is enlarged to its final
dimensions. Pilot-plant construction must be based on the confidence that all installed
systems are valid, with or without slight modifications to the plans.
When the final design is complete, a detailed project cost estimation must be prepared. This is
usually based on normally accepted sources such as bids from other recently built
detoxification plants, quotes from vendors and equipment suppliers, construction industry cost
estimation guides, etc.
All these possible alternatives, when appropriate, in detail, must be reflected in the contract
between the engineering firm and the owner or responsible party. Typically, the main contract
documents are the following:
− Bidding documents
− General and supplementary terms and conditions
− Technical specifications
− Contract drawings
The bidding documents describe the different items of the plant and are the basis for payment.
These documents should include additional information such as the proposed schedule and
milestones, possible alternatives to the price, etc., and an agreement among the parties on the
total price of the plant.
The general and supplementary terms and conditions must contain the standard and specific
clauses of the construction contract. Issues such as responsibility sharing among the parties,
payment procedures, settlement of disputes, insurance, guaranty, etc must also be included. It
is important to specify such issues as training of personnel, documentation to be provided,
technology confidentiality, initial plant performance check period and any other services
required by the contractor or engineering firm.
The technical specifications must contain the performance requirements and the criteria for
facility acceptance. Specific details of the equipment and materials to be used in the treatment
plant depending on the characteristics of the wastewater to be treated should also be included.
194
SOLAR DETOXIFICATION
The contract drawings should identify the conditions of the plant location and the possible
requirements and/or restrictions for construction. As appropriate, they may include plans,
sections and general details relevant to plant location.
The complete design and construction schedule for a solar detoxification facility could be
vary considerably, depending mainly on the requirements of the feasibility study and any
possible delays in obtaining the necessary permits. A sample schedule might be the following:
− Feasibility study: 2 to 5 months.
− Preliminary design: 1 to 2 months.
− Licensing: variable, depending on local regulations.
− Final design and contracts: 2 to 3 months.
− Vendor selection. Equipment and solar collector procurement: 4 to 6 months.
− Facility erection and installation: about 2 months.
− Start up, checkout and training: 1 to 2 months.
Finally, the documentation necessary for proper operation and maintenance of the facility
must be supplied.
9.6 EXAMPLE OF FINAL DESIGN AND PROJECT IMPLEMENTATION
This section describes the design and construction of the solar detoxification plant within the
project entitled “Solar detoxification technology in the treatment of persistent non-biodegradable
chlorinated water contaminants”. The project, partially financed by the DG-XII of EC, was
aimed at the treatment of wastewater containing C1 and C2 chlorinated hydrocarbons such as
methylene chloride, trichloroethylene, tetrachloroethylene, chloroform, methyl chloroform, etc.
For this project, a specific TiO2 catalyst was developed in the ENEL laboratories (Italy) by
synthesising titania powders with an innovative process in which a suitable reactant vapour
induced by a CO2 laser beam is pyrolysed. In the preliminary laboratory experiments, an
impressive 10 percent photon efficiency was obtained with this catalyst, when selected
chlorinated solvents at concentrations close to solubility were treated (Calza, Minero and
Pelizzetti, 1997). Real contaminated water generally contains chlorinated solvents at similar
concentrations. Figure 9.13 shows the manufacturing process for this catalyst.
195
SOLAR DETOXIFICATION
Figure 9.13 Laser device for TiO2 catalyst powders manufacturing. Courtesy of ENEL SpA
(Italy), 1999
This 10-percent photon efficiency represents a significant improvement in catalyst efficiency
over Degussa P-25 titania, attributable to both the higher specific surface area of the laser
powders and to better crystallisation due to the thermal treatment.
The design solar-collector was a 100-m2 CPC, having a flexible modular structure adjustable
to different angles of inclination (Figure 9.14) for easy on-site assembly and installation. The
collectors were erected based on the following design data:
− Acceptance angle:
90º
− Truncation angle:
90º
− Internal absorber radius:
14.6 mm
− External absorber radius:
16.0 mm
− Optical gap:
1.4 mm
− Sunlight concentration ratio:
1.0
Figure 9.14 belongs here
Modular collector structure to CPC easy assembly. Courtesy of AO SOL ENERGIAS
RENOVÁVEIS Lda. (Portugal), 1999
The CPC collectors were fabricated from a galvanised sheet frame containing 16 parallel
highly reflective anodised-aluminium CPC reflectors with 1.5-m-long reactor tubes. Each
tube had a connector at the ends to join it to the previous and following adjacent collector
tube forming a complete module of collectors connected in a row. Figure 9.15 shows the
CPC-trough manufacturing process. The reflector on which the glass reactor tube is
assembled is later mounted in a box frame. The overall unit is installed on the supporting
structure on-site (see also Figure 9.19).
196
197
Fig. 9.14
SOLAR DETOXIFICATION
Figures 9.15 (a), (b) and (c) Manufacturing of CPC shape reflector.
Courtesy of AO SOL ENERGIAS RENOVÁVEIS Lda. (Portugal), 1999
The detoxification plant was installed at the facilities of a waste management and treatment
company in Madrid (Spain). Design batch-system treatment efficiency was 2 m3 of water
contaminated with non-biodegradable chlorinated solvents, in approximately two to three
hours (depending on the solar irradiation). One of the first problems was the location within
the factory for aesthetic reasons, because the solar facility had to be oriented to the south and
because of the orientation of existing buildings. This is a common problem in any solar
installation in existing buildings since they are normally built without considering their
orientation. One possible solution to this problem is the installation of the solar facility on the
roof, however, in this case, it was not possible and the plant was installed on the ground as
shown in the layout in Figure 9.16. Obviously, an important factor that must always be
avoided is shadowing of the solar collector field by other buildings or constructions during the
year.
Figure 9.16 belongs here
Solar Detoxification plant layout. Courtesy of ECOSYSTEM S.A. (Spain), 1999
The main plant components designed were the civil engineering, i.e., foundations and pits,
pumps, piping and fittings, hydraulic system, tanks, automation and control, electrical and
mechanical installation and TiO2 recovery system. The main plant parameters are:
− 2 Modules (21 collectors each) in parallel rows,
−
total collector aperture area: 100 m2,
− total circuit volume: 800 L,
− total plant volume: 2000 L,
− catalyst configuration: slurry,
− completely airtight with air injection (oxygen supply).
The plant consisted of two parallel rows of 21 collectors and 31 m length each. East-West
orientation was chosen with a small structural tilt (1%) in the same orientation as a way to
dry-out and to avoid the accumulation of rain water on the CPC troughs (Figure 9.17).
Figure 9.17 belongs here
Solar Detoxification plant design. Front and lateral view. Courtesy of ECOSYSTEM S.A.
(Spain), 1999
Civil engineering must consider the possibility of an accident (e.g., broken glass reactor) and
spillage of hazardous water during treatment. For this possibility, a small sidewall and a sump
for containment and collection of possible leaks, were designed and constructed (Figure 9.18).
198
Fig. 9.16
199
200
Fig. 9.17
SOLAR DETOXIFICATION
Figures 9.18 (a) and (b) Solar Detoxification plant construction. Lateral wall and pit for
possible leaks containment and collection. Courtesy of HIDROCEN S.L. (Spain), 1999
Once the frame support has been prepared, the next step is the installation of the solar
collector (Figure 19). Collector inclination is equal to local latitude (40 degrees North) and
the distance between rows was calculated to minimize shadowing of collectors. To this end,
the angle of sunlight at noon on December 21st (lowest maximum sun elevation) was used as a
design parameter to define row separation.
Figures 9.19 (a) and (b) Supporting structure and CPC units installation. Courtesy of AO SOL
ENERGIAS RENOVÁVEIS Lda. (Portugal), 1999
The final system design was modular with glass collector reactor tubes connected in series by
HDPE quick-connections. Water flows simultaneously through all the parallel tubes and there
is no limit to the number of collector components in the modules. Pipes are made of PVC-C
and tanks are made of polyester-resin-reinforced glass. The hydraulic circuit was carefully
designed to obtain the highest volumetric efficiency with minimum “dark zones”. The
different sections of the pipes must be carefully calculated to guarantee similar flow rates in
all the reactor tubes. Nominal flow was turbulent (Re between 10000 and 20000) to avoid
catalyst settlement. Water input and output manifolds at the ends of the modules connect the
photoreactor array to the main feed pipe (Figure 9.20). All materials in contact with the water
to be treated must be carefully selected according to the nature of the contaminants and the
required pH.
201
SOLAR DETOXIFICATION
Figure 9.20 Photoreactor array input water manifold system. Source: CIEMAT (Spain), 1999
The Solar Detoxification facility was designed to be operated in batch mode. The water to be
treated is initially stored in the 2-m3 storage tank, from which it flows into the buffer tank and
solar collector loop, completely filling them by force of gravity, and then recirculates
continuously through the reactors until desired contaminant destruction is achieved. The TiO2
catalyst and chemical additives are prepared separately in small tanks and are fed into the
treatment loop in the equivalent of two recirculation cycles to guarantee complete
homogenisation. The total treatment-loop volume is about 800 L, 600 L being continuously
exposed to the solar radiation in the reactors. Once the desired destruction is obtained, the
water is transferred to the catalyst separation tank, the treatment circuit is filled again with
new wastewater to be treated (Figures 9.21) and the process is restarted.
Figures 9.21 (a), (b) and (c) (a) Installation of main tanks of Solar Detoxification facility:
buffer tank (smaller in right place), storage tank (left) and catalyst separation tank (conic tank
on the back). (b) Installation of tank level sensors. (c) Installation of catalyst separation
system. Source: CIEMAT (Spain), 1999
The plant was designed with full automatic control systems and minimum operation and
maintenance requirements. Achieved level of water treatment is indirectly measured by
measuring sunlight availability. In this way, a solar UV-A sensor is incorporated within the
202
SOLAR DETOXIFICATION
electronic control devices, with the function of solar UV integration from the beginning of the
treatment process (Figure 9.22). This sensor is connected to a Programmable Logic Controller
(PLC) and, once the level of energy to fulfil the treatment has been achieved (previously
determined from preliminary test for plant design, according to each specific contaminated
wastewater to be treated), the PLC stops the main pump, transfer the water to the catalyst
separation tank and advise the operator that the treatment has been completed.
Figures 9.22 (a) and (b) (a) Installation and testing of the UV-A sensor device (front left). (b)
Testing the PLC and electronic equipment. Courtesy of ECOSYSTEM S.A. (Spain), 1999
The PLC also receives other data signals (flow-rate, tank levels, temperatures, etc) for system
pump and valve control. Specifically developed software controls all normal operating
procedures and sequences, so very little direct human intervention is needed. Orders are
introduced through a keyboard and a printer indicates alarms and main system events.
Figures 9.23 (a) and (b) Two views of the completed Solar Detoxification treatment plant.
Source: CIEMAT (Spain), 1999
CHAPTER SUMMARY
Implementation of any Solar Detoxification project must follow a three-step sequence:
Feasibility Study, Preliminary Design and Final Engineering Design. The objective of the
Feasibility Study is the assessment of the practicability of the solar photocatalytic technology
for the treatment of the wastewater problem by preliminary testing. In addition, the feasibility
study must identify the recalcitrant hazardous compounds, possible pre- and post-treatment
203
SOLAR DETOXIFICATION
processes, the most appropriate photocatalytic process, the optimum process parameters and
the treatment factors. The objective of the preliminary design is to develop specific
parameters for the positioning, layout, and size of the solar detoxification facility from the
feasibility study previously performed. This phase should include sufficient engineering
details to proceed rapidly to the final design, including a schematic process diagram and a site
layout. The final figures for the volume of water to be treated, presence of hazardous
contaminants and their concentration, necessary collector surface, pre and post-treatment
processes, etc, must also be defined at this preliminary design level. The objective of the final
engineering design is to develop the necessary documentation to proceed with the
construction of the facility through selected contractors and vendors. This phase is usually
carried out for construction of the treatment plant by contract between the owner or
responsible party and the engineering firm responsible of the project.
BIBLIOGRAPHY AND REFERENCES
1. Bechtel Corporation. “Conceptual Design of a Photocatalytic Wastewater Treatment
Plant”. Sandia National Laboratory Report. SAND91-7005. 1991.
2. Blanco, J. et. al. "Solar detoxification plant for a hazardous plastic bottle recycling plant in
El Ejido: feasability study". 8th Symp. on Solar Thermal Conc. SolarPACES. 1996.
3. Blanco, J. et. al. “Compound Parabolic Concentrator Technology Development to
Commercial Solar Detoxification Applications”. International Solar Energy Society
Symposium. Jerusalem, Israel. 1999.
4. Calza P., Minero C. and Pelizzetti E. “Photocatalytic Transformations of Chlorinated
Methanes in the Presence of Electron and Hole Scavengers”. J. Chem. Soc. Faraday
Trans., 93, 3765-3771. 1997.
5. Goswami, D.; “Engineering of Solar Photocatalytic Detoxification and Disinfection
Processes”. Advances in Solar Energy. Vol. 10, pp. 165-209. (1995).
6. O’Brien & Gere Engineers, Inc. “Innovative Engineering Technologies for Hazardous
Waste Remediation”. International Thomson Publishing Inc. 1995.
7. Radian Corporation. “Conceptual Design Report for the Mobile Solar Detoxification Unit
–Draft Report”. Solar Energy Research Institute. Denver, CO, 1991.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. The reason for identifying possible pre-treatment processes is to make possible or enhance
photocatalytic wastewater degradation.
2. It may be possible to avoid pre and post-treatment processes, depending on the specific
conditions of the photocatalytic wastewater degradation process.
3. When TiO2-persulfate degradation is employed, specific post-treatment is required to
remove the excess sulphates in the discharge water.
4. Once the preliminary design is completed, no additional data should initially be required
to proceed with the detailed engineering development (final design).
5. The technical and economic feasibility of the solar detoxification process must be
demonstrated by the feasibility study and the preliminary design, respectively.
6. As Photo-Fenton uses more photons than the TiO2 process (photons up to 580 nm
compared to 390 nm of the solar spectrum, respectively), it is always the best option.
7. Final project design is usually associated with the construction of the treatment plant, but
not necessarily.
8. A technology confidentiality agreement among the parties should be included, if
appropriate, in the general conditions of the contract.
204
SOLAR DETOXIFICATION
9. Any specific documentation to be provided with the treatment facility, in addition to the
operation and maintenance manuals, should be defined in the Technical Specifications of
the contract.
10. When wastewater is treated by batch processing, the flow rate must be as low as possible
to save pump energy.
PART B.
1. Is the implementation of a specific pre-treatment process always recommended to increase
the efficiency of the photocatalytic treatment?
2. In which cases could water temperature play a significant role in the solar detoxification
process?
3. Why shouldn’t treatment factors be used to calculate the solar collector area for a specific
case of wastewater treatment?
4. An experimental solar detoxification system has 375 L of total volume and 250 L of
reactor volume (irradiated volume). What factor is used to transform the experiment time
to residence time?
5. Why is an increase in TOC concentration observed in many photocatalytic degradation
experiments?
6. One experimental system, made up of 25 m2 of solar collectors, can degrade 75% of the
TOC in 150 L of a specific wastewater in 150 minutes, with an average solar UV
irradiation of 33 W/m2. Another experimental system with 15 m2 of solar collectors can
also degrade 75% of the TOC in 100 L of the same wastewater in 120 minutes with an
average solar UV irradiation of 38 W/m2. Which of the two systems is more efficient?
7. The following results were obtained for a specific waste water in several photocatalytic
tests with TiO2-persulfate:
Test
1
2
3
4
5
Treatment factor (L h-1 m-2)
11
13
10
9
12
Average solar UV irradiation (W m-2)
28.5
33.4
31.8
26.9
33.2
Using these data, estimate the size of a solar detoxification plant necessary to treat 10,000
m3 of the same wastewater yearly operating 3000 h per year at a location with 24.5
WUV/m2 average solar irradiation.
8. Why is it always advisable to check the feasibility and preliminary design data before
beginning the final project design?
9. How can the risk of long cloudy periods (which could reduce the possibilities of
installation of a solar detoxification plant) be mitigated?
10. What are the main issues to be considered when designing the hydraulic loop for a solar
detoxification facility?
ANSWERS
Part A
1. True; 2. False; 3. False; 4. True; 5. True; 6. False; 7. True; 8. True; 9. False; 10. False.
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Part B
1. Not always, because potential gains in overall reaction rate must always be weighed
against additional associated costs.
2. Because when hazardous compounds with low boiling points, such as volatile organic
compounds (VOCs), are to be treated, possible transfer of the contaminants from liquid to
gas phase must be taken into account.
3. Because the solar radiation is not considered. Treatment factors are a good tool for
comparing degradation performance in experimental systems, but as local available solar
irradiation is an essential design factor, they cannot be used to calculate the required solar
field.
4. Using equation 9.1, the factor is 0.746:
tR =
280
tT = 0.746 tT
375
5. The initial presence of non-dissolved compounds in the wastewater.
6. The problem can be solved by using equation 9.3, modified as follows:
EUV =
S T UVG
VTOT
(9.4)
25 (150 x60 ) 33 x10 -3
EUV 1 =
= 49.5
150
kJ L−1
15 (120 x60 ) 38 x10 -3
= 41.04
100
kJ L−1
EUV 2 =
The second system may be considered more efficient than the first one, as it needs less
specific energy to achieve the same degradation.
7. From the five tests performed, the average treatment factor is 11 L h-1 m-2, and the average
solar irradiation, 30.76 W/m2. As there is a linear relationship with solar irradiation, if the
yearly average is 24.5, the equivalent treatment factor is:
11 x
24.5
= 8.76 L h -1 m −2
30.76
And the estimated solar field would be:
10000 10 3 L
= 380 m 2
−2
-1
8.76 L h m 3000 h
The final estimate, with a 25% increase, would be 475 m2.
8. Because final design and project implementation due not normally follow immediately
after the feasibility study and preliminary design phases, but some time later.
Furthermore, sometimes-important specific assumptions and design data must be
validated in a pilot plant previous to the final plant design.
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SOLAR DETOXIFICATION
9. A first approach is the design of an adequate buffer system. A second and more radical
approach is the design of hybrid systems, combining solar collectors and electric UV
lamps.
10. To guarantee the same flow rate in all the reactor tubes, to minimise the “dark volume”
(volume not exposed to solar radiation) and to avoid catalyst settlement when a
heterogeneous photocatalytic process is implemented.
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10 INTERNATIONAL COLLABORATION
AIMS
This unit describes some of the programmes and initiatives promoting national or
international collaboration in research, development and implementation of innovative
sustainable technologies, that include Solar Detoxification projects within their scope.
OBJECTIVES
At the end of this unit, you will appreciate the scale, nature and content of these programmes,
and some of the national and regional initiatives in progress around the world. You will
acquire basic information concerning the mechanisms and possibilities for proposing and
establishing international collaboration in the field of solar detoxification and you will also
know how to receive updated information on those possibilities.
NOTATION AND UNITS
Symbol
Description
CIEMAT
Centro de Investigaciones Energéticas, Medioambientales y
Tecnológicas (Spain)
DG-XII
Directorate General XII (of European Commission): Science,
Research and Development
DG-XVII
Directorate General XVII (of European Commission): Energy
DOE
Department of Energy (USA)
DOD
Department of Defense (USA)
EC
European Commission
EPA
Environmental Protection Agency (USA)
EU
European Union
ExCo
Executive Committee (SolarPACES)
FP5
Fifth Framework Programme (EU)
IEA
International Energy Agency
NREL
National Renewable Energy Laboratory (USA)
OEDC
Organization for Economic Co-operation and Development
PCO
Photocatalytic Oxidation
PSA
Plataforma Solar de Almería
PSI
Paul Scherrer Institute (Switzerland)
SANDIA
Sandia National Laboratories (USA)
SolarPACES
Solar Power And Chemical Energy Systems
RTD
Research and technological developments
SERI
Solar Energy Research Institute (USA)
SMEs
Small and medium-sized enterprises
SNL
Sandia National Laboratories (USA)
SSPS
Small Solar Power Systems
WHO
World Health Organisation
Units
10.1 INTERNATIONAL ENERGY AGENCY: THE SolarPACES PROGRAM
The IEA (International Energy Agency), founded in 1974, is the energy forum for
industrialised countries. Based in Paris, the IEA is an autonomous agency within the
framework of the Organisation for Economic Co-operation and Development (OEDC). An
important function of the IEA is the promotion of enhanced international collaboration on
energy research and the development and application of new and efficient energy
technologies. The IEA has set up more than 60 “Implementing Agreements” linking member
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Countries in R&D, technology demonstration and information initiatives. One of these
Implementing Agreements is called SolarPACES (Solar Power and Chemical Energy
Systems).
SolarPACES is one of the international co-operative programs managed under the umbrella of
the IEA to help find solutions to worldwide energy and environmental problems, bringing
together teams of national experts from around the world to focus on the development and
marketing of systems based on solar technologies.
The SolarPACES program was initiated in 1977 under its former name of SSPS (Small Solar
Power Systems). Two dissimilar solar facilities were designed in the project’s Stage 1 by ten
Contracting Parties from Austria, Belgium, Germany, Greece, Italy, Spain, United Kingdom
and the United States. All the countries, with the exception of the UK, continued the project
through Stage 2 (Building, testing and Evaluation) which was completed in Almería, in
southern Spain, at the end of 1984. In the course of the subsequent Stage 3, eight countries
(all but Greece) proceeded with solar-related research and development in various forms,
especially in advanced solar thermal and solar chemical applications. The two SSPS facilities
were transformed into what has since become the world’s most versatile solar test centre, the
Plataforma Solar de Almería (PSA), which continues to serve as the site of multiple cooperative international testing and development efforts.
In 1991, Germany, Spain, Switzerland and the USA decided to go on to a Stage 4 and sought
increased participation from both member and non-member countries. As of 1999, there are
fourteen members of SolarPACES: Australia, Brazil, Egypt, the European Commission (DG
XII and DG XVII), France, Germany, Israel, Mexico, Russia, South Africa, Spain,
Switzerland, the United Kingdom and the United States. In 1998 alone, contacts were
maintained with representatives of Azerbaijan, Chile, Ghana, India, Italy, Japan, Jordan,
South Africa, Turkey, Uzbekistan and Zimbabwe (1998 SolarPACES Annual Report).
Membership is open to all countries, subject to Executive Committee approval, and involves a
government (or its nominated contracting party) becoming a signatory to the program’s
“Implementing Agreement”, which defined the SolarPACES charter and conditions of
membership. The current Implementing Agreement, valid from 1996 until
December 31, 2001, is an amendment of the original one signed on September 23, 1977. The
Implementing Agreement may be extended by agreement of two or more participants, then
being applied only to those participants.
All SolarPACES activities are overseen by an Executive Committee (ExCo) composed of
individuals nominated from each member country. The ExCo meets twice yearly to formulate
strategic objectives, direct the program of work, review results and accomplishments, and
report to the IEA. An elected Chairperson presides over the ExCo meetings, and throughout
the year, an Executive Secretary deals with day-to-day program management.
The ongoing work and activities are co-ordinated through specific “Tasks” or areas of work,
defined within the Implementing Agreement. SolarPACES currently has three such on-going
tasks:
− Task I; Concentrating Solar Energy Power Systems.
− Task II; Solar Chemistry Research, where solar detoxification is included.
− Task III; Solar Technology and Applications.
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SOLAR DETOXIFICATION
An Operating Agent, nominated by the ExCo, is responsible for overseeing the work of each
Task and each member country nominates a National Co-ordinator within each of the three
Tasks. Each task maintains a detailed program of work that defines all task activities,
including their objectives, participants, plans and budgets. In addition to technical reports of
the activities and their participants, accomplishments and progress are summarised in the
SolarPACES annual report. Many SolarPACES activities involve close co-operation among
members countries (either through sharing of task activities or, occasionally, cost-sharing),
although some co-operation is limited to sharing of information and results with other
participants.
The activities formally identified within Task II (Solar Chemistry Research) are related with
the development of technologies and systems in the field of solar-driven thermochemical,
photochemical and electrochemical processes for the production of energy carriers, chemical
commodities and for the detoxification and recycling of waste materials. As indicated in the
current Implementing Agreement, Task II activities are divided into three sectors, Sector II.3
being completely devoted to solar detoxification activities and research.
(a)
Sector II.1: Solar production of Energy Carriers.
The objectives of this Sector are to:
− Explore new ideas and concepts for he thermochemical, photochemical and
electrochemical production of chemical fuels and chemical heat pipes for storage
and transportation of solar energy;
− Develop and test the required solar process technology;
− Assess their technical and economic feasibility and implementation;
− Set priorities of research and development needs;
(b)
Sector II.2: Solar Production of Chemical Commodities.
The objectives of this Sector are to:
− Identify chemical processes for the solar production of fine and bulk chemical
commodities;
− Develop and test the required solar process technologies;
− Assess their technical and economic feasibility and implementation;
(c)
Sector II.3: Solar Detoxification and Recycling.
The objectives of this Sector are to:
− Test and evaluate solar detoxification processes;
− Further develop and demonstrate solar detoxification systems up to commercial
level.
The core of the work of SolarPACES is development of new and advanced concentrating
solar technologies and solving the wide range of technical problems associated with their
commercialisation. This means that, from advanced solar concentrating technologies in
general to solar detoxification applications in particular, industrial participation plays a
critical role. Many of the Task’s international activities and teams involve industrial cooperation. In fact, in some countries (e.g., the UK and Australia), the SolarPACES contracting
party is an industrial company.
SolarPACES attempts to give added value to national work already funded by its member
governments. It is, therefore, not in itself a “big-budget” operation and normally does not
provide funding for work to be carried out in member countries. The small annual fee paid by
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SOLAR DETOXIFICATION
member countries is used to support a limited range of co-operative activities approved by the
ExCo, such as publication and distribution of documents, scholarships and activities
promoting international awareness.
The Task II Operating Agent is the Paul Scherrer Institute (PSI) of Switzerland, which coordinates the activities in close co-operation with the National Co-ordinators. Operating
Agent and National Co-ordinators normally meet once a year to review the progress of Task
activities, discuss technical issues and prepare future Task development.
Full additional information on activities, conferences, reports, newsletters and contact
addressed can be found at the following web site:
http://www.demon.co.uk/tfc/SolarPACES.html
10.2 THE EUROPEAN UNION
In recent years, the European Commission (EC) has become one of the most active
institutions in the financing and promoting of both solar detoxification research and
international collaboration. A considerably high number of projects related to photocatalytic
research has been approved and financed during the 90s (3rd and 4th Framework Programmes).
A large part of the information contained in this book was obtained from projects, networks
and activities promoted and partially financed by the European Commission, through these
Framework Programmes.
The Fifth Framework Programme, adopted in December 1998, defines the Community
activities in the field of research, technological development and demonstration for 19982002. Its differs notably from its predecessors in that it focuses on a limited number of
objectives and areas combining technological, industrial, economic, social and cultural
aspects. Environmental Protection is one of these priority areas, water treatment being one of
its specific objectives, thereby providing a good scenario for co-operative research,
development and demonstration initiatives related with solar detoxification of water.
The Fifth Framework Programme consists of seven Specific Programmes, of which four are
Thematic Programmes and three are Horizontal Programmes. The Thematic Programmes are:
− Quality of life and management of living resources
− User-friendly information society
− Competitive and sustainable growth
− Energy, environment and sustainable development
The Horizontal Programmes, complementing these Thematic Programmes, are:
− Confirming the international role of Community research
− Promotion of innovation and encouragement of participation of small and medium-sized
enterprises (SMEs)
− Improving the human research potential and socio-economic knowledge base
Exhaustive documentation related to all these Programmes is provided by the EC at the
following web sites:
− http://europa.eu.int/comm/dg12/index.html (European Commission DGXII)
− http://www.cordis.lu/fp5/home.html (Fifth Framework Work programmes)
− http://www.cordis.lu/home.html (CORDIS, big European database)
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The specific topic of water treatment is in the “Energy, environment and sustainable
development” programme. The strategic goal of this programme is to promote environmental
science and technology to improve quality of life and boost growth, competitiveness and
employment, while meeting the need for sustainable management of resources and protection
of the environment. Within this programme, research and technology development (RTD)
will concentrate on six key actions (two for the “energy” area, four to the “environment and
sustainable development” area). The first of the four key actions in the “environment” section
is “Sustainable management and quality of water”, specifically addressed to water treatment
and purification technologies, with the objectives, among others, of:
− developing improved waste-water treatment techniques and technologies,
− developing technologies for rational water reuse,
− developing technologies for water purification,
− enhancing waste-water treatments,
− minimising environmental impacts from waste water treatment.
Priority attention will be given to research initiatives addressed to waste water treatment and
re-use, water pollution abatement from contaminated land, landfills and sediments and ground
and surface waters diffuse pollution (persistent organic chemicals) abatement. The existing
budget for RTD initiatives related to the key action “Sustainable management and quality of
water”, for the period 1998-2002, is about 450 million Euro.
It may be observed that all these objectives and research initiatives are perfectly coherent with
the processes and technologies indicated in this book, providing an adequate framework for
international co-operation on solar detoxification applications and further research initiatives.
The “key action” concept is an important characteristic of the Fifth Framework Programme.
Its objective is to address the many and varied aspects of the economic and social issues to be
targeted, by integrating the entire spectrum of activities and disciplines needed to achieve the
specified objectives, using a problem-solving approach.
An important aspect of the overall European research strategy, in addition to the Fifth
Framework Programme’s basic support of European research, is international co-operation.
Entities of non-EU countries and international organisations may participate in all
Programmes, as well as in the Horizontal Programme “Confirming the international role of
Community research”. Conditions for participation of third countries in FP5 may differ from
one Programme to another depending on the status of the country, with regard to the
participation in EC research activities. Specific rules apply for the Programme “Confirming
the international role of Community research”.
In addition to EU member countries, institutions and entities from other states have a special
status when participating in EC research activities. Countries that have signed Association
Agreements may participate under the same conditions as EU member countries. Iceland,
Liechtenstein, Norway, Israel and candidates for EU-membership (currently Bulgaria,
Republic of Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania,
Slovakia and Slovenia) have Association Agreements either in force or expected to enter into
force during 1999. Switzerland has also concluded the Association Agreement negotiations.
Other countries, such as Argentina, Australia, Canada, China, Russia, South Africa and USA
have signed Co-operation Agreements with the EU for participation and collaboration in
research activities. In addition, some specific regions also have special relationship with the
EU, such as other European countries (Albania, Bosnia-Herzegovina, Former Yugoslav
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Republic of Macedonia Malta, Turkey and European Microstates and Territories), the socalled Mediterranean Partnership (Algeria, Republic of Cyprus, Egypt, Jordan, Lebanon,
Malta, Morocco, Palestine Authority, Syria, Tunisia and Turkey) or the European NIS
(Armenia, Azerbaijan, Belarus, Georgia, Moldova, Russia and Ukraine).
In some Work programmes, the developing countries are grouped into the following
geographic areas: African, Caribbean, Pacific (ACP) countries, Asian and Latin American
(ALA) countries, and the Mediterranean countries (MC), so it is always recommended that
up-to-date information and conditions for participation be obtained for the specific call to
which it is intended to submit a proposal.
FP5 is implemented, as past Framework Programmes were, through specific Work
Programmes, drawn up for each Programme and describing the specific activities and the
various research areas. The Work Programme is, regularly revised to ensure its continued
relevance in the light of evolving needs and developments, so potential proposers should
ensure that they are consulting the current version of the work programme when planning
their proposal. The Work Programme appearing at the Specific Programme Website is always
the current version. Work Programmes provide a means of focusing attention on areas or subareas, thereby optimising opportunities for launching collaborative projects and establishing
theme networks.
The EC partially finances RTD activities carried out under the Specific Programmes
implemented within its Framework Programmes. The types of activities normally aided are:
(a) Shared-cost activities
− Research and technological development (R&D) projects: projects obtaining new
knowledge for product process or service development or improvement, and/or to meet
the needs of Community policies.
− Demonstration projects: projects designed to prove the viability of new technologies
offering potential economic advantages, but which cannot be immediately
commercialised.
− Combined R&D and demonstration projects: projects combining the above elements.
− Support for access to research infrastructures: actions enhancing access to research
infrastructures for Community researchers.
− “SME Co-operative” research projects: projects enabling at least three mutually
independent SMEs from at least two Member States or one Member State and one
Associated State to jointly commission research carried out by a third party.
− “SME Exploratory” awards: support a project exploratory phase of up to 12 months (e.g.
feasibility studies, validation, partner search, etc).
(b) Training fellowships
These may be either fellowships, whereby individual researchers apply directly to the
Commission, or host fellowships, where institutions apply to host a number of researchers.
There is also a bursary for young researchers from Developing Countries. When preparing a
joint research proposal or concerted action proposal for submission to any of the programmes,
a consortium may include an application for an international co-operation-training bursary.
These bursaries are intended to allow young researchers from Developing Countries,
including Emerging Economies and Mediterranean Partner Countries to work for up to 6
months in a European research institute participating in a FP5 project. These bursary
applications must be submitted together with the proposal application and will be evaluated
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together with it. The bursary applicant must not be more than 40 years of age, must be a
national of one of the eligible countries and intending to return there at the end of the training
period. Applications from female researchers are encouraged.
(c) Research training networks and thematic networks
Training networks for promoting training-through-research especially of researchers at predoctoral and at post-doctoral level and thematic networks for bringing together e.g.
manufacturers, users, universities, research centres around a given objective.
(d) Concerted actions
Actions co-ordinating RTD projects already in receipt of funding, for example to exchange
experiences, to reach a critical mass, to disseminate results etc. These include co-ordination
networks between Community funded projects.
(e) Accompanying measures
Actions contributing to the implementation of a Specific Programme or the preparation of
future activities of the programme. They will also seek to prepare for or to support other
indirect RTD actions.
As previously indicated, when planning an RTD proposal for submission to one of the
programmes or to key actions, researchers should be aware of the conditions of participation
by entities from non-EU countries and international organisations.
10.3 THE CYTED PROGRAM
Another possibility for international collaboration is the CYTED Program (Programa
Iberoamericano de Ciencia y Tecnología para el Desarrollo). CYTED is the Latin-American
Science and Technology for Development Program. It was created in 1984 by institutional
agreement between Argentina, Bolivia, Brazil, Chile, Colombia, Costa Rica, Cuba, Ecuador,
El Salvador, Spain, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru,
Portugal, Dominican Republic, Uruguay and Venezuela.
The CYTED Program objective is to promote co-operation in development through applied
research and technology for transferable results to the production systems of the participant
countries. The program is addressed to universities, research centres, institutions and private
companies for the use of scarce resources to better advantage to modernise production
systems, improve the quality of life and enhance co-operative activities among Latin
American and European countries.
The Program is divided into 16 different subprograms, each directly managed by an
International Co-ordinator appointed by the General International Secretariat, made up of the
representatives of governmental research institutions in the participating countries and which
manages the overall Program. Each subprogram also has national representatives. There are
three different ways to participate:
- Field of Study Networks: to promote interaction, co-operation and transfer of knowledge
and technology among groups working on similar subjects.
- Pre-competitive Research Projects: research projects performed through the creation of an
international complementary team. Immediate market application potential is not
required.
- Innovative Projects: to promote technological development through co-operation between
business and research centres from different countries for industrial productivity and
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SOLAR DETOXIFICATION
competitiveness improvement. Innovation projects must be addressed to develop new
technologies, products, processes and services near the market or with existing potential
market.
Networks, Pre-competitive Research Projects and Innovative Projects must be within the
scope of the 16 sub-programs to be eligible. There are presently about 8600 Latin-American
scientist participating in CYTED Program activities, with more than 1000 universities, R&D
institutions and companies involved. Solar detoxification activities can be promoted within at
least the following two sub-programs:
- MATERIALS TECHNOLOGY. International co-ordinator: Miguel José Yacamán.
Consejo Nacional de Ciencia y Tecnología (CONACYT). Mexico.
- CATALYSIS AND ADSORBENTS. International co-ordinator: Paulino Andreu.
Petróleos de Venezuela, S.A. Venezuela.
Some solar detoxification initiatives are already in progress under the CYTED umbrella.
Among them are the “Latin-American Network of Semiconductor Oxides and Materials
Related to Optical Environmental Applications”, co-ordinated by Dr. Miguel Angel Blesa
(Comisión Nacional de Energía Atómica, Argentina), with partner institutions from
Argentina, Brazil, Mexico, Cuba and Spain.
The program philosophy is to share existing national research resources to create a synergistic
effect, reinforcing and consolidating national research. There is an agreement to this end
among all the participating countries by which companies and research institutions
participating in approved (“certified”) projects are financed nationally. The type of support
and the financial mechanisms are those normally used within each country to promote
development of scientific research and technology. Limited central financial support is only
provided some tasks for project co-ordination.
The normal procedure of a CYTED supported activity is the following:
1. A project is initiated by Latin-American company or research institution.
2. The National Co-ordinator of the appropriate CYTED sub-program is contacted.
3. A preliminary project proposal is defined.
4. An appropriate project partnership is sought. A collaboration agreement is reached.
5. A project proposal is prepared by the project co-ordinator following a specific format.
6. The CYTED National Co-ordinators involved confirms project eligibility.
7. The project is certified and approved by the CYTED General Secretariat.
8. National financing is requested through the National Co-ordinators.
9. The project is carried out with follow-up, conclusion and reporting.
Complete updated information about the CYTED Program can be found at the following web
address:
http://www.cicyt.es/ivpm/cyted.htm
10.4 MAIN RESEARCH ACTIVITIES
Although many scientists in different countries are very well known for their continuous
effort in the development of photocatalytic techniques and technologies (the
acknowledgement at the beginning of this book is just a small sample), only two countries
have had government-financed solar detoxification technology research and development
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programs with relevant industrial collaboration: The United Stated and Spain. Their research
programs are briefly described below.
10.4.1 United States
The U.S. Department of Energy sponsored a 10-year effort to apply photochemical
technology to destruction of environmental contaminants. The Solar Detoxification Project
was begun in the late 80s, initially funded by the Department of Energy (DOE) as part of its
Solar Thermal Program. In 1991 it was incorporated in a new Solar Industrial Program. The
goal of the project was to develop marketable solar detoxification technology by the mid1990s. Objectives of the project were to:
− Advance the state of development of photocatalytic water treatment chemistry so that it
could be adopted by industry,
− Bring the solar treatment of gas streams containing hazardous organic compounds to a
point where it could be transferred to industry, and
− Develop remedial solar treatment technology for contaminated soil.
Work in these areas at the Solar Energy Research Institute (SERI), which became the
National Renewable Energy Laboratory (NREL), and at Sandia National Laboratories (SNL)
had begun gradually in the mid-1980s. At the same time, there was a strong push, driven by
regulatory pressure, to develop new technologies for correcting past environmental
contamination of soil and of ground and surface water. The regulatory pressure created a
business environment encouraging development of new, environmentally friendly processes.
Many large and small companies studied a wide range of technologies. Solar technologies
were especially attractive because of the potential reduction in the cost of energy.
In order to reach the DOE goals, the following program elements were pursued:
− Technology Research, to develop process chemistry
− System Engineering, to develop reactors and solar concentrators and to create a solid
understanding of available solar resources applicable to the processes
− System and Market Assessment, to evaluate process cost and application information that
could guide R&D for the project.
Subcontractors from industry and academia were heavily involved in all of these areas.
Involving industry at an early phase was expected to smooth the path to commercialisation,
and universities were expected to provide more basic understanding as a foundation for the
technology. Initial research areas included aqueous phase applications, catalyst development,
and concentrating solar reactor development.
The project was initially directed toward the development of processes that would use
concentrating solar hardware so the existing knowledge base in solar companies could be
tapped. This led to a field test at Lawrence Livermore National Laboratory using parabolictrough reactors to treat contaminated ground water, as well as participation in the DOE, EPA,
DOD Tri-Agency Project to treat contaminated soil. The latter project involved the U. S.
Army, Environmental Protection Agency, and DOE. The NREL/SNL participation was to
assist with bench-scale solar testing of the high-flux process. Science Applications
International Corporation (SAIC) was the prime contractor for the Army. That project
culminated in a test of a reactor on a solar dish by SAIC.
The Photochemistry research team at NREL/SNL conducted research and development work
in all R&D areas: basic, applied, demonstration, and transfer to commercialization. Basic
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research included core Photocatalytic Oxidation (PCO) R&D and catalysts development
work, as well as conducting research into new areas of photochemistry such as photo-induced
adsorption and high-temperature solar PCO. Applied research projects consisted of
remediation of chloroethylenes in the gas and water phase, gas and water phase solar
photoreactor development, and application research including indoor air quality, hybrid
biological/PCO processes, and processes for treatment of munitions production wastewater.
Many of the projects were co-funded by other agencies and programs, including the Strategic
Environmental Research Defense Program, SEMATECH, and the U.S. Army.
A pilot-scale test conducted in 1991 at Lawrence Livermore National Laboratory treated
contaminated groundwater containing 200 parts per billion (ppb) of TCE. PCO reduced TCE
levels to below 5ppb, within the drinking water standard set by the Environmental Protection
Agency (EPA). The results of this field pilot test suggested that the benefits of concentrated
sunlight were insufficient to overcome the added costs of building concentrators. Efforts to
find a supported catalyst with activity close to that of slurried catalysts were also carried out.
A second field test was conducted at Tyndall Air Force Base using a non-concentrating photo
reactor system. The results of the Tyndall field test moved PCO research from the aqueous
phase into gas-phase research, because the data gathered from this test suggested that air
stripping the volatile organic pollutant compounds (VOCs), followed by PCO treatment,
could be more cost-effective and more efficient when the water to be treated contained a high
background level of substances which resulted in a reduction of catalyst activity.
In 1992, a co-operative research and development agreement was entered into with United
Technologies to develop PCO for air treating applications. In 1993, another agreement was
entered into with International Technology Corporation (IT) for the purpose of conducting
research and development to commercialise PCO remediation technologies coupled to air
stripping systems. In 1994, a third co-operative research and development agreement was
signed with SEMATECH to apply PCO to semiconductor manufacturing. Work with
SEMATECH resulted in a field demonstration at a semiconductor manufacturing planttreating emissions from a semiconductor manufacturing operation in 1996. In 1995 and 1996,
PCO applications were investigated for the Department of Defence for remediation of
trichloroethylene in water and air streams and for treating paint booth emissions. In 1997, two
field test demonstrations were completed for PCO/thermal treatment at a military paint booth
installation and PCO remediation of TCE contaminated groundwater. Other industrial
collaboration partners have been the International Fabricare Institute and the American Bakers
Association.
In 1996, the DOE Office of Industrial Technology terminated dedicated funding to the Solar
Detoxification Program. The Department of Defence project was successfully completed in
1997 after four demonstrations had been completed. PCO work at NREL continues in applied
areas of gas-phase PCO for DOE Industries of the Future applications, indoor air quality and
tandem processes such as PCO/biofiltration. This work is funded by a number of government
and private sector organisations.
10.4.2 Spain
The CIEMAT’s Department of Renewable Energies, a public research institution devoted to
energy and the environment belonging to the Spanish Ministry of Industry and Energy, has
been working on solar chemistry processes since 1987. Research on Solar Detoxification
applications also started at that time, mainly at the PSA, the CIEMAT’s solar research facility,
which is located in southeastern Spain. In 1990, through the EU-DGXII “Access to Large
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Installation Program”, the PSA designed and erected a large solar detoxification facility in cooperation with some relevant European photochemical research groups and the first solar tests
were successfully carried out.
Due to reasonable initial expectations for the technical feasibility of solar detoxification, in
1994, CIEMAT defined Solar Chemistry research as one of its areas of activity, focusing
mainly on water and gas-phase solar detoxification processes and applications. This research
activity was later transformed, in 1998, into a formal CIEMAT Research Project with the
more specific scientific and technological objectives of developing and transferring a feasible
solar detoxification technology to industry. The main project objective is the development of
a technology that would make the use of solar photons in environmental chemical
applications in general, and solar detoxification processes in particular, technically and
economically viable. This is to be done by assessing the scientific and technological bases
that make possible an engineering approach to photochemical processes using solar radiation
at pre-industrial scale. Specific project objectives are the following:
− Development and optimisation of solar water detoxification technology for the treatment
of hazardous non-biodegradable contaminants in industrial wastewater.
− Development and optimisation of gas-phase solar detoxification technology (including
engineering, photoreactor and catalyst) to the photocatalytic detoxification of VOCs from
industrial gaseous emissions.
− Assessment of technical and economic viability of other Solar Chemistry processes with
potential application in Spain and other countries with similar characteristics, such as high
temperature solar detoxification of hazardous wastes, solar reforming, solar gasification of
biomass wastes, solar synthesis of fine chemicals, etc).
An important part of the contents of this book is the result of CIEMAT activities in solar
detoxification of hazardous water contaminants during recent years.
During the entire period, a major source of scientific background has come from the EC
“Access to Large-Scale Scientific Installations” (1990-1993), “Human Capital and Mobility”
(1994-1995) and “Training and Mobility of Researchers” (1996-1998). These programs have
facilitated PSA Solar Detoxification Facility access for many relevant European universities
and other research groups and have made possible a continuous exchange of ideas leading to
photochemical process improvement. In addition to this scientific database, a large number of
national and international initiatives in solar detoxification, supported by Spanish and
European research programs, have provided CIEMAT an important complementary
technological background. Some important initiatives are still underway.
Industrial participation is also a fundamental pillar of the overall CIEMAT activity in solar
detoxification. Since 1993, it has collaborated with many Spanish companies in a large
number of small projects that have made it possible to test the feasibility of photocatalytic
degradation of industrial waste water and identify the most suitable targets. Moreover, these
tests have been used to verify the continuous modification and improvement of the solar
technology. Industrial collaboration with the CIEMAT Solar Detoxification project has
increased significantly since 1997, with the development of several important projects and
demonstration initiatives. Sections 9.2, 9.4 and 9.6 of this book are examples of this
collaboration.
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It may be affirmed that the result of this combined photochemical research from university
and industrial collaboration in hardware development and testing has lead to the current state
of the art of the solar detoxification technology, which this book has tried to reflect.
10.5 GUIDELINES TO SUCCESSFUL WATER TREATMENT PROJECTS IN
DEVELOPING COUNTRIES
Finally, because of the high rate of failure of water system projects in developing countries,
most of which are especially suitable for such applications since they are in the “sun belt”,
some guidelines for water treatment projects to be carried out in those countries are included
here as a complement to the previous sections.
Twenty guidelines indicating the requirements for water projects may be found in the
document Lessons Learned in Water, Sanitation and Health (reference 6). As these guidelines
are basically addressed to water and sanitation projects funded by international aid, some
important conclusions may be arrived at concerning solar detoxification water treatment
projects. These and other guidelines were obtained as the result of many years of experience
and underline the necessity of promoting co-operation with local authorities and institutions,
supporting local plans and requirements, rather than promoting the most convenient project
dictated by the developed country. Adequate information, training and local skill must also be
built up are to guarantee the long-term sustainability of any facility installed.
From such guidelines as these, lessons learned and general recommendations for addressing
water-related projects in developing countries, it may be affirmed that special emphasis must
be placed on training local people and involving local institutions for collaborative projects on
solar detoxification. If local technical or managerial skills are inadequate, the success of any
renewable energy project would be difficult, and this is more so in the case of solar
detoxification technologies that require adequate previous training. Also, the collaboration of
local institutions is very important in itself since any water treatment project is usually closely
related to sanitary and health issues.
A typical mistake is to focus the project on construction, forgetting the necessary previsions
and provisions to ensure the technical and financial sustainability of the renewable technology
installation. This issue must be foreseen from the beginning of the project. Therefore, it is
highly recommended that projects financed by foreign aid be designed in such a way that their
operation and maintenance costs as well as part of the initial construction cost can be borne by
the user or users.
Another relevant recommendation is that projects should not promote dependency on foreign
aid or foreign technical assistance. One possible way to avoid this when innovative
technologies, such as solar detoxification, are implemented is to promote the development of
the private sector on the area. Local companies, acting as interface between the users and the
foreign engineering company, could provide the necessary technical assistance and also
promote new projects and initiatives at local level.
As contaminated drinking water is a typical problem, water purification and disinfection
seems to be a solar detoxification technology application of interest in developing countries.
Water contaminants, the main cause of diseases in developing countries, have been divided
into five categories by the World Health Organisation (WHO): biological, inorganic, organic,
aesthetic and radioactive contaminants. With regard to organic constituents of health
significance, main WHO recommendation is to encourage efforts to protect water sources
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from organic contamination, as their treatment tends to be complicated. Whenever possible,
these industrial and agricultural contaminants must be treated at their source. Solar
detoxification is always an alternative to be considered for this.
Among the denominated Advanced Oxidation Processes (AOPs), based on catalytic and
photochemical techniques, Solar Detoxification has become especially attractive to the
treatment of water contaminants. This has been due to the synergistic combination of solving
difficult environmental problems by using solar energy with the possibilities of creating new
jobs and activities. Solar Detoxification could be considered, then, as a good example of the
concept of sustainable development which, as sooner the better, mankind must achieve.
SUMMARY OF THE CHAPTER
Along this chapter, a review of different existing mechanisms to promote international
collaboration in the subject of solar detoxification has been made: the International Energy
Agency SolarPACES project, the Research Programmes of the European Commission and the
Latin American CYTED network. All three mechanisms are currently (1999) promoting
and/or supporting activities and projects on solar detoxification topics with important
international collaboration. United States and Spain were the countries with more activity
during the nineties on solar detoxification technology development, resulting in the Present
State of the Art of the technology. A brief review of the governmental activities of both
countries is also provided within the chapter. International collaboration with developing
countries must be addressed with special care due to the high rate of failure of water related
projects. Several general recommendations and lesson learned, from the wide experience of
many international organisations, are particularised in the case of solar detoxification
initiatives, when addressed to be implemented at developing countries.
BIBLIOGRAPHY AND REFERENCES
1. Blake, D. M.; Carlson-Boyd, L. E.; Lee Recca, L.; Kissell, G. “Photochemical Pollution
Control: The Final Report of the Solar Industrial Program. Solar Detoxification Project”.
U.S. Department of Energy. NREL/SANDIA read-only compact disk (CD-ROM). 1997.
2. IEA. “Implementing Agreement for the establishment of a project on Solar Power and
Chemical Energy Systems (SolarPACES)”. International Energy Agency. 1996 update of
1977 document.
3. Niewoehner, J.; Larson, R.; Azrag, E.; Hailu, T.; Horner, J.; VanArsdale, P.
“Opportunities for Renewable Energy Technologies in Water Supply in Developing
Country Villages”. National Renewable Energy Laboratory. NREL/SR-430-22359. 1997.
4. SolarPACES. “Towards the 21st Century, IEA/SolarPACES Strategic Plan”. SolarPACES
Brochure, March 1996.
5. SolarPACES. “Solar Thermal Power and Solar Chemical Energy Systems, SolarPACES
Program of the International Energy Agency”. SolarPACES Brochure, Birmingham,
United Kingdom, September 1994, and 1998 update.
6. WASH. “Lessons Learned in Water, Sanitation, and Health; Thirteen Years of Experience
in Developing Countries”. Water and Sanitation for Health Project (WASH). Updated
Edition; Alexandria, VA. 1993.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. IEA headquarters are located in London.
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2. SolarPACES is one of the more than 60 Implementing Agreements of the International
Energy Agency.
3. The PSA was built during the initial stages of the SolarPACES Project to test and evaluate
the different solar technologies to the electricity production existing at that moment.
4. The assistance to the SolarPACES periodic meetings is restricted to the Executive
Committee members, Operating Agents and National Co-ordinators from the different
country members.
5. The Fifth Framework Programme of the European Union (1998-2002) consists of seven
Specific Programmes, of which four are Thematic Programmes and three are Horizontal
Programmes.
6. “Sustainable management and quality of water” is one of the four key actions of the
environment section of the EU-FP5 “Energy, environment and sustainable development”
Work programme.
7. Participation from institutions, organisations and companies from non-EU countries is
possible in all EU-FP5 Programmes.
8. An application for an international Cupertino training bursary (EU-FP5 Programmes) can
be made to any of the programmes, independently of the submission of a joint research
proposal or concerted action initiative.
9. CYTED is a Latin American network of governmental research organisations to promote,
support and reinforce national research initiatives.
10. When addressing international collaborative projects to be implemented at developing
countries it is highly recommended to get information from the experience of similar or
related initiatives previously implemented in the same area.
PART B.
1. How would you define an Implementing Agreement of the International Energy Agency?
2. What is SolarPACES?
3. Indicate the initial members of the present stage of the SolarPACES project, initiated in
1991.
4. How many countries have signed the SolarPACES Implementing Agreement in 1999?
5. Indicate the different work areas defined at the SolarPACES Implementing Agreement.
6. Which specific sector of SolarPACES Implementing Agreement includes the research
activities on Solar Detoxification?
7. Indicate the name of the thematic programme, within the EU-FP5, more directly related
with environmental research.
8. Indicate the name of the Horizontal Programme, within the EU-FP5, specifically
addressed to promote International Collaboration.
9. Indicate the type of actions normally supported by the different EU-FP5 Programmes.
10. Indicate the different shared-cost actions allowed within the different EU-FP5
Programmes.
ANSWERS
Part A
1. False; 2. True; 3. True; 4. False; 5. True; 6. True; 7. True; 8. False; 9. True; 10. True.
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Part B
1. An “Implementing Agreement” is a specific contract between several IEA members to
collaborate in the research, development and application on a determined energy related
issue.
2. SolarPACES (Solar Power And Chemical Energy Systems) is one of the IEA
Implementing Agreements focused on the development and marketing of systems based
on solar technologies.
3. Germany, Spain, Switzerland and the United Stated.
4. Twelve countries (Australia, Brazil, Egypt, France, Germany, Israel, Mexico, Russia,
Spain, Switzerland, United Kingdom and United States) plus the European Commission
(DG XII and DG XVII).
5. Three on-going Tasks or thematic areas of work are defined within the SolarPACES
Implementing Agreement:
− Task I; Concentrating Solar Energy Power Systems.
− Task II; Solar Chemistry Research.
− Task III; Solar Technology and Applications.
6. The Sector II.3
7. Energy, environment and sustainable development.
8. Confirming the international role of Community research.
9. Shared-cost actions. Training fellowships. Research training networks and thematic
networks. Concerted actions. Accompanying measures.
10. The different shared-cost actions allowed within the EU-FP5 Programmes are:
− Research and technological development (R&D) projects.
− Demonstration projects.
− Combined R&D and demonstration projects.
− Support for access to research infrastructures.
− “SME Co-operative” research projects.
− “SME Exploratory” awards.
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10 INTERNATIONAL COLLABORATION
AIMS
This unit describes some of the programmes and initiatives promoting national or
international collaboration in research, development and implementation of innovative
sustainable technologies, that include Solar Detoxification projects within their scope.
OBJECTIVES
At the end of this unit, you will appreciate the scale, nature and content of these programmes,
and some of the national and regional initiatives in progress around the world. You will
acquire basic information concerning the mechanisms and possibilities for proposing and
establishing international collaboration in the field of solar detoxification and you will also
know how to receive updated information on those possibilities.
NOTATION AND UNITS
Symbol
Description
CIEMAT
Centro de Investigaciones Energéticas, Medioambientales y
Tecnológicas (Spain)
DG-XII
Directorate General XII (of European Commission): Science,
Research and Development
DG-XVII
Directorate General XVII (of European Commission): Energy
DOE
Department of Energy (USA)
DOD
Department of Defense (USA)
EC
European Commission
EPA
Environmental Protection Agency (USA)
EU
European Union
ExCo
Executive Committee (SolarPACES)
FP5
Fifth Framework Programme (EU)
IEA
International Energy Agency
NREL
National Renewable Energy Laboratory (USA)
OEDC
Organization for Economic Co-operation and Development
PCO
Photocatalytic Oxidation
PSA
Plataforma Solar de Almería
PSI
Paul Scherrer Institute (Switzerland)
SANDIA
Sandia National Laboratories (USA)
SolarPACES
Solar Power And Chemical Energy Systems
RTD
Research and technological developments
SERI
Solar Energy Research Institute (USA)
SMEs
Small and medium-sized enterprises
SNL
Sandia National Laboratories (USA)
SSPS
Small Solar Power Systems
WHO
World Health Organisation
Units
10.1 INTERNATIONAL ENERGY AGENCY: THE SolarPACES PROGRAM
The IEA (International Energy Agency), founded in 1974, is the energy forum for
industrialised countries. Based in Paris, the IEA is an autonomous agency within the
framework of the Organisation for Economic Co-operation and Development (OEDC). An
important function of the IEA is the promotion of enhanced international collaboration on
energy research and the development and application of new and efficient energy
technologies. The IEA has set up more than 60 “Implementing Agreements” linking member
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Countries in R&D, technology demonstration and information initiatives. One of these
Implementing Agreements is called SolarPACES (Solar Power and Chemical Energy
Systems).
SolarPACES is one of the international co-operative programs managed under the umbrella of
the IEA to help find solutions to worldwide energy and environmental problems, bringing
together teams of national experts from around the world to focus on the development and
marketing of systems based on solar technologies.
The SolarPACES program was initiated in 1977 under its former name of SSPS (Small Solar
Power Systems). Two dissimilar solar facilities were designed in the project’s Stage 1 by ten
Contracting Parties from Austria, Belgium, Germany, Greece, Italy, Spain, United Kingdom
and the United States. All the countries, with the exception of the UK, continued the project
through Stage 2 (Building, testing and Evaluation) which was completed in Almería, in
southern Spain, at the end of 1984. In the course of the subsequent Stage 3, eight countries
(all but Greece) proceeded with solar-related research and development in various forms,
especially in advanced solar thermal and solar chemical applications. The two SSPS facilities
were transformed into what has since become the world’s most versatile solar test centre, the
Plataforma Solar de Almería (PSA), which continues to serve as the site of multiple cooperative international testing and development efforts.
In 1991, Germany, Spain, Switzerland and the USA decided to go on to a Stage 4 and sought
increased participation from both member and non-member countries. As of 1999, there are
fourteen members of SolarPACES: Australia, Brazil, Egypt, the European Commission (DG
XII and DG XVII), France, Germany, Israel, Mexico, Russia, South Africa, Spain,
Switzerland, the United Kingdom and the United States. In 1998 alone, contacts were
maintained with representatives of Azerbaijan, Chile, Ghana, India, Italy, Japan, Jordan,
South Africa, Turkey, Uzbekistan and Zimbabwe (1998 SolarPACES Annual Report).
Membership is open to all countries, subject to Executive Committee approval, and involves a
government (or its nominated contracting party) becoming a signatory to the program’s
“Implementing Agreement”, which defined the SolarPACES charter and conditions of
membership. The current Implementing Agreement, valid from 1996 until
December 31, 2001, is an amendment of the original one signed on September 23, 1977. The
Implementing Agreement may be extended by agreement of two or more participants, then
being applied only to those participants.
All SolarPACES activities are overseen by an Executive Committee (ExCo) composed of
individuals nominated from each member country. The ExCo meets twice yearly to formulate
strategic objectives, direct the program of work, review results and accomplishments, and
report to the IEA. An elected Chairperson presides over the ExCo meetings, and throughout
the year, an Executive Secretary deals with day-to-day program management.
The ongoing work and activities are co-ordinated through specific “Tasks” or areas of work,
defined within the Implementing Agreement. SolarPACES currently has three such on-going
tasks:
− Task I; Concentrating Solar Energy Power Systems.
− Task II; Solar Chemistry Research, where solar detoxification is included.
− Task III; Solar Technology and Applications.
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An Operating Agent, nominated by the ExCo, is responsible for overseeing the work of each
Task and each member country nominates a National Co-ordinator within each of the three
Tasks. Each task maintains a detailed program of work that defines all task activities,
including their objectives, participants, plans and budgets. In addition to technical reports of
the activities and their participants, accomplishments and progress are summarised in the
SolarPACES annual report. Many SolarPACES activities involve close co-operation among
members countries (either through sharing of task activities or, occasionally, cost-sharing),
although some co-operation is limited to sharing of information and results with other
participants.
The activities formally identified within Task II (Solar Chemistry Research) are related with
the development of technologies and systems in the field of solar-driven thermochemical,
photochemical and electrochemical processes for the production of energy carriers, chemical
commodities and for the detoxification and recycling of waste materials. As indicated in the
current Implementing Agreement, Task II activities are divided into three sectors, Sector II.3
being completely devoted to solar detoxification activities and research.
(a)
Sector II.1: Solar production of Energy Carriers.
The objectives of this Sector are to:
− Explore new ideas and concepts for he thermochemical, photochemical and
electrochemical production of chemical fuels and chemical heat pipes for storage
and transportation of solar energy;
− Develop and test the required solar process technology;
− Assess their technical and economic feasibility and implementation;
− Set priorities of research and development needs;
(b)
Sector II.2: Solar Production of Chemical Commodities.
The objectives of this Sector are to:
− Identify chemical processes for the solar production of fine and bulk chemical
commodities;
− Develop and test the required solar process technologies;
− Assess their technical and economic feasibility and implementation;
(c)
Sector II.3: Solar Detoxification and Recycling.
The objectives of this Sector are to:
− Test and evaluate solar detoxification processes;
− Further develop and demonstrate solar detoxification systems up to commercial
level.
The core of the work of SolarPACES is development of new and advanced concentrating
solar technologies and solving the wide range of technical problems associated with their
commercialisation. This means that, from advanced solar concentrating technologies in
general to solar detoxification applications in particular, industrial participation plays a
critical role. Many of the Task’s international activities and teams involve industrial cooperation. In fact, in some countries (e.g., the UK and Australia), the SolarPACES contracting
party is an industrial company.
SolarPACES attempts to give added value to national work already funded by its member
governments. It is, therefore, not in itself a “big-budget” operation and normally does not
provide funding for work to be carried out in member countries. The small annual fee paid by
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member countries is used to support a limited range of co-operative activities approved by the
ExCo, such as publication and distribution of documents, scholarships and activities
promoting international awareness.
The Task II Operating Agent is the Paul Scherrer Institute (PSI) of Switzerland, which coordinates the activities in close co-operation with the National Co-ordinators. Operating
Agent and National Co-ordinators normally meet once a year to review the progress of Task
activities, discuss technical issues and prepare future Task development.
Full additional information on activities, conferences, reports, newsletters and contact
addressed can be found at the following web site:
http://www.demon.co.uk/tfc/SolarPACES.html
10.2 THE EUROPEAN UNION
In recent years, the European Commission (EC) has become one of the most active
institutions in the financing and promoting of both solar detoxification research and
international collaboration. A considerably high number of projects related to photocatalytic
research has been approved and financed during the 90s (3rd and 4th Framework Programmes).
A large part of the information contained in this book was obtained from projects, networks
and activities promoted and partially financed by the European Commission, through these
Framework Programmes.
The Fifth Framework Programme, adopted in December 1998, defines the Community
activities in the field of research, technological development and demonstration for 19982002. Its differs notably from its predecessors in that it focuses on a limited number of
objectives and areas combining technological, industrial, economic, social and cultural
aspects. Environmental Protection is one of these priority areas, water treatment being one of
its specific objectives, thereby providing a good scenario for co-operative research,
development and demonstration initiatives related with solar detoxification of water.
The Fifth Framework Programme consists of seven Specific Programmes, of which four are
Thematic Programmes and three are Horizontal Programmes. The Thematic Programmes are:
− Quality of life and management of living resources
− User-friendly information society
− Competitive and sustainable growth
− Energy, environment and sustainable development
The Horizontal Programmes, complementing these Thematic Programmes, are:
− Confirming the international role of Community research
− Promotion of innovation and encouragement of participation of small and medium-sized
enterprises (SMEs)
− Improving the human research potential and socio-economic knowledge base
Exhaustive documentation related to all these Programmes is provided by the EC at the
following web sites:
− http://europa.eu.int/comm/dg12/index.html (European Commission DGXII)
− http://www.cordis.lu/fp5/home.html (Fifth Framework Work programmes)
− http://www.cordis.lu/home.html (CORDIS, big European database)
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The specific topic of water treatment is in the “Energy, environment and sustainable
development” programme. The strategic goal of this programme is to promote environmental
science and technology to improve quality of life and boost growth, competitiveness and
employment, while meeting the need for sustainable management of resources and protection
of the environment. Within this programme, research and technology development (RTD)
will concentrate on six key actions (two for the “energy” area, four to the “environment and
sustainable development” area). The first of the four key actions in the “environment” section
is “Sustainable management and quality of water”, specifically addressed to water treatment
and purification technologies, with the objectives, among others, of:
− developing improved waste-water treatment techniques and technologies,
− developing technologies for rational water reuse,
− developing technologies for water purification,
− enhancing waste-water treatments,
− minimising environmental impacts from waste water treatment.
Priority attention will be given to research initiatives addressed to waste water treatment and
re-use, water pollution abatement from contaminated land, landfills and sediments and ground
and surface waters diffuse pollution (persistent organic chemicals) abatement. The existing
budget for RTD initiatives related to the key action “Sustainable management and quality of
water”, for the period 1998-2002, is about 450 million Euro.
It may be observed that all these objectives and research initiatives are perfectly coherent with
the processes and technologies indicated in this book, providing an adequate framework for
international co-operation on solar detoxification applications and further research initiatives.
The “key action” concept is an important characteristic of the Fifth Framework Programme.
Its objective is to address the many and varied aspects of the economic and social issues to be
targeted, by integrating the entire spectrum of activities and disciplines needed to achieve the
specified objectives, using a problem-solving approach.
An important aspect of the overall European research strategy, in addition to the Fifth
Framework Programme’s basic support of European research, is international co-operation.
Entities of non-EU countries and international organisations may participate in all
Programmes, as well as in the Horizontal Programme “Confirming the international role of
Community research”. Conditions for participation of third countries in FP5 may differ from
one Programme to another depending on the status of the country, with regard to the
participation in EC research activities. Specific rules apply for the Programme “Confirming
the international role of Community research”.
In addition to EU member countries, institutions and entities from other states have a special
status when participating in EC research activities. Countries that have signed Association
Agreements may participate under the same conditions as EU member countries. Iceland,
Liechtenstein, Norway, Israel and candidates for EU-membership (currently Bulgaria,
Republic of Cyprus, Czech Republic, Estonia, Hungary, Latvia, Lithuania, Poland, Romania,
Slovakia and Slovenia) have Association Agreements either in force or expected to enter into
force during 1999. Switzerland has also concluded the Association Agreement negotiations.
Other countries, such as Argentina, Australia, Canada, China, Russia, South Africa and USA
have signed Co-operation Agreements with the EU for participation and collaboration in
research activities. In addition, some specific regions also have special relationship with the
EU, such as other European countries (Albania, Bosnia-Herzegovina, Former Yugoslav
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Republic of Macedonia Malta, Turkey and European Microstates and Territories), the socalled Mediterranean Partnership (Algeria, Republic of Cyprus, Egypt, Jordan, Lebanon,
Malta, Morocco, Palestine Authority, Syria, Tunisia and Turkey) or the European NIS
(Armenia, Azerbaijan, Belarus, Georgia, Moldova, Russia and Ukraine).
In some Work programmes, the developing countries are grouped into the following
geographic areas: African, Caribbean, Pacific (ACP) countries, Asian and Latin American
(ALA) countries, and the Mediterranean countries (MC), so it is always recommended that
up-to-date information and conditions for participation be obtained for the specific call to
which it is intended to submit a proposal.
FP5 is implemented, as past Framework Programmes were, through specific Work
Programmes, drawn up for each Programme and describing the specific activities and the
various research areas. The Work Programme is, regularly revised to ensure its continued
relevance in the light of evolving needs and developments, so potential proposers should
ensure that they are consulting the current version of the work programme when planning
their proposal. The Work Programme appearing at the Specific Programme Website is always
the current version. Work Programmes provide a means of focusing attention on areas or subareas, thereby optimising opportunities for launching collaborative projects and establishing
theme networks.
The EC partially finances RTD activities carried out under the Specific Programmes
implemented within its Framework Programmes. The types of activities normally aided are:
(a) Shared-cost activities
− Research and technological development (R&D) projects: projects obtaining new
knowledge for product process or service development or improvement, and/or to meet
the needs of Community policies.
− Demonstration projects: projects designed to prove the viability of new technologies
offering potential economic advantages, but which cannot be immediately
commercialised.
− Combined R&D and demonstration projects: projects combining the above elements.
− Support for access to research infrastructures: actions enhancing access to research
infrastructures for Community researchers.
− “SME Co-operative” research projects: projects enabling at least three mutually
independent SMEs from at least two Member States or one Member State and one
Associated State to jointly commission research carried out by a third party.
− “SME Exploratory” awards: support a project exploratory phase of up to 12 months (e.g.
feasibility studies, validation, partner search, etc).
(b) Training fellowships
These may be either fellowships, whereby individual researchers apply directly to the
Commission, or host fellowships, where institutions apply to host a number of researchers.
There is also a bursary for young researchers from Developing Countries. When preparing a
joint research proposal or concerted action proposal for submission to any of the programmes,
a consortium may include an application for an international co-operation-training bursary.
These bursaries are intended to allow young researchers from Developing Countries,
including Emerging Economies and Mediterranean Partner Countries to work for up to 6
months in a European research institute participating in a FP5 project. These bursary
applications must be submitted together with the proposal application and will be evaluated
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together with it. The bursary applicant must not be more than 40 years of age, must be a
national of one of the eligible countries and intending to return there at the end of the training
period. Applications from female researchers are encouraged.
(c) Research training networks and thematic networks
Training networks for promoting training-through-research especially of researchers at predoctoral and at post-doctoral level and thematic networks for bringing together e.g.
manufacturers, users, universities, research centres around a given objective.
(d) Concerted actions
Actions co-ordinating RTD projects already in receipt of funding, for example to exchange
experiences, to reach a critical mass, to disseminate results etc. These include co-ordination
networks between Community funded projects.
(e) Accompanying measures
Actions contributing to the implementation of a Specific Programme or the preparation of
future activities of the programme. They will also seek to prepare for or to support other
indirect RTD actions.
As previously indicated, when planning an RTD proposal for submission to one of the
programmes or to key actions, researchers should be aware of the conditions of participation
by entities from non-EU countries and international organisations.
10.3 THE CYTED PROGRAM
Another possibility for international collaboration is the CYTED Program (Programa
Iberoamericano de Ciencia y Tecnología para el Desarrollo). CYTED is the Latin-American
Science and Technology for Development Program. It was created in 1984 by institutional
agreement between Argentina, Bolivia, Brazil, Chile, Colombia, Costa Rica, Cuba, Ecuador,
El Salvador, Spain, Guatemala, Honduras, Mexico, Nicaragua, Panama, Paraguay, Peru,
Portugal, Dominican Republic, Uruguay and Venezuela.
The CYTED Program objective is to promote co-operation in development through applied
research and technology for transferable results to the production systems of the participant
countries. The program is addressed to universities, research centres, institutions and private
companies for the use of scarce resources to better advantage to modernise production
systems, improve the quality of life and enhance co-operative activities among Latin
American and European countries.
The Program is divided into 16 different subprograms, each directly managed by an
International Co-ordinator appointed by the General International Secretariat, made up of the
representatives of governmental research institutions in the participating countries and which
manages the overall Program. Each subprogram also has national representatives. There are
three different ways to participate:
- Field of Study Networks: to promote interaction, co-operation and transfer of knowledge
and technology among groups working on similar subjects.
- Pre-competitive Research Projects: research projects performed through the creation of an
international complementary team. Immediate market application potential is not
required.
- Innovative Projects: to promote technological development through co-operation between
business and research centres from different countries for industrial productivity and
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competitiveness improvement. Innovation projects must be addressed to develop new
technologies, products, processes and services near the market or with existing potential
market.
Networks, Pre-competitive Research Projects and Innovative Projects must be within the
scope of the 16 sub-programs to be eligible. There are presently about 8600 Latin-American
scientist participating in CYTED Program activities, with more than 1000 universities, R&D
institutions and companies involved. Solar detoxification activities can be promoted within at
least the following two sub-programs:
- MATERIALS TECHNOLOGY. International co-ordinator: Miguel José Yacamán.
Consejo Nacional de Ciencia y Tecnología (CONACYT). Mexico.
- CATALYSIS AND ADSORBENTS. International co-ordinator: Paulino Andreu.
Petróleos de Venezuela, S.A. Venezuela.
Some solar detoxification initiatives are already in progress under the CYTED umbrella.
Among them are the “Latin-American Network of Semiconductor Oxides and Materials
Related to Optical Environmental Applications”, co-ordinated by Dr. Miguel Angel Blesa
(Comisión Nacional de Energía Atómica, Argentina), with partner institutions from
Argentina, Brazil, Mexico, Cuba and Spain.
The program philosophy is to share existing national research resources to create a synergistic
effect, reinforcing and consolidating national research. There is an agreement to this end
among all the participating countries by which companies and research institutions
participating in approved (“certified”) projects are financed nationally. The type of support
and the financial mechanisms are those normally used within each country to promote
development of scientific research and technology. Limited central financial support is only
provided some tasks for project co-ordination.
The normal procedure of a CYTED supported activity is the following:
1. A project is initiated by Latin-American company or research institution.
2. The National Co-ordinator of the appropriate CYTED sub-program is contacted.
3. A preliminary project proposal is defined.
4. An appropriate project partnership is sought. A collaboration agreement is reached.
5. A project proposal is prepared by the project co-ordinator following a specific format.
6. The CYTED National Co-ordinators involved confirms project eligibility.
7. The project is certified and approved by the CYTED General Secretariat.
8. National financing is requested through the National Co-ordinators.
9. The project is carried out with follow-up, conclusion and reporting.
Complete updated information about the CYTED Program can be found at the following web
address:
http://www.cicyt.es/ivpm/cyted.htm
10.4 MAIN RESEARCH ACTIVITIES
Although many scientists in different countries are very well known for their continuous
effort in the development of photocatalytic techniques and technologies (the
acknowledgement at the beginning of this book is just a small sample), only two countries
have had government-financed solar detoxification technology research and development
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programs with relevant industrial collaboration: The United Stated and Spain. Their research
programs are briefly described below.
10.4.1 United States
The U.S. Department of Energy sponsored a 10-year effort to apply photochemical
technology to destruction of environmental contaminants. The Solar Detoxification Project
was begun in the late 80s, initially funded by the Department of Energy (DOE) as part of its
Solar Thermal Program. In 1991 it was incorporated in a new Solar Industrial Program. The
goal of the project was to develop marketable solar detoxification technology by the mid1990s. Objectives of the project were to:
− Advance the state of development of photocatalytic water treatment chemistry so that it
could be adopted by industry,
− Bring the solar treatment of gas streams containing hazardous organic compounds to a
point where it could be transferred to industry, and
− Develop remedial solar treatment technology for contaminated soil.
Work in these areas at the Solar Energy Research Institute (SERI), which became the
National Renewable Energy Laboratory (NREL), and at Sandia National Laboratories (SNL)
had begun gradually in the mid-1980s. At the same time, there was a strong push, driven by
regulatory pressure, to develop new technologies for correcting past environmental
contamination of soil and of ground and surface water. The regulatory pressure created a
business environment encouraging development of new, environmentally friendly processes.
Many large and small companies studied a wide range of technologies. Solar technologies
were especially attractive because of the potential reduction in the cost of energy.
In order to reach the DOE goals, the following program elements were pursued:
− Technology Research, to develop process chemistry
− System Engineering, to develop reactors and solar concentrators and to create a solid
understanding of available solar resources applicable to the processes
− System and Market Assessment, to evaluate process cost and application information that
could guide R&D for the project.
Subcontractors from industry and academia were heavily involved in all of these areas.
Involving industry at an early phase was expected to smooth the path to commercialisation,
and universities were expected to provide more basic understanding as a foundation for the
technology. Initial research areas included aqueous phase applications, catalyst development,
and concentrating solar reactor development.
The project was initially directed toward the development of processes that would use
concentrating solar hardware so the existing knowledge base in solar companies could be
tapped. This led to a field test at Lawrence Livermore National Laboratory using parabolictrough reactors to treat contaminated ground water, as well as participation in the DOE, EPA,
DOD Tri-Agency Project to treat contaminated soil. The latter project involved the U. S.
Army, Environmental Protection Agency, and DOE. The NREL/SNL participation was to
assist with bench-scale solar testing of the high-flux process. Science Applications
International Corporation (SAIC) was the prime contractor for the Army. That project
culminated in a test of a reactor on a solar dish by SAIC.
The Photochemistry research team at NREL/SNL conducted research and development work
in all R&D areas: basic, applied, demonstration, and transfer to commercialization. Basic
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research included core Photocatalytic Oxidation (PCO) R&D and catalysts development
work, as well as conducting research into new areas of photochemistry such as photo-induced
adsorption and high-temperature solar PCO. Applied research projects consisted of
remediation of chloroethylenes in the gas and water phase, gas and water phase solar
photoreactor development, and application research including indoor air quality, hybrid
biological/PCO processes, and processes for treatment of munitions production wastewater.
Many of the projects were co-funded by other agencies and programs, including the Strategic
Environmental Research Defense Program, SEMATECH, and the U.S. Army.
A pilot-scale test conducted in 1991 at Lawrence Livermore National Laboratory treated
contaminated groundwater containing 200 parts per billion (ppb) of TCE. PCO reduced TCE
levels to below 5ppb, within the drinking water standard set by the Environmental Protection
Agency (EPA). The results of this field pilot test suggested that the benefits of concentrated
sunlight were insufficient to overcome the added costs of building concentrators. Efforts to
find a supported catalyst with activity close to that of slurried catalysts were also carried out.
A second field test was conducted at Tyndall Air Force Base using a non-concentrating photo
reactor system. The results of the Tyndall field test moved PCO research from the aqueous
phase into gas-phase research, because the data gathered from this test suggested that air
stripping the volatile organic pollutant compounds (VOCs), followed by PCO treatment,
could be more cost-effective and more efficient when the water to be treated contained a high
background level of substances which resulted in a reduction of catalyst activity.
In 1992, a co-operative research and development agreement was entered into with United
Technologies to develop PCO for air treating applications. In 1993, another agreement was
entered into with International Technology Corporation (IT) for the purpose of conducting
research and development to commercialise PCO remediation technologies coupled to air
stripping systems. In 1994, a third co-operative research and development agreement was
signed with SEMATECH to apply PCO to semiconductor manufacturing. Work with
SEMATECH resulted in a field demonstration at a semiconductor manufacturing planttreating emissions from a semiconductor manufacturing operation in 1996. In 1995 and 1996,
PCO applications were investigated for the Department of Defence for remediation of
trichloroethylene in water and air streams and for treating paint booth emissions. In 1997, two
field test demonstrations were completed for PCO/thermal treatment at a military paint booth
installation and PCO remediation of TCE contaminated groundwater. Other industrial
collaboration partners have been the International Fabricare Institute and the American Bakers
Association.
In 1996, the DOE Office of Industrial Technology terminated dedicated funding to the Solar
Detoxification Program. The Department of Defence project was successfully completed in
1997 after four demonstrations had been completed. PCO work at NREL continues in applied
areas of gas-phase PCO for DOE Industries of the Future applications, indoor air quality and
tandem processes such as PCO/biofiltration. This work is funded by a number of government
and private sector organisations.
10.4.2 Spain
The CIEMAT’s Department of Renewable Energies, a public research institution devoted to
energy and the environment belonging to the Spanish Ministry of Industry and Energy, has
been working on solar chemistry processes since 1987. Research on Solar Detoxification
applications also started at that time, mainly at the PSA, the CIEMAT’s solar research facility,
which is located in southeastern Spain. In 1990, through the EU-DGXII “Access to Large
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Installation Program”, the PSA designed and erected a large solar detoxification facility in cooperation with some relevant European photochemical research groups and the first solar tests
were successfully carried out.
Due to reasonable initial expectations for the technical feasibility of solar detoxification, in
1994, CIEMAT defined Solar Chemistry research as one of its areas of activity, focusing
mainly on water and gas-phase solar detoxification processes and applications. This research
activity was later transformed, in 1998, into a formal CIEMAT Research Project with the
more specific scientific and technological objectives of developing and transferring a feasible
solar detoxification technology to industry. The main project objective is the development of
a technology that would make the use of solar photons in environmental chemical
applications in general, and solar detoxification processes in particular, technically and
economically viable. This is to be done by assessing the scientific and technological bases
that make possible an engineering approach to photochemical processes using solar radiation
at pre-industrial scale. Specific project objectives are the following:
− Development and optimisation of solar water detoxification technology for the treatment
of hazardous non-biodegradable contaminants in industrial wastewater.
− Development and optimisation of gas-phase solar detoxification technology (including
engineering, photoreactor and catalyst) to the photocatalytic detoxification of VOCs from
industrial gaseous emissions.
− Assessment of technical and economic viability of other Solar Chemistry processes with
potential application in Spain and other countries with similar characteristics, such as high
temperature solar detoxification of hazardous wastes, solar reforming, solar gasification of
biomass wastes, solar synthesis of fine chemicals, etc).
An important part of the contents of this book is the result of CIEMAT activities in solar
detoxification of hazardous water contaminants during recent years.
During the entire period, a major source of scientific background has come from the EC
“Access to Large-Scale Scientific Installations” (1990-1993), “Human Capital and Mobility”
(1994-1995) and “Training and Mobility of Researchers” (1996-1998). These programs have
facilitated PSA Solar Detoxification Facility access for many relevant European universities
and other research groups and have made possible a continuous exchange of ideas leading to
photochemical process improvement. In addition to this scientific database, a large number of
national and international initiatives in solar detoxification, supported by Spanish and
European research programs, have provided CIEMAT an important complementary
technological background. Some important initiatives are still underway.
Industrial participation is also a fundamental pillar of the overall CIEMAT activity in solar
detoxification. Since 1993, it has collaborated with many Spanish companies in a large
number of small projects that have made it possible to test the feasibility of photocatalytic
degradation of industrial waste water and identify the most suitable targets. Moreover, these
tests have been used to verify the continuous modification and improvement of the solar
technology. Industrial collaboration with the CIEMAT Solar Detoxification project has
increased significantly since 1997, with the development of several important projects and
demonstration initiatives. Sections 9.2, 9.4 and 9.6 of this book are examples of this
collaboration.
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It may be affirmed that the result of this combined photochemical research from university
and industrial collaboration in hardware development and testing has lead to the current state
of the art of the solar detoxification technology, which this book has tried to reflect.
10.5 GUIDELINES TO SUCCESSFUL WATER TREATMENT PROJECTS IN
DEVELOPING COUNTRIES
Finally, because of the high rate of failure of water system projects in developing countries,
most of which are especially suitable for such applications since they are in the “sun belt”,
some guidelines for water treatment projects to be carried out in those countries are included
here as a complement to the previous sections.
Twenty guidelines indicating the requirements for water projects may be found in the
document Lessons Learned in Water, Sanitation and Health (reference 6). As these guidelines
are basically addressed to water and sanitation projects funded by international aid, some
important conclusions may be arrived at concerning solar detoxification water treatment
projects. These and other guidelines were obtained as the result of many years of experience
and underline the necessity of promoting co-operation with local authorities and institutions,
supporting local plans and requirements, rather than promoting the most convenient project
dictated by the developed country. Adequate information, training and local skill must also be
built up are to guarantee the long-term sustainability of any facility installed.
From such guidelines as these, lessons learned and general recommendations for addressing
water-related projects in developing countries, it may be affirmed that special emphasis must
be placed on training local people and involving local institutions for collaborative projects on
solar detoxification. If local technical or managerial skills are inadequate, the success of any
renewable energy project would be difficult, and this is more so in the case of solar
detoxification technologies that require adequate previous training. Also, the collaboration of
local institutions is very important in itself since any water treatment project is usually closely
related to sanitary and health issues.
A typical mistake is to focus the project on construction, forgetting the necessary previsions
and provisions to ensure the technical and financial sustainability of the renewable technology
installation. This issue must be foreseen from the beginning of the project. Therefore, it is
highly recommended that projects financed by foreign aid be designed in such a way that their
operation and maintenance costs as well as part of the initial construction cost can be borne by
the user or users.
Another relevant recommendation is that projects should not promote dependency on foreign
aid or foreign technical assistance. One possible way to avoid this when innovative
technologies, such as solar detoxification, are implemented is to promote the development of
the private sector on the area. Local companies, acting as interface between the users and the
foreign engineering company, could provide the necessary technical assistance and also
promote new projects and initiatives at local level.
As contaminated drinking water is a typical problem, water purification and disinfection
seems to be a solar detoxification technology application of interest in developing countries.
Water contaminants, the main cause of diseases in developing countries, have been divided
into five categories by the World Health Organisation (WHO): biological, inorganic, organic,
aesthetic and radioactive contaminants. With regard to organic constituents of health
significance, main WHO recommendation is to encourage efforts to protect water sources
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from organic contamination, as their treatment tends to be complicated. Whenever possible,
these industrial and agricultural contaminants must be treated at their source. Solar
detoxification is always an alternative to be considered for this.
Among the denominated Advanced Oxidation Processes (AOPs), based on catalytic and
photochemical techniques, Solar Detoxification has become especially attractive to the
treatment of water contaminants. This has been due to the synergistic combination of solving
difficult environmental problems by using solar energy with the possibilities of creating new
jobs and activities. Solar Detoxification could be considered, then, as a good example of the
concept of sustainable development which, as sooner the better, mankind must achieve.
SUMMARY OF THE CHAPTER
Along this chapter, a review of different existing mechanisms to promote international
collaboration in the subject of solar detoxification has been made: the International Energy
Agency SolarPACES project, the Research Programmes of the European Commission and the
Latin American CYTED network. All three mechanisms are currently (1999) promoting
and/or supporting activities and projects on solar detoxification topics with important
international collaboration. United States and Spain were the countries with more activity
during the nineties on solar detoxification technology development, resulting in the Present
State of the Art of the technology. A brief review of the governmental activities of both
countries is also provided within the chapter. International collaboration with developing
countries must be addressed with special care due to the high rate of failure of water related
projects. Several general recommendations and lesson learned, from the wide experience of
many international organisations, are particularised in the case of solar detoxification
initiatives, when addressed to be implemented at developing countries.
BIBLIOGRAPHY AND REFERENCES
1. Blake, D. M.; Carlson-Boyd, L. E.; Lee Recca, L.; Kissell, G. “Photochemical Pollution
Control: The Final Report of the Solar Industrial Program. Solar Detoxification Project”.
U.S. Department of Energy. NREL/SANDIA read-only compact disk (CD-ROM). 1997.
2. IEA. “Implementing Agreement for the establishment of a project on Solar Power and
Chemical Energy Systems (SolarPACES)”. International Energy Agency. 1996 update of
1977 document.
3. Niewoehner, J.; Larson, R.; Azrag, E.; Hailu, T.; Horner, J.; VanArsdale, P.
“Opportunities for Renewable Energy Technologies in Water Supply in Developing
Country Villages”. National Renewable Energy Laboratory. NREL/SR-430-22359. 1997.
4. SolarPACES. “Towards the 21st Century, IEA/SolarPACES Strategic Plan”. SolarPACES
Brochure, March 1996.
5. SolarPACES. “Solar Thermal Power and Solar Chemical Energy Systems, SolarPACES
Program of the International Energy Agency”. SolarPACES Brochure, Birmingham,
United Kingdom, September 1994, and 1998 update.
6. WASH. “Lessons Learned in Water, Sanitation, and Health; Thirteen Years of Experience
in Developing Countries”. Water and Sanitation for Health Project (WASH). Updated
Edition; Alexandria, VA. 1993.
SELF-ASSESSMENT QUESTIONS
PART A. True or False?
1. IEA headquarters are located in London.
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2. SolarPACES is one of the more than 60 Implementing Agreements of the International
Energy Agency.
3. The PSA was built during the initial stages of the SolarPACES Project to test and evaluate
the different solar technologies to the electricity production existing at that moment.
4. The assistance to the SolarPACES periodic meetings is restricted to the Executive
Committee members, Operating Agents and National Co-ordinators from the different
country members.
5. The Fifth Framework Programme of the European Union (1998-2002) consists of seven
Specific Programmes, of which four are Thematic Programmes and three are Horizontal
Programmes.
6. “Sustainable management and quality of water” is one of the four key actions of the
environment section of the EU-FP5 “Energy, environment and sustainable development”
Work programme.
7. Participation from institutions, organisations and companies from non-EU countries is
possible in all EU-FP5 Programmes.
8. An application for an international Cupertino training bursary (EU-FP5 Programmes) can
be made to any of the programmes, independently of the submission of a joint research
proposal or concerted action initiative.
9. CYTED is a Latin American network of governmental research organisations to promote,
support and reinforce national research initiatives.
10. When addressing international collaborative projects to be implemented at developing
countries it is highly recommended to get information from the experience of similar or
related initiatives previously implemented in the same area.
PART B.
1. How would you define an Implementing Agreement of the International Energy Agency?
2. What is SolarPACES?
3. Indicate the initial members of the present stage of the SolarPACES project, initiated in
1991.
4. How many countries have signed the SolarPACES Implementing Agreement in 1999?
5. Indicate the different work areas defined at the SolarPACES Implementing Agreement.
6. Which specific sector of SolarPACES Implementing Agreement includes the research
activities on Solar Detoxification?
7. Indicate the name of the thematic programme, within the EU-FP5, more directly related
with environmental research.
8. Indicate the name of the Horizontal Programme, within the EU-FP5, specifically
addressed to promote International Collaboration.
9. Indicate the type of actions normally supported by the different EU-FP5 Programmes.
10. Indicate the different shared-cost actions allowed within the different EU-FP5
Programmes.
ANSWERS
Part A
1. False; 2. True; 3. True; 4. False; 5. True; 6. True; 7. True; 8. False; 9. True; 10. True.
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Part B
1. An “Implementing Agreement” is a specific contract between several IEA members to
collaborate in the research, development and application on a determined energy related
issue.
2. SolarPACES (Solar Power And Chemical Energy Systems) is one of the IEA
Implementing Agreements focused on the development and marketing of systems based
on solar technologies.
3. Germany, Spain, Switzerland and the United Stated.
4. Twelve countries (Australia, Brazil, Egypt, France, Germany, Israel, Mexico, Russia,
Spain, Switzerland, United Kingdom and United States) plus the European Commission
(DG XII and DG XVII).
5. Three on-going Tasks or thematic areas of work are defined within the SolarPACES
Implementing Agreement:
− Task I; Concentrating Solar Energy Power Systems.
− Task II; Solar Chemistry Research.
− Task III; Solar Technology and Applications.
6. The Sector II.3
7. Energy, environment and sustainable development.
8. Confirming the international role of Community research.
9. Shared-cost actions. Training fellowships. Research training networks and thematic
networks. Concerted actions. Accompanying measures.
10. The different shared-cost actions allowed within the EU-FP5 Programmes are:
− Research and technological development (R&D) projects.
− Demonstration projects.
− Combined R&D and demonstration projects.
− Support for access to research infrastructures.
− “SME Co-operative” research projects.
− “SME Exploratory” awards.
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Solar Detoxification
by
Julian Blanco Galvez, Head of Solar Chemistry
and
Sixto Malato Rodriguez, Researcher in the Solar Chemistry Area,
Plataforma Solar de Almeria
Spain
SYNOPSIS
AIMS
The main object of the book is the translation of scientific,
technological and engineering knowledge and experiences to make
possible solar applications of water treatment within the "solar
belt" of the world. As the book will define the necessary boundary
conditions and the limits of the solar photocatalytic processes, it
is the author’s main objective that, after its lecture, skilled people
could arrange the necessary infrastructure to carry out not only
similar or related applications as the explained within the book,
but also complete different ones.
SCOPE OF THE BOOK
The book is divided into two parts, with five chapters each. First
one addresses the theory and fundamentals of the water
decontamination by means of solar energy. The objective of this
part is to provide enough background to the reader for the second
part of the book, addresses the practical applications and systems
engineering of the process.
LEVEL OF THE BOOK
The book is basically descriptive and available to a very wide range
of students. Also, an important quantity of drawings, graphics and
photos will make it more pleasant besides its technical and
scientific rigor. Approximately 75% of the book could be
considered descriptive with a 25% of a more deepen study of
specific subjects.
PRE-REQUISITE KNOWLEDGE OF THE READER
The reader of the book must be a technician with university or
bachelor degree, or a technical university student. Nevertheless,
as the book will be written in a wide descriptive way since the
beginning of the different subjects to be treated, no previous
specific knowledge will be required (such as in solar or
photocatalysis matters).
MOTIVATION FOR WRITING THE BOOK
Up to now, there not exist a single book compiling the extensive
work performed on the engineering and applications of the solar
detoxification process, besides the wide number of papers and
articles published on the subject. Our main motivation is the
compilation, in a comprehensive and extensive way, of all this
work making it accessible not only to people interested in solar
and photocatalytic applications, but also to all people interested
into learning how environmental technology could help to solve
environmental problems in general.