CHEMICAL OXIDATION
The use of oxidizing agents without the need of
microorganisms for the reactions to proceed
oxidizing agents : O3, H2O2, Cl2 or HOCl or O2 etc
catalysts : pH , transition metals, light , ..etc
Types of oxidation Processes
1. Conventional oxidation processes
- Use common oxidizing agents such as ozone,
chlorine without production of highly reactive species
2. Oxidation processes carried out at high
temperature/ high pressure
- Produce highly reactive species, hydroxyl radicals
(HO·)
3. Advanced oxidation processes (AOPs)
- Produce highly reactive species, hydroxyl radicals
(HO ·) using oxidizing agents and catalysts
Oxidation rate observed
HO· > O3 > H2O2 > HOCl > ClO2 > KMnO4 > Cl2 > O2
Application
o For nonbiodegradable , toxic organic compounds
o For compounds that inhibit microbial growth
o Effective for destruction of many inorganic compounds
o Elimination of odorous compounds e.g. H2S
Extent of degradation
1. Primary degradation
- a destruction change in the parent compounds
2. Acceptable degradation
- a structural change in the parent compounds to the extent
that toxicity is reduced
3. Ultimate degradation (mineralization)
- conversation of organic carbon to inorganic carbon CO2
4. Unacceptable degradation (fusing)
- a structural change in the parent compounds resulting in
an increase in toxicity
Stoichiometry
Stoichiometric relationship between an oxidant and the
compounds to be treated
→ To estimate the required oxidant dosages for the
treatment
General approach
Express the half reaction for each oxidant in terms
of “free reactive oxygen” O·
Exp. Oxygen , O2
→
2O·
For convenient :
1. Use the electrochemical half-reaction (reduction reaction)
of the oxidant
2. Balance e- with half reaction of water (that gives O· in this
approach)
H2O → O· + 2 H+ + 2 eExp. Ozone
( O· = ½ O2)
O3 + 2H+ + 2e- → O2 + H2O
H2O → O· + 2 H+ + 2 eO3
→ O· + O2
(O· = ½ O2)
For ultimate conversion
CaHbOc + d O·
→
a CO2 + (b/2) H2O
Balance equation using free reactive oxygen produced from
an oxidant
¾¾ moles oxidant required for conversion of a mole of pollutant
Ex. The ultimate conversion of phenol using O3
C6H5OH
+ 14 O3 → 6 CO2 + 14 O2 + 3 H2O
Note : Wastewater contains a wild variety of compounds
Adapt free reactive oxygen approach to the surrogate COD
Oxidant demand = (2/n)(MW/32) COD
COD = Chemical oxygen demand (mg O/L)
MW = Molecular weight of oxidant
n = mole O/mole oxidant
Exp. For O3
O3 stoichiometric demand = (2/1)(48/32) COD
= 3 COD
Exp. Estimate stoichiometric dosage of HClO needed
to treat an effluent having COD = 288 mg/L
HOCl + H+ + 2 e-
→ Cl- + H2O
H2O → O· + 2 H+ + 2 e-
(O· = ½ O2)
1. Mole of O per mole of HOCl
HOCl → Cl- + O· + H+
2. Oxidant demand
Oxidant demand = (2/n)(MW/32) COD
= (2/1) (52/32) (288)
= 936
mg/L
How to choose an oxidant for treatment
the stoichiometric requirement -- yield of free reactive oxygen
( e.g. Moles of O per kg oxidant)
Establish a Ranking of oxidants based on
- cost
- effectiveness of oxidant
Choose a suitable oxidant
Applicability
Apart from O2, most oxidants are expensive and not
competitive with biological treatment for high strength,
large-volume wastewater.
☺ Chemical oxidation processes are designed for toxic,
inhibitory and refractory compounds
- to reduce toxicity
- to increase biodegradability of the parents compounds
at dosages far less than required for ultimate degradation
Coupling chemical oxidation and biological treatment
Oxidants
ƒ O2 (in wet air oxidation)
ƒ Ozone (O3)
ƒ Hydrogen peroxide (H2O2)
ƒ Fenton’s reagent (Fe(II)/ H2O2)
ƒ H2O2/UV
ƒ O3/H2O2
Advanced Oxidation Processes (AOPs)
Ozone :
* powerful oxidant
* unstable gas which decomposes to O2 at
normal temperature
* decomposition is accelerated by contact with
solid surfaces, chemical substances and by heat
* generated by electric discharge of air, O2
The corona discharge device can be fabricated and
configured in many different ways.
The primary feature is to generate a corona between
two electrode surfaces
air or oxygen pass between these electrodes
high-energy electrons bombard gas molecules
gas molecules are ionized forming a light emitting
gaseous plasma referred to as a corona
Reaction of ozone
O3
(g)
O3 (g)
M
O3 (aq)
Added to water
S
OH•
oxidized M
Direct reaction
M
Φ
R•
OH M : organic / inorganic molecule
S : scavenger
Φ : by-products
Indirect reaction
Direct reactions of ozone
Ozone can be electrophilic and nucleophilic
I . Reaction with organic compounds
Examples :
- oxidation of alcohols to aldehydes, organic acids
- substitution of an oxygen atom onto an aromatic ring
- clevage of carbon double bonds
II. Reactions with inorganic compounds
a. Oxidation of ammonia
4 O3 + NH3 →
NO3- +
4 O2 + H3O+
b. Oxidation of iron and manganese
2 Fe3+(aq)
Fe 2+ (aq)
O3
Mn2+ (aq)
O3
2Fe(OH)3 (s)
O2
Mn4+(aq)
O2
c. Oxidation of nitrite
NO2- + O3
MnO2 (s)
NO3- + O2
Indirect reactions of ozone
• Free radicals reactions
Radicals produced will react with compounds in water
Applications
• Phenol and aromatic hydrocarbon destruction
• Color removal
• Drinking water purification
• Water bottling plants
• Swimming pools
• Laundry recycling
Hydrogen peroxide
- Colorless liquid at room temperature
- decomposes easily to give O2
- the decomposition is slow in dilute solution
or in pure solution well conserved in dark
Reactivity of H2O2 : 2 types
• Direct or molecular reactivity
• indirect or radical reactivity
As an oxidant , H2O2 can react with number of organic
and inorganic pollutants. Examples are shown below.
I. Direct reaction
a) Treatment of sulfide , H2S
rapid oxidation : H2S + H2O2 → S + 2H2O
b) Treatment of cyanides
H2O2 + CN- + H+ →
CNO- + H2O
c) Purification of iron and manganes containing groundwater
H2O2 + 2Fe2+ + 2H+ → 2Fe3+ + 2H2O
↓
Fe(OH)3 (S)
II. Direct reaction : Reaction of hydroxyl radicals formed in
H2O2 decomposition
Limit : - oxidation by H2O2 alone is not effective for high
concentration of certain contaminants
e.g. Highly chlorinated compounds
: - Low rate of reaction at reasonable [H2O2]
Advanced Oxidation Processes (AOPs)
“The advanced oxidation process can be used to
decompose many hazardous chemical compounds
to acceptable levels.
The term advanced oxidation processes refers
specifically to processes in which oxidation of organic
contaminants occurs primarily through reactions with
hydroxyl radicals.”
Source: http://www.spartanwatertreatment.com/advanced-oxidation-processes.html
The most widely applied AOPs are:
ƒ Peroxide/ultraviolet light (H2O2/UV),
ƒ Ozone/ultraviolet light (O3/UV),
ƒ Hydrogen peroxide/ozone (H2O2/O3)
ƒ Hydrogen peroxide/ozone/ultraviolet (H2O2/O3/UV)
H2O2/UV process
- Irradiation of UV light ( λ < 365 nm) can break H-O bond
chain reactions
Initiation : H2O2 +
Propagation :
Termination :
hν
H2O2 +
→
2 OHx
ε = 18.6 L mol-1 cm-1
2 OHx → H2O + HOOx
HOOx + HOOx → H2O2 +
O2
O3/UV process
O3 decomposes to produce radicals at high pH values
(photolysis).
Initiation : O3 + OHOH2•
hν
OH2• + O2• -
O2• - + H+
Propagation : O3 + O2• O3 + OH•
Termination : OH2• + OH•
2O2 + OH•
OH2• + O2
O2 + H2O
Radicals produced will react with compounds in water
Source: http://www.spartanwatertreatment.com/advanced-oxidation-UV-Ozone.html
Chlorine
Use in water and wastewater treatment e.g. Color removal
At room temperature, Cl2 can dissolved in water giving
HClO.
Cl2 + H2O
HOCl + H2O
HClO + Cl- + H+
ClO- + H3O+
ka = 1.6-3.2 x 10-8
Hypochlorous acid
hypochlorite ion
• Nature of species in water depends on pH
(HClO, ClO-, H2ClO+, Cl2O)
• Cl2 and its derivatives can react with organic
Matters in water by :
- oxidation reaction
- addition / substitution reaction
==> results in formation of organochlorinated compounds
==> reduce the use of chlorine
Applications of chlorine
•Treatment of CN-
- change CN- to CNO- and finally to N2
• Reactions with NH3, NH4+
- result in formation of chloroamines (NH2Cl, NHCl2, NCl3)
- possible reactions leading to disappearance of
chloroamines :
4 NH2Cl + 3Cl2 + H2O
2 NH2Cl + HClO
NH2Cl + NHCl
N2 + N2O + 10HCl
N2 + H2O + 3HCl
N2 + 3HCl
Overall reactions for total degradation of NH3
2 NH3 + 3 HOCl
N2 + 3H2O + 3HCl
2 NH3 + 3 Cl2
N2 + 6H + 6Cl-
Molar ratio Cl2 : NH3 = 3 : 2
Mass ratio Cl2 : NH3 = 7.6 : 1
CHLORINE DEMAND
amount of Cl2 used in reactions
= Cl2 added to water - total available residual
chlorine remaining at the end of the specific time (Cl2, HClO,
ClO-)
CHLORINE DEMAND
Chlorine residuals
Zero demand residual
Chlorine demand
B
A
D
Cl2 added (mg/L)
Point A : low chlorine residual
= > consumption of chlorine by reducing compounds
e.g. Fe2+, Mn2+, H2S and organic matters
Above point A : Formation of chlorinated org compounds
e.g. chloroamines
Point B : All compounds has been reacted
B – D : some chlorinated org compounds are oxidized
(then chlorine residual is reduced to Cl-)
Point D : Breakpoint (most of chlorinated organic
Compounds are oxidized)
Beyond D : - presence of some resistant chlorinated
compounds
- all added chlorine residual is free
available chlorine (Cl2, HClO, ClO-)
Chemicals
Hypochlorites (salt of HClO)
- NaOCl, Ca(OCl)2 Æ give OCl- use in small installation e.g. Swimming pools
Liquid chlorine
- used in water treatment plant e.g. In US
ClO2 - prepared by following reactions
2 NaClO2 + Cl2
2ClO2 + 2NaCl
2 NaClO2 + 4HCl
4ClO2 + 5NaCl + 2H2O
CHOICE OF USAGE
- Sterilization of water
ClO2 is superior to chlorine in destruction of spores,
bacteria’s, virus and other pathogen organisms
- Industrial water treatment
- chlorine are more reactive than ClO2 and will react with
most organic compounds (ClO2 does not react with NH3 or
NH4+)
References
http://www.iwawaterwiki.org/xwiki/bin/view/Articles/CHEMICALOXIDATIO
NAPPLICATIONSFORINDUSTRIALWASTEWATERS
http://www.lenntech.com/products/chemicals/water-treatment-chemicals.htm
http://openlearn.open.ac.uk/mod/oucontent/view.php?id=399252&section=1.4.7
( 6 Feb 2011)