Environmental Chemistry and Analysis
Prof. M.S.Subramanian
MODULE 7.3
Reactions and fate of Hazardous wastes
Segregation of hazardous wastes
3
Transport of hazardous wastes
5
Reactions of hazardous waste
5
Hazardous wastes in the geosphere
6
Hazardous wastes in the hydrosphere
7
Hazardous wastes in the atmosphere
10
Hazardous wastes in the biosphere
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
MODULE 7.3
Reactions and fate of Hazardous wastes
A hazardous substance is defined according to Resource Conservation
and Recovery Act (RCRA) as any solid or combination of solids that because of
its quantity, concentration, or physical, chemical, or infectious characteristics,
may cause or significantly contribute to an increase in mortality or an increase in
serious irreversible or incapacitating reversible illness, or pose a substantial
present or potential hazard to human health or the environment when improperly
managed.
A hazardous waste may be defined as a hazardous substance that
1. Is discarded, accumulated, stored , or physically, chemically, or biologically
treated prior to being discarded; or
2. Has served its original intended use and sometimes is discarded; or
3. Is a manufacturing or mining by-product and sometimes is discarded.
The hazardous substance and waste can be identified by testing to determine if it
possesses any one of the following four characteristics.
1. Ignitibility, which identifies wastes that pose a fire hazard during routine
management. Fires not only pose immediate dangers of heat and smoke, but
also can spread harmful particles over wide areas.
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
2. Corrosivity, which identifies wastes requiring special containers because of
their ability to corrode standard materials, or requiring segregation from other
wastes because of their ability to dissolve toxic contaminants.
3. reactivity (or explosiveness) which identifies wastes that, during routine
management, tend to react spontaneously, to react vigorously with air or water,
to be unstable to shock or heat, to generate toxic gases or to explode.
4. toxicity, which identifies wastes that, when improperly managed, may release
toxicants in sufficient quantities to pose a substantial hazard to human health or
the environment.
The effect of toxic chemicals in the environment can produce wide range
of health effects. They can be acute, such as responses to hydrogen sulphide, or
they can be chronic, such as cancer or pulmonary fibrosis from asbestos
exposure. Table 1 lists some major health effects categories which are of
concern.
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
Table 1 Health effects associated with pollutant overdoses
Observed Health
Sample producing
effect
the effect
Cancer
Vinyl chloride
Types of responses
Tumors, death
Irritations, scarring of
Fibrosis
Cotton dust
lungs, emphysema,
pneumonia
Asphyxiation
Hydrogen sulphide
Dizziness, nausea,
disorientation, death
Reproductive effects due
Mutagenesis
DDT
to chromosomal
changes,
Teratogenesis
Dimethyl acetimide
Systemic poisoning
Lead
Birth defects, fatal death
organ or brain damage,
anemia, death
The purpose of risk assessment is to insure with a reasonable margin of
safety, that human health effects do not result from exposures caused by
production or waste management practices.
Segregation of hazardous wastes:
The hazardous waste materials are first segregated based upon their
physical forms such as organic materials, aqueous materials and sludges. These
forms determine the course of action that would be taken in the treatment and
disposal of these wastes. The level of segregation of hazardous wastes (fig.1)
that should be followed is very important in the treatment, storage and disposal of
different kinds of wastes. It is relatively easy to deal with wastes that are highly
segregated.
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
Segregated waste
Spent halogenated
solvents from
cleaning parts
Partially mixed waste unsegregated waste
Mixed organic
materials containing
little or no water
Spent hydrocarbon
solvents
Dilute sludge
consisting of mixed
organic and
inorganic Wastes
Waste solution from
electroplating
Spent steel pickling
liquor
Dilute mixed
inorganic sludge
Waste water
treatment sludge
from pigment
production
Spent aqueous
chromate sludge
Fig1. Illustration of waste segregation
(Redrawn by permission of Lewis Publishers, Chelsea, Michigan 48118, USA from
Fundamentals of Environmental Chemistry, S.E.Manahan, p.661,1993)
A waste that has been concentrated is generally much easier and more
economical to handle than the one that is dispersed in a large quantity of water
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
or soil. Dealing with hazardous wastes is facilitated when the original quantities
of wastes are minimised and the wastes remain separated and concentrated.
Transport of hazardous wastes:
The physical properties of hazardous wastes such as volatility and
solubility contribute to a large extent to their transport in the environment. Highly
volatile hazardous wastes are more likely to be transported through the
atmosphere and the more soluble ones will be carried by water.
Again the compound volatility determines the distribution of hazardous
waste constituent between atmosphere, geosphere or hydrosphere. Usually in
the hydrosphere, and often in soil, hazardous waste constituents are dissolved in
water; therefore, the tendency of water to hold the constituent is a factor of its
mobility. For example, although ethylalcohol has a higher evaporation rate and
lower boiling temperature than toluene, the vapour of the latter is more readily
evolved from the soil because of its limited solubility in water in comparison with
ethanol.
Chemical factors also contribute to the transport of wastes. For example
the clay minerals in the soil tend to bind some elements more strongly (cadmium,
mercury, lead and zinc); some elements moderately (potassium,magnesium,
iron, silicon and NH4+ ) and some other elements (sodium, calcium, manganese
and boron) very poorly. It should be noted that retention of iron and manganese
is a strong function of their oxidation state in that the reduced forms of Fe and Mn
are poorly retained, whereas the oxidised forms of Fe2O3• xH2O and MnO2 are
very insoluble and stay on soil as solids.
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
Reactions of hazardous waste:
The hazardous wastes also undergo chemical reactions with the
environment constituent to give rise to secondary pollutants which may be even
more harmful. An example (which has already been discussed) is the oxidation of
primary pollutant sulphur dioxide,
2SO2 + O2 + 2H2O → 2H2SO4 ………………………………………(1)
(Several steps and intermediates)
to yield secondary pollutant sulphuric acid, the important constituent of acid rain.
Hazardous wastes in the geosphere:
The primary environmental concern regarding hazardous wastes in the
geosphere is that it may contaminate ground water acquifiers through leakage
from wastes. The hazardous waste leachate from landfill, can be attennuated by
soil or rock by various sorption mechanisms. The distribution of the solute
between ground water and leachate is expressed by the term distribution
coefficient, which is the ratio of the concentration of the solute in the solid phase
to that of the concentration solute in water. The degree of attenuation depends
on the surface area of the solid and the content of the organic matter (humus).
The presence of hydrous metal oxides and the chemical characteristics of the
leachate also affect the attenuation to a very large extent.
For example the sorption of metal ion by soil in acidic leachate is very
poor, since precipitation reactions such as the one below.
M2 + + 2OH → M(OH)2 (s) ………………………………………..(2)
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
are reversed in acid.
M(OH)2 (s) + 2H+ → M2 + + 2H2O ………………………………………….(3)
Also organic solvents present in leachate tend to present the sorption of
the organic hazardous waste species by soil.
Disposal of nuclear wastes along with chelating agents used for
decontamination can result in the decrease in sorption of the metal ions by the
soil. For example the disposal of intermediate-level nuclear wastes containing
60
Co along with salts of EDTA accompanied by naturally occurring humic acid
resulted in the desorption of the metal ion from the soil. In the absence of
chelates, the following sorption mechanism occurs in the soil.
2 Soil} − H+ + Co2 + → (Soil} − )2 Co2 + + 2H+ ……………………………...(4)
Co2 + + 2OH− → Co(OH)2 (s) …………………………………………..…(5)
However in the presence of chelate the metal cations can be converted to
complex anions CoY2-(where Y4- is EDTA) which are not strongly retained by the
negatively charged functional groups in the soil.
Hazardous wastes in the hydrosphere:
The main sources for hazardous wastes in the hydrosphere are land fill,
precipitation from the atmosphere with rain fall,deliberate release to streams and
bodies of water, run off from soil, and mobilisation from sediments.
Different physical, chemical and biological processes determine the
transformation and ultimate fates of hazardous chemical species in the
hydrosphere. They are precipitation reactions which involve the aggregation of
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
colloidal particles suspended in water; oxidation reduction reactions which are
generally mediated by microorganisms; hydrolysis reactions which cleave the
molecule with the addition of water; sorption processes involving the adsorption
of hazardous solutes on sediments, suspended minerals and organic matter,
photolysis reactions involving degradation and miscellaneous reactions.
The hydrolysis of waste containing acetic anhydride is shown by the following
equation:
H3C C = 0
2H3CCOOH............(6)
0 +HOH
H3C C = 0
Acetic anhydride reacts with water including the water in the skin. It is this
great affinity for water that makes it hazardous. Once in the aqueous
environment, acetic anhydride is converted vary rapidly to essentially harmless
acetic acid. The rates at which compounds hydrolyse vary widely and many
compounds such as ethers,esters hydrolyse very slowly.
Sorption processes quite often remove low level hazardous
materials. Many heavy metals are sorbed by or co-precipitated with hydrated
iron(III) oxide (Fe2O3.xH2O) or managanese (IV) oxide (MnO2.xH2O) . Oxidationreduction reaction are very important means of transformation of hazardous
wastes in water.
Precipitation is one of the most common means of isolating hazardous
components from unsegregated wastes.. More complicated species generally
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
result when precipitates are formed in the aquatic environment. For example for
lead carbonate more complicated species like 2PbCO3. Pb(OH)2
generally
result. It may be co-precipitated as a minor constituent of some other compound,
or be sorbed by the surface of another solid.
Natural and waste waters contain carbonate, sulphate and
hydroxides as major ions. With cationic pollutants they form precipitates. Basic
salts containing OH - ion along with some other anions are very common in solids
formed
by
precipitation
from
water.
A
typical
example
is
azurite,
2CuCO3. Cu(OH)2. Two or more metal ions may be present in a compound, as in
the case with chalcopyrite, CuFeS2.
Often biochemical processes determine the fate of hazardous
chemical species in the hydrosphere. Micro organisms mediate the biochemical
reactions for the biodegradation of organic wastes. Bacteria produce organic
acids and chelating agents such as citrate, which have the effect of solubilising
hazardous heavy metal ions. Some mobile methylated forms, such as
compounds of methylated arsenic and mercury are produced by bacterial action.
Bacteria produce organic acids and chelating agents, such as citrate, which have
the effect of solubilising hazardous heavy metal ions. Some mobile methylated
forms, such as compounds of methylated arsenic and mercury are produced by
bacterial action. The photolysis reactions are initiated by the absorption of light
but the effect of photolytic processes on the destruction of hazardous wastes in
the hydrosphere is minimal.
Ground water is the most vulnerable part of hydrosphere to
damage from hazardous wastes and can become almost irreversibly
contaminated by the improper land disposal of hazardous wastes.
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
Hazardous wastes in the atmosphere :
Hazardous wastes in the atmosphere can be classified as primary
pollutants and secondary pollutants. Examples of primary pollutants include toxic
organic vapours (vinylchloride), corrosive acid gases (HCl), and toxic inorganic
gases such as H2S which is released by the accidental mixing of HCl from waste
steel pickling liquor and waste metal sulphides.
2HCl + FeS → FeCl2 + H2S(g) ………………………………………………..(7)
Primary pollutants are the most dangerous and pose hazards to workers
involved in disposal or clean up or people living adjacent to the site.
One example of secondary air pollutant
from hazardous waste is the
formation of corrosive sulphuric acid from sulphur dioxide which is released from
the action of waste strong acids on sulfites that subsequently is oxidised in the
atmosphere to corrosive sulphuric acid,
SO2 + 1 O2 + H2O → H2SO4 (aerosol) ……………………………………(8)
2
Similarly nitrogen dioxide (itelf a toxic primary air pollutant) produced by
the reaction of waste nitric acid with reducing agents such as metals is oxidised
to corrosive nitric acid or converted to corrosive nitrate salts:
4 HNO3 + Cu → Cu(NO3 )2 + 2NO2 (g) + 2H2O ……………………………….(9)
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
1
2NO2 (g) + O2 + H2O → 2HNO3 (aerosol) ………………………………...(10)
2
HNO3 (aerosol) + NH3 (g) → NH4NO3 (aerosol) ……………………………(11)
The third example is the photochemical oxidation of
unsaturated organic
compounds with atomic oxygen or hydroxyl radical in air.
R − CH = CH2 + HO⋅ → RCH2 CH2O ⋅ ……………………………………(12)
to yield reactive radicals that participate in chain reactions to eventually yield
ozone, organic oxidants, noxious aldehydes, and other products characteristic of
photochemical smog.
Hazardous wastes in the biosphere:
Microorganisms present in the soil can alter the waste constituents by
organic matter decomposition and assimilation. Biological degradation exploits
the ability of the micro-organisms to produce enzymes that metabolise specific
compounds. Some microorganisms breakdown a contaminant to carbon dioxide,
water and methane, inorganic salts, and/or biomass, from which they derive
carbon and energy for growth. In general, soils have a high capacity for organic
carbon decomposition. Hazardous wastes, however, contain organics that are
not readily degradable. Organics such as chlorinated hydrocarbons are relatively
resistant to decomposition in soils.
The biological conversion of a toxic compound to a less toxic species,
which may still he a relatively complex is called detoxification. An example is the
enzymatic conversions of paraoxon, a highly toxic organophosphate insecticide
to p-nitrophenol which has only about 1/200 of the toxicity of the parent
compound.
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
A common method of degradation for highly chlorinated, highly recalcirant
compound is cometabolism. Cometabolism is the degradation of a compound by
a microorganism that derives no nutritional benefit from that compound but
occurs concurrently with normal metabolic processes. An example of
cometabolism of hazardous wastes is provided by the white rot fungus,
Phanerochaete
chrysosporium.
This
organism,
degrades
a
number
of
organochlorine compounds -including DDT, PCBS, and chlorodioxins - under the
appropriate conditions. The same enzyme system is used by the fungus in
breaking down lignin in plant material under normal conditions.
Naturally occuring compounds undergo biodegradation easily and
anthropogenic compounds resist biodegradation. This is due to the absence of
enzymes that can bring about an initial attack on the compound. Many physical
and chemical characteristics of a compound such as hydrophobicity, solubility,
volatility and affinity for lipids assist biodegradation. Organic structural groups
such as branched carbon chains, ether linkages, metasubstituted benzene rings,
chlorine, amines, methoxy groups, sulfonates, and nitro groups, resist
biodegradation.
Several group of microorganisms either partially or completely degrade
hazardous organic compounds. The Pseudomonas family which are aerobic
bacteria are the most widespread and most adaptable to degradation of synthetic
compounds. These degrade biphenyl, naphthalene, DDT, and many other
compounds. Anaerobic bacteria are known to catabolize biomass through
hydrolytic processes, breaking down proteins lipids and saccharides. They can
reduce nitro compounds to amines, degrade nitrosamines, promote reductive
dechlorination, reduce epoxide groups to alkenes, and break down aromatic
structures. The degradation of variety of organic compounds, including
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Indian Institute of Technology Madras
Environmental Chemistry and Analysis
Prof. M.S.Subramanian
degradation-resistant alkanes, and ligno cellulose are carried by actinomycetes
which are similar to bacteria and fungi. Long-chain and complex hydrocarbons
are attacked more effectively by fungi than by bacteria. Fungi are also successful
in attacking PCB compounds thus making way for its attack by other
microorganisms. Phototropic microorganisms like algae, photosynthetic bacteria
and blue-green algae tend to concentrate organophilic compounds in their lipid
stores and induce photochemical degradation of the stored compounds. Thus all
classes of synthetic organic compounds are atleast partially degraded by
different microorganisms.
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Indian Institute of Technology Madras