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11 TRANSPORT, EFFECTS, AND FATES OF HAZARDOUS WASTES
The distribution of hazardous-waste compounds between the atmosphere and the
geosphere or hydrosphere is largely a function of compound volatility. Usually, in
the hydrosphere, and often in soil, hazardous-waste compounds are dissolved in
water; therefore, the tendency of water to hold the compound is a factor in its
mobility. For example, although ethyl alcohol has a higher vapor pressure and lower
boiling temperature (77.8˚C) than toluene (110.6 ˚C), vapor of the latter compound
is more readily evolved from soil because of its limited solubility in water compared
with ethanol, which is totally miscible with water.
As an illustration of chemical factors involved in transport of wastes, consider
cationic inorganic species consisting of common metal ions. These inorganic species
can be divided into three groups based upon their retention by clay minerals.
Elements that tend to be highly retained by clay include cadmium, mercury, lead,
and zinc. Potassium, magnesium, iron, silicon, and NH4+ ions are moderately
retained by clay, whereas sodium, chloride, calcium, manganese, and boron ions are
poorly retained. The retention of the last three elements is probably biased in that
they are leached from clay, so that negative retention (elution) is often observed. It
should be noted, however, that the retention of iron and manganese is a strong
function of oxidation state in that the reduced forms of Mn and Fe are relatively
poorly retained, whereas the oxidized forms of Fe2O3•xH2O and MnO2 are very
insoluble and stay on soil as solids.
Effects of Hazardous Wastes
The effects of hazardous wastes in the environment can be divided among effects
on organisms, effects on materials, and effects on the environment. These are
addressed briefly here and in greater detail in later sections.
The ultimate concern with wastes has to do with their toxic effects on animals,
plants, and microbes. Virtually all hazardous-waste substances are poisonous to a
degree, some extremely so. The toxicity of a waste is a function of many factors,
including the chemical nature of the waste, the matrix in which it is contained,
circumstances of exposure, the species exposed, manner of exposure, degree of
exposure, and time of exposure. The toxicities of hazardous wastes are discussed in
more detail in Chapter 23.
As defined in Section 21.6, many hazardous wastes are corrosive to materials,
usually because of extremes of pH or because of dissolved salt content. Oxidant
wastes can cause combustible substances to burn uncontrollably. Highly reactive
wastes can explode, causing damage to materials and structures. Contamination by
wastes, such as by toxic pesticides in grain, can result in substances’ becoming unfit
In addition to their toxic effects in the biosphere, hazardous wastes can damage
air, water, and soil. Wastes that get into air can cause deterioration of air quality,
either directly or by the formation of secondary pollutants. hazardous-waste compounds dissolved in, suspended in, or floating as surface films on the surface of
water can render it unfit for use and for sustenance of aquatic organisms.
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Soil exposed to hazardous wastes can be severely damaged by alteration of its
physical and chemical properties and ability to support plants. For example, soil
exposed to concentrated brines from petroleum production may become unable to
support plant growth so that the soil becomes extremely susceptible to erosion.
Fates of Hazardous Wastes
The fates of hazardous-waste substances are addressed in more detail in
subsequent sections. As with all environmental pollutants, such substances eventually reach a state of physical and chemical stability, although that may take many
centuries to occur. In some cases, the fate of a hazardous-waste material is a simple
function of its physical properties and surroundings.
The fate of a hazardous-waste substance in water is a function of the substance’s
solubility, density, biodegradability, and chemical reactivity. Dense, water-immiscible liquids may simply sink to the bottoms of bodies of water or aquifers and
accumulate there as “blobs” of liquid. This has happened, for example, with hundreds of tons of PCB wastes that have accumulated in sediments in the Hudson River
in New York State. Biodegradable substances are broken down by bacteria, a
process for which the availability of oxygen is an important variable. Substances that
readily undergo bioaccumulation are taken up by organisms, exchangeable cationic
materials become bound to sediments, and organophilic materials may be sorbed by
organic matter in sediments.
The fates of hazardous-waste substances in the atmosphere are often determined
by photochemical reactions. Ultimately, such substances may be converted to
nonvolatile, insoluble matter and precipitate from the atmosphere onto soil or plants.
21.12 HAZARDOUS WASTES AND THE ANTHROSPHERE
As the part of the environment where humans process substances, the anthrosphere is the source of most hazardous wastes. These materials may come from
manufacturing, transportation activities, agriculture, and any one of a number of
activities in the anthrosphere. Hazardous wastes may be in any physical form and
may include liquids, such as spent halogenated solvents used in degreasing parts;
semisolid sludges, such as those generated from the gravitation separation of oilwater-solids mixtures in petroleum refining; and solids, such as baghouse dusts from
the production of pesticides.
Releases of hazardous wastes from the anthrosphere commonly occur through
incidents such as spills of liquids, accidental discharge of gases or vapors, fires, and
explosions.7 Resource Conservation and Recovery Act (RCRA) regulations designed
to minimize such accidental releases from the anthrosphere and to deal with them
when they occur are contained in 40 CFR 265.31 (Title 40 of the Code of Federal
Regulations, Part 265.31). Under these regulations, hazardous-waste generators are
required to have specified equipment, trained personnel, and procedures that protect
human health in the event of a release, and that facilitate remediation if a release
occurs. An effective means of communication for summoning help and giving
emergency instruction must be available. Also required are firefighting capabilities
including fire extinguishers and adequate water. To deal with spills, a facility is
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required to have on hand absorbents, such as granular vermiculite clay, or absorbents
in the form of pillows or pads. Neutralizing agents for corrosive substances that may
be used should be available as well.
As noted above, hazardous wastes originate in the anthrosphere. However, to a
large extent, they move, have effects, and end up in the anthrosphere as well. Large
quantities of hazardous substances are moved by truck, rail, ship, and pipeline. Spills
and releases from such movement, ranging from minor leaks from small containers
to catastrophic releases of petroleum from wrecked tanker ships, are a common
occurrence. Much effort in the area of environmental protection can be profitably
devoted to minimizing and increasing the safety of the transport of hazardous
substances through the anthrosphere.
In the United States, the transportation of hazardous substances is regulated
through the U.S. Department of Transportation (DOT). One of the ways in which
this is done is through the manifest system of documentation designed to
accomplish the following goals:
• Act as a tracking device to establish responsibility for the generation,
movement, treatment, and disposal of the waste.
• By requiring the manifest to accompany the waste, such as during truck
transport, it provides information regarding appropriate actions to take
during emergencies such as collisions, spills, fires, or explosions.
• Act as the basic documentation for recordkeeping and reporting.
Many of the adverse effects of hazardous substances occur in the anthrosphere.
One of the main examples of such effects occurs as corrosion of materials that are
strongly acidic or basic or that otherwise attack materials. Fire and explosion of
hazardous materials can cause severe damage to anthrospheric infrastructure.
The fate of hazardous materials is often in the anthrosphere. One of the main
examples of a material dispersed in the anthrosphere consists of lead-based anticorrosive paints that are spread on steel structural members.
21.13 HAZARDOUS WASTES IN THE GEOSPHERE
The sources, transport, interactions, and fates of contaminant hazardous wastes
in the geosphere involve a complex scheme, some aspects of which are illustrated in
Figure 21.4. As illustrated in the figure, there are numerous vectors by which
hazardous wastes can get into groundwater. Leachate from a landfill can move as a
waste plume carried along by groundwater, in severe cases draining into a stream or
into an aquifer where it may contaminate well water. Sewers and pipelines may leak
hazardous substances into the geosphere. Such substances seep from waste lagoons
into geological strata, eventually contaminating groundwater. Wastes leaching from
sites where they have been spread on land for disposal or as a means of treatment
can contaminate the geosphere and groundwater. In some cases, wastes are pumped
into deep wells as a means of disposal.
The movement of hazardous-waste constituents in the geosphere is largely by the
action of flowing water in a waste plume, as shown in Figure 21.4. The speed and
degree of waste flow depend upon numerous factors. Hydrologic factors such as
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water gradient and permeability of the solid formations through which the waste
plume moves are important. The rate of flow is usually rather slow, typically several
centimeters per day. An important aspect of the movement of wastes through the
geosphere is attenuation by the mineral strata. This occurs because waste compounds are sorbed to solids by various mechanisms. A measure of the attenuation
can be expressed by a distribution coefficient, Kd,
Kd = Cs
where CS and C W are the equilibrium concentrations of the constituent on solids and
in solution, respectively. This relationship assumes relatively ideal behavior of the
hazardous substance that is partitioned between water and solids (the sorbate). A
more empirical expression is based on the Freundlich equation,
CS = KF Ceq1/n
where and K F and 1/n are empirical constants.
Aquifer containing groundwater
Figure 21.4 Sources and movement of hazardous wastes in the geosphere.
Several important properties of the solid determine the degree of sorption. One
obvious factor is surface area. The chemical nature of the surface is also important.
Among the important chemical factors are presence of sorptive clays, hydrous metal
oxides, and humus (particularly important for the sorption of organic substances).
In general, sorption of hazardous-waste solutes is higher above the water table in
the unsaturated zone of soil. This region tends to have a higher surface area and to
favor aerobic biodegradation processes.
The chemical nature of the leachate is important in sorptive processes of
hazardous substances in the geosphere. Organic solvents or detergents in leachates
will solubilize organic materials, preventing their retention by solids. Acidic leachates tend to dissolve metal oxides,
M(OH)2(s) + 2H+ → M2+ + 2H2O
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thus preventing sorption of metals in insoluble forms. This is a reason that leachates
from municipal landfills, which contain weak organic acids, are particularly prone to
transport metals. Solubilization by acids is particularly important in the movement of
Heavy metals are among the most dangerous hazardous-waste constituents that
are transported through the geosphere. Many factors affect their movement and
attenuation. The temperature, pH, and reducing nature (as expressed by the negative
log of the electron activity, pE) of the solvent medium are important. The nature of
the solids, especially the inorganic and organic chemical functional groups on the
surface, the cation-exchange capacity, and the surface area of the solids largely
determine the attenuation of heavy-metal ions. In addition to being sorbed and
undergoing ion exchange with geospheric solids, heavy metals may undergo
oxidation-reduction processes, precipitate as slightly soluble solids (especially
sulfides), and in some cases, such as occurs with mercury, undergo microbial
methylation reactions that produce mobile organometallic species.
The importance of chelating agents interacting with metals and increasing their
mobilities has been illustrated by the effects of chelating ethylenediaminetetraacetic
acid (EDTA) on the mobility of radioactive heavy metals, especially 60Co.8 The
EDTA and other chelating agents, such as diethylenetriaminepentaacetic acid
(DTPA) and nitrilotriacetic acid (NTA), were used to dissolve metals in the decontamination of radioactive facilities and were codisposed with radioactive materials at
Oak Ridge National Laboratory (Tennessee) during the period 1951–1965.
Unexpectedly high rates of radioactive metal mobility were observed, which was
attributed to the formation of anionic species such as 60CoT - (where T3- is the
chelating NTA anion). Whereas unchelated cationic metal species are strongly
retained on soil by precipitation reactions and cation exchange processes,
Co2+ + 2OH- → Co(OH)2(s)
2Soil}-H+ + Co2+ → (Soil}-)2Co2+ + 2H +
anion bonding processes are very weak, so that the chelated anionic metal species
are not strongly bound. Naturally occurring humic acid chelating agents may also be
involved in the subsurface movement of radioactive metals. It is now generally
accepted that poorly biodegradable, strong chelating agents, of which EDTA is the
prime example, will facilitate transport of metal radionuclides, whereas oxalate and
citrate will not do so because they form relatively weak complexes and are readily
biodegraded.9 Biodegradation of chelating agents tends to be a slow process under
Soil can be severely damaged by hazardous-waste substances. Such materials
may alter the physical and chemical properties of soil and thus its ability to support
plants. Some of the more catastrophic incidents in which soil has been damaged by
exposure to hazardous materials have arisen from soil contamination from SO2
emitted from copper or lead smelters, or from brines from petroleum production.
Both of these contaminants stop the growth of plants and, without the binding effects
of viable plant root systems, topsoil is rapidly lost by erosion.
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Unfortunate cases of the improper disposal of hazardous wastes into the
geosphere have occurred throughout the world. For example, in December 1998 it
was alleged that Formosa Plastics had disposed of 3000 tons of mercury-containing
wastes in Cambodia, the result of byproduct sludge generated by the chloralkali
electrolytic prcess for generating chlorine and sodium hydroxide. Subsequently,
illegal dump sites containing mercury were found in many places in Taiwan, causing
major environmental concerns.10
21.14 HAZARDOUS WASTES IN THE HYDROSPHERE
Hazardous-waste substances can enter the hydrosphere as leachate from waste
landfills, drainage from waste ponds, seepage from sewer lines, or runoff from soil.
Deliberate release into waterways also occurs, and is a particular problem in
countries with lax environmental enforcement. There are, therefore, numerous ways
by which hazardous materials may enter the hydrosphere.
For the most part, the hydrosphere is a dynamic, moving system, so that it
provides perhaps the most important variety of pathways for moving hazardouswaste species in the environment. Once in the hydrosphere, hazardous-waste species
can undergo a number of processes by which they are degraded, retained, and
transformed. These include the common chemical processes of precipitationdissolution, acid-base reactions, hydrolysis, and oxidation-reduction reactions. Also
included is a wide variety of biochemical processes which, in most cases, reduce
hazards, but in some cases, such as the biomethylation of mercury, greatly increase
the risks posed by hazardous wastes.
The unique properties of water have a strong influence on the environmental
chemistry of hazardous wastes in the hydrosphere. Aquatic systems are subject to
constant change. Water moves with groundwater flow, stream flow, and convection
currents. Bodies of water become stratified so that low-oxygen reducing conditions
may prevail in the bottom regions of a body of water, and there is a constant
interaction of the hydrosphere with the other environmental spheres. There is a
continuing exchange of materials between water and the other environmental
spheres. Organisms in water may have a strong influence on even poorly biodegradable hazardous-waste species through bioaccumulation mechanisms.
Figure 21.5 shows some of the pertinent aspects of hazardous-waste materials in
bodies of water, with emphasis upon the strong role played by sediments. An
interesting kind of hazardous-waste material that may accumulate in sediments
consists of dense, water-immiscible liquids that can sink to the bottom of bodies of
water or aquifers and remain there as “blobs” of liquid. Hundreds of tons of PCB
wastes have accumulated in sediments in the Hudson River in New York State and
are the subject of a heated debate regarding how to remediate the problem.
Hazardous-waste species undergo a number of physical, chemical, and biochemical processes in the hydrosphere that strongly influence their effects and fates. The
major ones are listed below:
• Hydrolysis reactions are those in which a molecule is cleaved with addition
of a molecule of H 2O. An example of a hydrolysis reaction is the hydrolysis
of dibutyl phthalate, Hazardous Waste Number U069:
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C O C4H9
C O C4H9
Another example is the hydrolysis of bis(chloromethyl)ether to produce HCl
Cl C O C Cl + H2O
2H C H + 2HCl
Compounds that hydrolyze are normally those, such as esters and acid anhydrides, originally formed by joining two other molecules with the loss of
• Precipitation reactions, such as the formation of insoluble lead sulfide
from soluble lead(II) ion in the anaerobic regions of a body of water:
Pb2+ + HS- → PbS(s) + H+
An important part of the precipitation process is normally aggregation of
the colloidal particles first formed to produce a cohesive mass. Precipitates
are often relatively complicated species, such as the basic salt of lead
carbonate, 2PbCO3•Pb(OH)2. Heavy metals, a common ingredient of
hazardous-waste species precipitated in the hydrosphere, tend to form
hydroxides, carbonates, and sulfates with the OH-, HCO 3-, and SO42- ions
Dissolved hazardous organics
stirred by waves
Settling microparticles of biomass and mineral matter
Sorption of hazardous
substances in sediment
Release of hazardouswaste species from
Deep unstirred sediments
Figure 21.5 Aspects of hazardous wastes in surface water in the hydrosphere. The deep unstirred
sediments are anaerobic and the site of hydrolysis reactions and reductive processes that may act
on hazardous-waste constituents sorbed to the sediment.
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that commonly are present in water, and sulfides are likely to be formed in
bottom regions of bodies of water where sulfide is generated by anaerobic
bacteria. Heavy metals are often coprecipitated as a minor constituent of
some other compound, or are sorbed by the surface of another solid.
• Oxidation-reduction reactions commonly occur with hazardous-waste
materials in the hydrosphere, generally mediated by microorganisms. An
example of such a process is the oxidation of ammonia to toxic nitrite ion
mediated by Nitrosomonas bacteria:
NH3 + 3/2O2 → H+ + NO2-(s) + H2O
• Biochemical processes, which often involve hydrolysis and oxidationreduction reactions. Organic acids and chelating agents, such as citrate,
produced by bacterial action may solubilize heavy metal ions. Bacteria
also produce methylated forms of metals, particularly mercury and
• Photolysis reactions and miscellaneous chemical phenomena. Photolysis
of hazardous-waste compounds in the hydrosphere commonly occurs on
surface films exposed to sunlight on the top of water.
Hazardous-waste compounds have a number of effects on the hydrosphere.
Perhaps the most serious of these is the contamination of groundwater, which in
some cases can be almost irreversible. Waste compounds accumulate in sediments,
such as river or estuary sediments. Hazardous-waste compounds dissolved in,
suspended in, or floating as surface films on the surface of water can render it unfit
for use and for sustenance of aquatic organisms.
Many factors determine the fate of a hazardous-waste substance in water.
Among these are the substance’s solubility, density, biodegradability, and chemical
reactivity. As discussed above and in Section 21.16, biodegradation largely determines the fates of hazardous-waste substances in the hydrosphere. In addition to biodegradation, some substances are concentrated in organisms by bioaccumulation
processes and may become deposited in sediments as a result. Organophilic materials may be sorbed by organic matter in sediments. Cation-exchanging sediments
have the ability to bind cationic species, including cationic metal ions and organics
that form cations.
21.15 HAZARDOUS WASTES IN THE ATMOSPHERE
Hazardous-waste chemicals can enter the atmosphere by evaporation from
hazardous-waste sites, by wind erosion, or by direct release. Hazardous-waste chemicals usually are not evolved in large enough quantities to produce secondary air
pollutants. (Secondary air pollutants are formed by chemical processes in the
atmosphere. Examples are sulfuric acid formed from emissions of sulfur oxides and
oxidizing photochemical smog formed under sunny conditions from nitrogen oxides
and hydrocarbons.) Therefore, species from hazardous-waste sources are usually of
most concern in the atmosphere as primary pollutants emitted in localized areas at a
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hazardous-waste site. Plausible examples of primary air pollutant hazardous-waste
chemicals include corrosive acid gases, particularly HCl; toxic organic vapors, such
as vinyl chloride (U043); and toxic inorganic gases, such as HCN potentially
released by the accidental mixing of waste cyanides:
H2SO4 + 2NaCN → Na2SO4 + 2HCN(g)
Primary air pollutants such as these are almost always of concern only adjacent to
the site or to workers involved in site remediation. One such substance that has been
responsible for fatal poisonings at hazardous-waste sites, usually tanks that are
undergoing cleanup or demolition, is highly toxic hydrogen sulfide gas, H2S.
An important characteristic of a hazardous-waste material that enters the
atmosphere is its pollution potential. This refers to the degree of environmental
threat posed by the substance acting as a primary pollutant, or to its potential to
cause harm from secondary pollutants.
Another characteristic of a hazardous-waste material that determines its threat to
the atmosphere is its residence time, which can be expressed by an estimated
atmospheric half-life, τ1/2. Among the factors that go into estimating atmospheric
half-lives are water solubilities, rainfall levels, and atmospheric mixing rates.
Hazardous-waste compounds in the atmosphere that have significant water
solubilities are commonly removed from the atmosphere by dissolution in water.
The water may be in the form of very small cloud or fog particles or it may be
present as rain droplets.
Some hazardous-waste species in the atmosphere are removed by adsorption
onto aerosol particles. Typically, the adsorption process is rapid so that the lifetime
of the species is that of the aerosol particles (typically a few days). Adsorption onto
solid particles is the most common removal mechanism for highly nonvolatile
constituents such as benzo[a]pyrene.
Dry deposition is the name given to the process by which hazardous-waste
species are removed from the atmosphere by impingement onto soil, water, or plants
on the earth’s surface. These rates are dependent upon the type of substance, the
nature of the surface that they contact, and weather conditions.
A significant number of hazardous-waste substances leave the atmosphere much
more rapidly than predicted by dissolution, adsorption onto particles, and dry
deposition, meaning that chemical processes must be involved. The most important
of these are photochemical reactions, commonly involving hydroxyl radical, HO•.
Other reactive atmospheric species that may act to remove hazardous-waste
compounds are ozone (O3), atomic oxygen (O), peroxyl radicals (HOO•), alkylperoxyl radicals (ROO•), and NO3. Although its concentration in the troposphere is
relatively low, HO• is so reactive that it tends to predominate in the chemical
processes that remove hazardous-waste species from air. Hydroxyl radical undergoes
abstraction reactions that remove H atoms from organic compounds,
R-H + HO• → R• + H 2O
and may react with those containing unsaturated bonds by addition as illustrated by
the following reaction:
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The free radical products are very reactive. They react further to form oxygenated
species, such as aldehydes, ketones, and dehalogenated organics, eventually leading
to the formation of particles or water-soluble materials that are readily scavenged
from the atmosphere.
Direct photodissociation of hazardous-waste compounds in the atmosphere may
occur by the action of the shorter wavelength light that reaches to the troposphere
and is absorbed by a molecule with a light-absorbing group called a chromophore:
R–X + hν → R• + X •
Among the factors involved in assessing the effectiveness of direct absorption of
light to remove species from the atmosphere are light intensity, quantum yields
(chemical reactions per quantum absorbed), and atmospheric mixing. The
requirement of a suitable chromophore limits direct photolysis as a removal
mechanism for most compounds other than conjugated alkenes, carbonyl compounds, some halides, and some nitrogen compounds, particularly nitro compounds,
all of which commonly occur in hazardous wastes.
21.16 HAZARDOUS WASTES IN THE BIOSPHERE
Microorganisms, bacteria, fungi, and, to a certain extent, protozoa may act metabolically on hazardous-waste substances in the environment. Most of these subtances are anthropogenic (made by human activities), and most are classified as
xenobiotic molecules that are foreign to living systems. Although by their nature
xenobiotic compounds are degradation resistant, almost all classes of them—nonhalogenated alkanes, halogenated alkanes (trichloroethane, dichloromethane), nonhalogenated aryl compounds (benzene, naphthalene, benzo[a]pyrene), halogenated
aryl compounds (hexachlorobenzene, pentachlorophenol), phenols (phenol, cresols),
polychlorinated biphenyls, phthalate esters, and pesticides (chlordane, parathion)—
can be at least partially degraded by various microorganisms.
Bioaccumulation occurs in which wastes are concentrated in the tissue of
organisms. It is an important mechanism by which wastes enter food chains.
Biodegradation occurs when wastes are converted by biological processes to
generally simpler molecules; the complete conversion to simple inorganic species,
such as CO2, NH 3, SO42-, and H2PO4-/HPO4-, is called mineralization. The production of a less toxic product by biochemical processes is called detoxification. An
example is the bioconversion of highly toxic organophosphate paraoxon to pnitrophenol, which is only about l/200 as toxic:
H5C2 O P O
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Microbial Metabolism in Waste Degradation
The following terms and concepts apply to the metabolic processes by which
microorganisms biodegrade hazardous-waste substances:
• Biotransformation is the enzymatic alteration of a substance by
• Metabolism is the biochemical process by which biotransformation is
• Catabolism is an enzymatic process by which more-complex molecules
are broken down into less complex ones.
• Anabolism is an enzymatic process by which simple molecules are
assembled into more-complex biomolecules.
Two major divisions of biochemical metabolism that operate on hazardous-waste
species are aerobic processes that use molecular O 2 as an oxygen source and
anaerobic processes , which make use of another oxidant. For example, when sulfate ion acts as an oxidant (electron receptor) the transformation SO42- → H2S
occurs. (This has the benefit of providing sulfide, which precipitates insoluble metal
sulfides in the presence of hazardous waste heavy metals.) Because molecular
oxygen does not penetrate to such depths, anaerobic processes predominate in the
deep sediments, as shown in Figure 21.5.
For the most part, anthropogenic compounds resist biodegradation much more
strongly than do naturally occurring compounds. Given the nature of xenobiotic
substances, there are very few enzyme systems in microorganisms that act directly
on these substances, especially in making an intial attack on the molecule. Therefore,
most xenobiotic compounds are acted upon by a process called cometabolism,
which occurs concurrently with normal metabolic processes. An interesting example
of cometabolism is provided by the white rot fungus, Phanerochaete
chrysosporium, which has been promoted for the treatment of hazardous organochlorides such as PCBs, DDT, and chlorodioxins. This fungus uses dead wood as a
carbon source and has an enzyme system that breaks down wood lignin, a
degradation-resistant biopolymer that binds the cellulose in wood. Under appropriate
conditions, this enzyme system attacks organochloride compounds and enables their
The susceptibility of a xenobiotic hazardous-waste compound to biodegradation
depends upon its physical and chemical characteristics. Important physical characteristics include water solubility, hydrophobicity (aversion to water), volatility, and
lipophilicity (affinity for lipids). In organic compounds, certain structural groups—
branched carbon chains, ether linkages, meta-substituted benzene rings, chlorine,
amines, methoxy groups, sulfonates, and nitro groups—impart particular resistance
Microorganisms vary in their ability to degrade hazardous-waste compounds;
virtually never does a single microorganism have the ability to completely
mineralize a waste compound. Abundant aerobic bacteria of the Pseudomonas
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