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11 TRANSPORT, EFFECTS, AND FATES OF HAZARDOUS WASTES

11 TRANSPORT, EFFECTS, AND FATES OF HAZARDOUS WASTES

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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.



Chemical Factors

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

for use.

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.



© 2001 CRC Press LLC



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



© 2001 CRC Press LLC



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



© 2001 CRC Press LLC



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

Cw



(9.13.1)



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



(9.13.2)



where and K F and 1/n are empirical constants.

Landfill



Land

spreading



Stream

Sewer



Waste

lagoon

Water table



Waste plume

Leakage



Leachate



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



© 2001 CRC Press LLC



(9.13.3)



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-metal ions.

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)



(9.13.4)



2Soil}-H+ + Co2+ → (Soil}-)2Co2+ + 2H +



(9.13.5)



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

subsurface conditions.

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.



© 2001 CRC Press LLC



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:



© 2001 CRC Press LLC



O

C O C4H9



O

C OH



+ 2H2O



+ 2HOC4H9



C O C4H9

O



C OH

O



Another example is the hydrolysis of bis(chloromethyl)ether to produce HCl

and formaldehyde:

H

H

Cl C O C Cl + H2O

H

H



O

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

H2O.

• 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

and inorganics



Shallow sediment

stirred by waves

Settling microparticles of biomass and mineral matter

Sorption of hazardous

substances in sediment



Photosynthetic generation

of biomass

Release of hazardouswaste species from

sediments

CO2 CH4

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.



© 2001 CRC Press LLC



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

arsenic.

• 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)



(21.15.1)



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



(9.15.2)



and may react with those containing unsaturated bonds by addition as illustrated by

the following reaction:



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R



H

C



H



C



R

H C



.



.



+ HO

H



H

C OH

H



(9.15.3)



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 •



(9.15.4)



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:

O

H5C2 O P O

H5C2 O



© 2001 CRC Press LLC



NO2

NO2



H2O,{O}



other

+ products



Enzymes

OH



(21.16.1)



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

microorganisms.

• Metabolism is the biochemical process by which biotransformation is

carried out.

• 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

mineralization.

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

to biodegradation.

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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