Natural and Enhanced Remediation Systems - Chapter 2 ppt

©2001 CRC Press LLCContaminant and Environmental CharacteristicsCONTENTS 2.1 Introduction2.2 Contaminant Characteristics 2.2.1 Physical/Chemical Properties2.2.1.1 Boiling Point2.2.1.2 Va

Suthersan, Suthan S “Contaminant and Environmental Characteristics”

Natural and Enhanced Remediation Systems

Edited by Suthan S SuthersanBoca Raton: CRC Press LLC, 2001

©2001 CRC Press LLC

Contaminant and Environmental

CharacteristicsCONTENTS

2.1 Introduction2.2 Contaminant Characteristics 2.2.1 Physical/Chemical Properties2.2.1.1 Boiling Point2.2.1.2 Vapor Pressure 2.2.1.3 Henry’s Law Constant 2.2.1.4 Octanol/Water Partition Coefficients2.2.1.5 Solubility in Water

2.2.1.6 Hydrolysis2.2.1.7 Photolytic Reactions in Surface Water2.2.2 Biological Characteristics

2.2.2.1 Cometabolism2.2.2.2 Kinetics of Biodegradation2.3 Environmental Characteristics

2.3.1 Sorption Coefficient 2.3.1.1 Soil Sorption Coefficients2.3.1.2 Factors Affecting Sorption Coefficients2.3.2 Oxidation-Reduction Capacities of Aquifer Solids2.3.2.1 pe and pH

2.3.2.2 REDOX Poise2.3.2.3 REDOX Reaction References

Water is scientifically very different in comparison to other liquids With its rare and distinctive property of being denser as a liquid than as a solid, it is different Water is different in that it is the only chemical compound found naturally in solid, liquid, or gaseous states at ambient conditions Water is sometimes called the universal solvent This is a fitting name, especially when you consider that water is

a powerful reagent, which is capable in time of dissolving everything on earth.

2.1 INTRODUCTION

The primary management goal during remediation of a contaminated site is toobtain closure, that is, to achieve a set of conditions that is considered environmen-tally acceptable and which will ensure that no future action will be required at thesite A substantial ongoing national debate associated with site closure centers onthe definition of “how clean is clean” for contaminated subsurface media The keyissue in this debate is, “What concentration of residual contaminant in the subsurface,particularly adsorbed to the soil, is environmentally acceptable?”

In this context, the term contaminant availability becomes an important concept;

it refers to the rate and extent to which the chemical will be released from thesubsurface into the environment and/or is bioavailable to ecological and humanreceptors The dissemination of a contaminant after its release into the environment

is determined by its partition among the water, soil and sediment, and atmosphericphases, and its degradability via biotic and/or abiotic means These processes deter-mine both the impact and the extent of its dissemination

Within the context of overall site management, measurements of contaminant availability are not intended to replace other approaches, required regulatorily, toachieve site closure; rather, they are meant to broaden the range of options or toolsavailable to environmental professionals This chapter will discuss the basis andparameters for the development of procedures and determination of partitioning,transport, and fate of various types of contaminants in the subsurface These param-eters will also provide the basis for the development of the tools to determinecontaminant availability and incorporate those estimations into a decision framework

to define environmentally acceptable endpoints for the different media In addition,how these parameters and characteristics influence contaminant fate and transportand how they impact remediation system design are woven together in the discus-sions in subsequent chapters

The reactions that contaminants undergo in the natural environment, such as tion, desorption, precipitation, complexation, biodegradation, biotransformation,hydrolysis, oxidation-reduction, and dissolution, are critical in determining their fateand mobility in the subsurface environment Reaction time scales can vary from micro-seconds for many ion association reactions microseconds and milliseconds for someion exchange and sorption reactions, to days, weeks, or months for some microbiallycatalyzed reactions, or years for many mineral solution and crystallization reactions.Both transport and chemical reaction processes can affect the reaction rates inthe subsurface environment Transport processes include: (1) transport in the solutionphase, across a liquid film at the particle/liquid interface (film diffusion), and in

sorp-liquid-filled macropores, all of which are nonactivated diffusion processes and occur

in mobile regions; (2) particle diffusion processes, which include diffusion of sorbateoccluded in micropores (pore diffusion) and along pore-wall surfaces (surface dif-fusion) and diffusion processes in the bulk of the solid, all of which are activateddiffusion processes (Figure 2.1).1 Pore and surface diffusion within the immediateregion can be referred to as intra-aggregate (intraparticle) diffusion and diffusion inthe solid can be called interparticle diffusion The actual chemical reaction at thesurface, e.g., adsorption, is usually instantaneous The slowest of the chemicalreaction and transport process is the ratelimiting reaction

As an introduction to the various organic compounds which end up as inants once discharged into the environment, Table 2.1 gives the basic structure ofthe different compounds

contam-Figure 2.1 Transport processes in solid-liquid soil reactions (adapted from Sparks, 1998).

Liquid (Groundwater)

Transport in the Soil Solution (Macro Pores) Transport Across a Liquid Film at the Solid-Liquid Interface Transport in a Liquid-Filled Macropore

Diffusion of a Sorbate at the Surface of the Solid Diffusion of a Sorbate Occluded in a Micropore Diffusion in the Bulk of the Solid

1 2 3 4 5 6

Br OH

RNH - + 4

R N X

1

Alkane Alkene Alkyne Chloride Bromide Alcohol Ether Amine Quaternary 2

ammonium salt

Ethane Ethene Ethyne Chloroethane Bromomethane Ethanol Ethoxyethane 1-Aminopropane Decyltrimethyl- Ammonium

Ammonium Decyltrimethyl- Propylamine Diethyl ether Ethyl alcohol Methyl bromide Ethyl chloride Acetylene Ethylene Ethane

3

3

CH CH

2 3 CH

CH HC

CH CH Cl2 2

3

CH Br

2 3

CH CH OH

2 3

CH CH OCH CH

2 3

CH CH CH NH

2 3

CH (CH ) N(CH ) Cl

2 3

2 2 + -

Acetic acid

CH CH CH O H CH O

3

Table 2.1 Some Common Functional Groups.

(Continued)

©2001 CRC Press LLC

Table 2.1 (Cont.)

O C

Acetic acid Ethyl ethanoate

Ester OR'

O OR'

C

O

Acetonitrile Nitromethane Methyl mercaptan

Dimethyl disulfide Dimethyl disulfide

Methanethiol Nitromethane Ethanenitrile

(sulfide) Disulfide

Thiol Nitro Nitrile

1

R R R

SH

NO

Example

Common Name IUPAC Name

Formula

General Name General

2

O

C NH

O C

2

3

O C

Acid chloride Cl

O Cl

C

3

O C CH Acid anhydride

O C

O S

O

S

O S R

Dimethyl sulfone Dimethyl sulfone

3

The italicized portion indicates the group.

A primary (1 °) amine; there are also secondary (2°), R NH, and tertiary (3°), R N, amines.

Another name is propanamine.

1

2

©2001 CRC Press LLC

2.2 CONTAMINANT CHARACTERISTICS 2.2.1 Physical/Chemical Properties

2.2.1.1 Boiling Point

The boiling point is defined as the temperature at which a liquid’s vapor pressure equals the pressure of the atmosphere on the liquid.2 If the pressure is exactly 1atmosphere (101,325 Pa), the temperature is referred to as “the normal boiling point.”Pure chemicals have a unique boiling point, and this fact can be used in somelaboratory investigations to check on the identity and/or purity of a material Mixtures

of two or more compounds have a boiling point range

For organic compounds, boiling points range from –162 to over 700∞C, but formost chemicals of interest the boiling points are in the range of 300 to 600∞C.2Having a value for a chemical’s boiling point, whether measured or estimated, issignificant because it defines the uppermost temperatures at which the chemical canexist as a liquid Also, the boiling point itself serves as a rough indicator of volatility,with higher boiling points indicating lower volatility at ambient temperatures Theboiling point is associated with a number of molecular properties and features Mostimportant is molecular weight; boiling points generally increase with this parameter.Next is the strength of the intermolecular bonding because boiling points increasewith increasing bonding strength This bonding, in turn, is associated with processesand properties such as hydrogen bonding, dipole moments, and acid/base behavior

2.2.1.2 Vapor Pressure

The vapor pressure of a chemical is the pressure its vapor exerts in equilibriumwith its liquid or solid phase.2 Vapor pressure’s importance in environmental workresults from its effects on the transport and partitioning of chemicals among theenvironmental media (air, water, and soil) The vapor pressure expresses and controlsthe chemical’s volatility The volatilization of a chemical from the water surface isdetermined by its Henry’s Law Constant, which can be estimated from the ratio of

a chemical’s vapor pressure to its water solubility The volatilization of a chemicalfrom the soil surface is determined largely by its vapor pressure, although this istempered by its sorption to the soil matrix and its Henry’s Law Constant betweenthe soil water content and air

A substance’s vapor pressure determines whether it will occur as a free molecule

in the vapor phase or will be associated with the solid phase For volatile substancesthat boil at or below 100∞C, the vapor pressure is likely to be known, but, for manyhigh-boiling substances with low vapor pressure, the value may be unknown orpoorly known An estimation procedure may be needed to help convert the knownvapor pressure at the normal boiling point (i.e., 1 atmosphere) to the vapor pressure

at the lower temperatures of environmental importance For some of these highboiling compounds, the actual boiling point may also be unknown, since the sub-stance may decompose before it boils

2.2.1.3 Henry’s Law Constant

Along with the octanol-water and octanol-air partition coefficients, the Henry’sLaw Constant determines how a chemical substance will partition among the threeprimary media of accumulation in the environment, namely air, water, and organicmatter present in soils, solids, and biota Volatile organic compounds (VOCs) withlarge values of Henry’s Law Constant evaporate appreciably from soils and water,and their fate and effects are controlled primarily by the rate of evaporation and therate of subsequent atmospheric processes For such chemicals, an accurate value ofthis parameter KAW is essential Even a very low value of KAW for example, 0.001,can be significant and must be known accurately, because the volume of the acces-sible atmosphere is much larger than that of water and soils by at least a factor of1000; thus even a low atmospheric concentration can represent a significant quantity

of chemical Further, the rate of evaporation from soils and water is profoundlyinfluenced by KAW because that process involves diffusion in water and air phases

in series, or in parallel, and the relative concentrations which can be established inthese phases control these diffusion rates.2,3

Accurate values of KAW are thus essential for any assessment of the behavior ofexisting chemicals or prediction of the likely behavior of new chemicals Air-waterpartitioning can be viewed as the determination of the solubility of a gas in water

as a function of pressure, as first studied by William Henry in 1803 A plot ofconcentration or solubility of a chemical in water expressed as mole fraction x, vs.partial pressure of the chemical in the gaseous phase P, is usually linear at low partialpressures, at least for chemicals which are not subject to significant dissociation orassociation in either phase This linearity is expressed as Henry’s Law The Henry’sLaw Constant (H) which in modern SI units has dimensions of Pa/(mol fraction).For environmental purposes, it is more convenient to use concentration units in water

CW of mol /m3 yielding H with dimensions of Pa m3/mol

P (Pa) = H (Pa m3/mol) CW (mol/m3) (2.1)The partial pressure can be converted into a concentration in the air phase CA byinvoking the ideal gas law:

Where n is mols, V is volume (m3), R is the gas constant (8.314 Pa m3/mol K) and

T is absolute temperature (K)

CA = P/RT = (H/RT) CW = KAWCW (2.3)The dimensionless air-water partition coefficient KAW (which can be the ratio in units

of mol/m3 or g/m3 or indeed any quantity/volume combination) is thus (H/RT)

A plot of CA vs CW is thus usually linear with a slope of KAW as Figure 2.2

illustrates For organic chemicals which are sparingly soluble in water, these centrations are limited on one axis by the water solubility and on the other by the

con-maximum achievable concentration in the air phase which corresponds to the vaporpressure, as Figure 2.2 shows To the right of or above the saturation limit, a separateorganic phase is present Strictly speaking, this saturation vapor pressure is that ofthe organic phase saturated with water, not the pure organic phase.2,3

The usefulness of the ratio of the concentration of a solute between water andoctanol as a model for its transport between phases in a physical or biological systemhas long been recognized.2,4,5 It is expressed as P OCT = C O /C W = K OW This ratio isessentially independent of concentration, and is usually given in logarithmic terms(log POCT or log KOW) The importance of bioconcentration in environmental hazardassessment and the utility of this hydrophobic parameter in its prediction led to anintense interest in the measurement of POCT and also its prediction from molecularstructure (So many calculation methods have been published in the last five yearsthat it is not possible to examine them all in detail.)

Solubility in water is one of the most important physical chemical properties of

a substance, having numerous applications to the prediction of its fate and its effects

in the environment It is a direct measurement of hydrophobicity, i.e., the tendency

of water to exclude the substance from solution It can be viewed as the maximumconcentration which an aqueous solution will tolerate before the onset of phaseseparation

Figure 2.2 Description of Henry’s Law Constant.

Slope = Kaw

= H/RT

Substances which are readily soluble in water, such as lower molecular weightalcohols, will dissolve freely in water if accidentally spilled and will tend to remain

in aqueous solution until degraded On the contrary, sparingly soluble substancesdissolve more slowly and, when in solution, have a stronger tendency to partitionout of aqueous solution into other phases They tend to have larger air–water partitioncoefficients or Henry’s Law Constants, and they tend to partition more into solidand biotic phases such as soils, sediments, and fish As a result, it is common tocorrelate partition coefficients from water to those media with solubility in water.Solubility normally is measured by bringing an excess amount of a pure chemicalphase into contact with water at a specified temperature, so that equilibrium isachieved and the aqueous phase concentration reaches a maximum value It is rare

to encounter a single compound as the contaminant present in the groundwater at acontaminant site

(2.4)where,

Ci* = equilibrium solute concentration for component i in the mixture

Ci0 = equilibrium solute concentration for component i as a pure compound

xi = mole fraction of compound i in the mixture

gi = activity coefficient of compound i in the mixture

Possible equilibrium situations may exist, depending on the nature of the ical phase, each of which requires separate theoretical treatment and leads to differentequations for expressing solubility These equations form the basis of the correlationsdiscussed later

chem-Single compound is an immiscible liquid (e.g., Benzene)

Substances such as polychlorinated biphenyls (PCBs) can have activity cients exceeding 1 million Hydrophobicity thus is essentially an indication of themagnitude of g Some predictive methods focus on estimating g, from which solu-bility can be deduced

coeffi-C*i =C xi0 igi

Single compound is a miscible substance (e.g., Ethanol)

If the activity coefficient is relatively small, i.e., < 20, it is likely that the liquid

is miscible with water and no solubility can be measured The relevant descriptor

of hydrophobicity in such cases is the activity coefficient Correlations of otherenvironmental partitioning properties with solubility are then impossible.2

Solubility is a function of temperature because both vapor pressure and g aretemperature dependent Usually g falls with increasing temperature, thus solubilityincreases This implies that the process of dissolution is endothermic Exceptionsare frequent and in some cases, such as benzene, there may be a solubility minimum

as a function of the temperature at which the enthalpy of dissolution is zero.2

Under natural conditions, dissolved organic matter such as humic and fulvicacids frequently increases the apparent solubility This is the result of sorption ofthe chemical to organic matter which is sufficiently low in molecular mass to beretained permanently in solution The true solubility or concentration in the pureaqueous phase probably is not increased The apparent solubility is the sum of thetrue or dissolved concentration and the quantity which is sorbed

The solubility of substances such as carboxylic acids, which dissociate or formions in solution, is also a function of pH, a common example being pentachlorophe-nol Data must thus be at a specific pH Alternatively, the solubility of the parent(nonionic) form may be given, and pKa or pKb given, to permit the ratio of ionic tononionic forms to be calculated as

Hydrolytic processes provide the baseline loss rate for any chemical in anaqueous environment Although various hydrolytic pathways account for significantdegradation of certain classes of organic chemicals, other organic structures arecompletely inert Strictly speaking, hydrolysis should involve only the reactantspecies water provides — that is, H+, OH– and H2O — but the complete pictureincludes analogous reactions and thus the equivalent effects of other chemical speciespresent in the local environment, such as HS– in anaerobic bogs, Cl– in seawater,and various ions in laboratory buffer solutions

Hydrolysis results in reaction products that may be more susceptible to radation, as well as more soluble The likelihood that a halogenated solvent will

biodeg-undergo hydrolysis depends in part on the number of halogen substituents More

halogen substituents on a compound will decrease the chance for hydrolysis reactions

to occur and will therefore decrease the rate of the reaction Hydrolysis rates can

generally be described using first-order kinetics, particularly in groundwater where

water is the dominant nucleophile Bromine substituents are more susceptible to

hydrolysis than chlorine substituents As the number of chlorine atoms in the

mol-ecule increases, dehydrohalogenation may become more important.12,47

Dehydrohalogenation is an elimination reaction involving halogenated alkanes

in which a halogen is removed from one carbon atom, followed by subsequent

removal of a hydrogen atom from an adjacent carbon atom In this two-step reaction

an alkene is produced Although the oxidation state of the compound decreases due

to the removal of a halogen, the loss of a hydrogen atom increases it This results

in no external electron transfer, and there is no net change in the oxidation state of

the reacting molecule.47 Contrary to the patterns observed for hydrolysis, the

like-lihood of dehydrohalogenation increases with the number of halogen constituents

Under normal environmental conditions, monohalogenated aliphatics apparently do

not undergo dehydrohalogenation The compounds 1,1,1-TCA and 1,1,2-TCA are

known to undergo dehydrohalogenation and are transformed to 1,1-DCE, which is

then reductively dechlorinated to VC and ethene Tetrachloroethanes and

pentachlo-roethanes are transformed to TCE and PCE via dehydrohalogenation pathways.47

Methods to predict the hydrolysis rates of organic compounds for use in the

environmental assessment of pollutants have not advanced significantly since the

first edition of the Lyman Handbook.8 Two approaches have been used extensively

to obtain estimates of hydrolytic rate constants for use in environmental systems.2

The first and potentially more precise method is to apply quantitative

structure/activ-ity relationships (QSARs).2,9 To develop such predictive methods, one needs a set

of rate constants for a series of compounds that have systematic variations in structure

and a database of molecular descriptors related to the substituents on the reactant

molecule The second and more widely used method is to compare the target

compound with an analogous compound or compounds containing similar functional

groups and structure, to obtain a less quantitative estimate of the rate constant

Predictive methods can be applied for assessing hydrolysis for simple one-step

reactions where the product distribution is known Generally, however, pathways are

known only for simple molecules Often, for environmental studies, the investigator

is interested in not only the parent compound but also the intermediates and products

Therefore, estimation methods may be required for several reaction pathways

Some preliminary examples of hydrolysis reactions illustrate the very wide range

of reactivity of organic compounds For example, triesters of phosphoric acid

hydro-lyze in near-neutral solution at ambient temperatures with half-lives ranging from

several days to several years,10 whereas the halogenated alkanes such as

tetrachlo-roethane, carbon tetrachloride, and hexachloroethane have half-lives of about 2

hours, 50 years, and 1000 millennia (at pH = 7, and 25ºC), respectively.11,12 On the

other hand, pure hydrocarbons from methane through the PAHs are not hydrolyzed

under any circumstances that are environmentally relevant

Hydrolysis can explain the attenuation of contaminant plumes in aquifers where

the ratio of rate constant to flow rate is sufficiently high Thus 1,1,1-trichloroethane

(TCA) has been observed to disappear from a mixed chlorinated hydrocarbon plume

over time, while trichloroethene and its biodegradation product

cis-1,2-dichloroet-hene persist The hydrolytic loss of organophosphate pesticides in sea water, as

determined from both laboratory and field studies, suggests that these compounds

will not be long-term contaminants despite runoff into streams and, eventually,

the sea

2.2.1.7 Photolytic Reactions in Surface Water

Photolysis (or photolytic reaction) can be defined as any chemical reaction that

occurs only in the presence of light Environmental photoreactions necessarily take

place in the presence of sunlight, which has significant photon fluxes only above

295 nm in the near ultraviolet (UV) range, extending into the infrared region of the

electromagnetic spectrum.2,13 Environmental photoreactions occur in surface waters,

on solid ground, and in the atmosphere, sometimes rapidly enough to make them

the dominant environmental transformation processes for many organic compounds

In the atmosphere, for example, photooxidation, mediated by hydroxyl radical (OH•),

is the dominant removal process for more than 90% of the organic compounds

found there

Photolytic reactions are often complex reactions that not only control the fate

of many chemicals in air and surface water, but also often produce products with

chemical, physical, and biological properties quite different from those of their parent

compounds: more water soluble, less volatile, and less likely to be taken up by biota

Photooxidation removes many potentially harmful chemicals from the environment,

although occasionally more toxic products form in oil slicks and from pesticides.14

Biogeochemical cycling of organic sulfur compounds in marine systems involves

photooxidation on a grand scale in surface waters, as well as in the troposphere.2

Environmental photoreactions can be divided into two broad categories of

reac-tions: direct and indirect A direct photoreaction occurs when a photon is absorbed

by a compound leading to formation of excited or radical species, which can react

in a variety of different ways to form stable products In dilute solution, rate constants

for these reactions are the products of the rate constants for light absorption and the

reaction efficiencies An indirect photoreaction occurs when a sunlight photon is

absorbed by one compound or group of compounds to form oxidants of excited

states, which then react with or transfer energy to other compounds present in the

same environmental compartment to form new products For example, NO2 and O3

in air form hydroxyl radicals (OH•), and humic acids in water form singlet oxygen

and oxyradicals, when they absorb sunlight photons These oxidants react with other

chemicals in thermal (dark) reactions, and the rates for these processes follow simple

bimolecular kinetics

Direct Photoreactions: Only a small proportion of synthetic organic compounds

absorb UV light in the sunlight region of the spectrum (above 295 nm) and then

photolyze at significant rates.13 Most aliphatic and oxygenated compounds, such as

alcohols, acids, esters, and ethers, absorb only in the far UV region (below 220 nm),

and simple benzene derivatives with alkyl groups or one heteroatom substituent

absorb strongly only in the far and middle UV region Nitro or polyhalogenated

benzenes, naphthalene derivatives, polycyclic aromatics and aromatic amines,

nitroalkanes, azaolkanes, ketones, and aldehydes absorb sunlight between 300 and

450 nm; polycyclic and azoaromatics (dyes), as well as quinones, also absorb visible

light, in some cases to beyond 700 nm.2,13

The rate of a direct photoprocess depends only on the product of the rate of light

(photon) absorption by compound C,(IA) and the efficiency with which the absorbed

light is used to effect reaction (quantum yield, Ø):13

(2.8)

Under most environmental conditions, chemicals are present in surface water or

air at low concentrations, so their light absorbing properties lead to simple kinetic

expressions for direct photolysis in water.13

Indirect Photoreactions: Indirect photolysis is most important for compounds

that absorb little or no sunlight Light absorption by chromophores (sensitizers) other

than the compound of interest begin the process, forming intermediate (and transient)

oxidants or excited states that affect chemical changes in the compound of

inter-est.2,15,16 Examples of sensitizers that serve this purpose are dissolved organic matter

(DOM or humic acid) and nitrate ion in water, and ozone and NO2 in the atmosphere

Transient species formed by indirect photoreactions in water include singlet oxygen

and peroxy radicals, both of which are relatively selective and electrophilic As a

result, only electron-rich compounds, such as phenols, furans, aromatic amines,

polycyclic aromatic hydrocarbons (PAHs), and alkyl sulfides can undergo relatively

rapid indirect photoprocesses with these oxidants Nitroaromatics, though not

oxi-dized, appear to be sensitized by triplet DOM or scavenged by solvated electrons

Many of these compounds (e.g., PAHs, nitroaromatics, and aromatic amines) also

undergo rapid direct photoreactions.2,16

By contrast, OH• radical, which dominates tropospheric photochemistry, oxidizes

all classes of organic compounds (except perhalogenated compounds such as PCE),

including alkanes, olefins, alcohols, and simple aromatics.160,166 Aqueous OH•

rad-ical, derived mainly from the photolysis of nitrate ion, plays an important role in

converting marine DOM to simpler carbonyl compounds, even though the average

concentration is extremely low (<2 ¥ 10–8).17 OH• also appears important in degrading

synthetic chemicals in a variety of nitrate-bearing freshwaters, where the OH•

con-centrations appear to be one to two orders of magnitude higher.2,13

In many cases, detailed pathways for forming these oxidants and reductants

remain unclear, but identities of several of the transients are fairly well established.2,13

Transient species are transient because they react rapidly with themselves or with a

variety of natural organic and metal species in natural waters,2 balancing formation

rates to give low average concentrations

Rate dc

dt

= = Efficiency ¥ Photons Absorbed / time =∆ IA

2.2.2 Biological Characteristics

It is generally conceded that biological reactions are of the greatest significance

in determining the fate and persistence of organic compounds in most natural aquaticecosystems It is essential at the start to make a clear distinction between biodegra-dation and biotransformation Biodegradation is a process in which the destruction

of a chemical is accomplished by the action of a living microorganism Duringbiotransformation, on the other hand, only a restricted number of metabolic reactions

is accomplished, and the basic framework of the molecule remains essentiallyintact.18 Even though biodegradation and biotransformation, considered as alterna-tives, are not mutually exclusive

Biodegradation can be categorized into three types that have importance in anecosystem setting:

Primary Biodegradation: biodegradation to the minimum extent necessary to change

the identity of the compound.

Ultimate Biodegradation: biodegradation to water, carbon dioxide, and inorganic

com-pounds (if elements other than C, H, and O are present) This is also called alization Under anaerobic conditions, methane may be formed in addition to carbon dioxide during fermentation reactions.

miner-Acceptable Biodegradation: biodegradation to the minimum extent necessary to remove

some undesirable property of the compound, such as toxicity Conversion of vinyl chloride to ethene is an example and in many instances this can also be considered

biotransformation.

Although biological degradation conceivably might be accomplished by anyliving organism, available information indicates that, by far, the most significantbiological systems involved in ultimate biodegradation of contaminants are bacteriaand fungi Critical and necessary conditions necessary for biodegradation of con-taminants to take place are summarized below:

• A microbial population must exist that has the necessary enzymes to bring about the biodegradation.

• This population must be present in the environment where the contaminant is present.

• The contaminant must be accessible to the microorganisms having the requisite enzymes, and most of the time this requires the contaminant to be available in the dissolved phase.

• If the initial enzyme bringing about the degradation is extracellular, the bonds acted upon by that enzyme must be exposed for the catalytic enzyme to function.

• Should the enzyme catalyzing the initial degradation be intracellular, the molecule must penetrate the surface of the cell to the internal sites where the enzyme acts Alternatively, for the transformation to proceed further, the products of an extra- cellular reaction must penetrate the cell.

• Because the population or biomass of bacteria or fungi acting on many synthetic compounds is initially small, conditions in the environment must be optimum to allow for proliferation of the potentially active microorganisms.

The initial concentration of the microbial population and the contaminant what affects the growth and proliferation; a lag period often occurs between theaddition of a chemical and the onset of biodegradation This lag period, usuallyattributed to the need for acclimation,19,20 could result from enzyme induction, genetransfer or mutation, predation by protozoa, or growth in the population of respon-sible organisms.

some-The initial species present, their relative concentrations, the induction of theirenzymes, and their ability to acclimate once exposed to a chemical are likely to varyconsiderably, depending upon environmental parameters such as temperature, salin-ity, pH, oxygen concentration (aerobic or anaerobic), redox potential, concentrationand nature of various substrates and nutrients, concentration of heavy metals (tox-icity), and effects (synergistic and antagonistic) of associated microflora.21 Many ofthese parameters affect the biodegradation of contaminants in the environment.One important parameter is the chemical substrate concentration A number ofchemicals have biodegradation rates proportional to substrate concentration, butthere are also examples of thresholds and inhibitions.22 Recently, the bioavailability

of the chemical to the catalytic enzyme has been identified as a major factor indetermining biodegradability in nature Several studies have demonstrated that,although a chemical freshly added to soil is biodegraded at a moderate rate, thebiodegradation rate for some chemicals present in the soil sample for a long time

is very low.19 Thus, depending on the chemical, the longer a chemical remains inthe soil, the greater the potential for it to become sequestered and less bioavailable.Some microorganisms are capable of biodegrading contaminants without popu-lation growth In this process, known as “cometabolism,”19 the microorganismdegrades the contaminant from which it derives no carbon or energy; instead, it issustained on other organic substrates and nutrients

2.2.2.1 Cometabolism

The transformation of an organic compound by a microorganism that is unable

to use the substrate as a source of energy or as one of its growth substrate is termed

cometabolism The active populations thus derive no nutritional benefit from the

substrates they cometabolize Energy sufficient to fully sustain growth is not acquiredeven if the conversion is an oxidation and releases energy In addition, the C, N, S,

or P that may be in the molecule is not used as a source of these elements for growth

and energy deriving purposes Because of the prefix co, which often is appended to

a word to indicate that something is done jointly or together (as in copilot orcooperate), there has been some debate regarding the use of the term cometabolism.Specifically, some classical microbiologists argue that the term should be appliedonly to circumstances in which a substrate that is not used for growth is metabolized

in the presence of a second substrate that is used to support multiplication.19ing to this view, the transformation of a substance that is not used as a nutrient orenergy source but which occurs in the absence of a chemical supporting growth

Accord-should be designated by another term, for example, fortuitous metabolism However, the prefix co also has another meaning, namely, “the same or similar.” The latter

usage implies that the cometabolic transformation is similar to some other metabolic

reaction, which is consistent with one explanation for the phenomenon Fortuitousmetabolism is, indeed, a more attractive term because it suggests an explanation forcometabolism, but the term will be used here as in the original definition, if for noother reason than it has gained wide acceptance.

The term cooxidation is sometimes used in studies of pure cultures of bacteria,

referring specifically to oxidations of substrates that do not support growth in thepresence of a second compound that does support multiplication Cooxidation hashistorical precedence in the debate but since it is restricted to oxidation, the worddoes not have sufficient breadth to include many reactions that are not oxidations.19

In summary, two types of reactions called cometabolism take place in the ronment In one, the cometabolized compound is transformed only in the presence

envi-of a second substrate, which indeed may be the compound that supports growth.For heterotrophs, the energy-providing substrate is organic; for autotrophs, it isinorganic In the other type, the compound is metabolized even in the absence of asecond substrate

Important reasons for using the more general definition, and even for maintainingcometabolism as a term apart from bioconversion or biotransformation, are theenvironmental consequences of cometabolism Cometabolic reactions have impacts

in nature that are different from growth-linked biodegradations, and when the formations take place, it is usually totally unclear whether the microorganisms do

trans-or do not have a second substrate available on which they are growing

A large number of chemicals are subject to cometabolism in nature Amongcometabolic conversions that appear to involve a single enzyme, the reactions may

be hydroxylations, oxidations, denitrations, deaminations, hydrolyses, acylations, orcleavages of ether linkages; however, many of the conversions are complex andinvolve several enzymes Some of the unique cometabolic reactions brought about

by bacteria and fungi in nature come as no surprise in view of the vast array ofgrowth linked biological transformations that heterotrophic bacteria and fungi arecapable of in nature An example which has no significant importance in contaminantremoval is the methane monooxygenase of methanotrophic bacteria which is able

to oxidize alkanes, alkenes, secondary alcohols, methylene chloride, chloroform,dialkyl ethers, cycloalkanes, and various aromatic compounds.19

Caution needs to be exercised in concluding that cometabolism is occurringmerely because an organism cannot be isolated from an environment in which achemical is undergoing a biological reaction.19 The isolation of bacteria acting onspecific substrates is usually performed by enriching the organism in a medium whenthe only C source is the test chemical, and the agar medium used to plate theenrichments contains that single organic supplement Yet, many bacteria that areable to grow at the expense of that substrate will not develop in such simple mediabecause they require amino acids, B vitamins, or other growth factors These essentialgrowth factors are not routinely included in such liquid media, and hence bacteriaand fungi needing them fail to proliferate If the only organisms in the environmentable to metabolize a contaminant need these growth factors, no isolate will beobtained, and an erroneous conclusion will be reached that the compound is come-tabolized If a chemical supports the growth of many species, some will undoubtedlyrequire no growth factors (these organisms are called prototrophs), and they will be

enriched and ultimately can be isolated If the compound is acted on by only onespecies, in contrast, it is likely that the responsible organism will need amino acids,

B vitamins, or other growth factors; these species are termed auxotrophs Hence,the failure to isolate a bacterium or fungus capable of using the contaminant as thesole C source for growth is not sufficient evidence for cometabolism

Mechanisms of Cometabolic Reactions: Several reasons have been advanced to

explain cometabolism, that is, why an organic chemical that is a substrate does notsupport growth but is converted to products that accumulate Some of these reasonshave experimental support: (1) the initial enzyme converts the substrate to a productthat is not further transformed by other enzymes in the microorganism to yield themetabolic intermediates ultimately used for biosynthesis and energy production; (2)the initial substrate is transformed to products that inhibit the activity of later enzymes

in mineralization or that suppress growth of the organisms; and (3) the organism needs

a second substrate to bring about some particular reaction

It could be speculated that the first explanation is the most common The basisfor this explanation is the fact that many enzymes act on several structurally relatedsubstrates; thus, an enzyme naturally present in the cell will catalyze reactions andalter synthetic chemicals that are not typical cellular intermediates These enzymesare not absolutely specific for their substrates Consider a normal metabolic sequenceinvolving the conversion of A to B by enzyme a, B to C by enzyme b, and C to D

by enzyme c in a sequence that ultimately yields CO2 energy for biosynthesisreactions and intermediates that are converted to cell constituents.19

(2.9)The first enzyme a may have a low substrate specificity and act on a moleculestructurally similar to A, namely, A.1 The product (B1) would differ from B in thesame way that A differs from A1 However, if enzyme b is unable to act on B1(because the structural features controlling which substrate it modifies differ fromthose controlling the substrate specificity of enzyme a), B1 will accumulate:19

(2.10)

In addition, CO2 and energy will not be generated and, because cellular carbon

is not formed, the organisms do not multiply The formation of B1 is thus entirelyfortuitous.19

In instances where the contaminant concentration is high, cometabolism mayresult from the conversion of the parent compound to toxic products In the sequencejust depicted, if the rate of reaction catalyzed by enzyme a is faster than the processcatalyzed by enzyme b, B will accumulate because it is not destroyed as readily as

it is generated For example, a strain of pseudomonas that grows on benzoate but

not 2-fluorobenzoate converts the latter to fluorinated products that are toxic.23 Theinhibitor that accumulates may affect a single enzyme important for the furthermetabolism of the toxin

AÆ Æ Æ ÆÆÆa B b C c D CO energy cell C2 + + –

Æ

æ Æaæ

A1 B1

In some instances, an organism may not be able to metabolize an organiccompound because it needs a second substrate to bring about a particular reaction.The second substrate may provide something that is present in insufficient supply

in the cells for the reaction to proceed — for example, an electron donor for thetransformation.19

The above explanation is linked to the existence of enzymes acting on more than

a single substrate Many enzymes are not absolutely specific for a single substrate

As a rule, they act on a series of closely related molecules, but some carry out asingle type of reaction on a variety of somewhat dissimilar molecules The followingare examples of single enzymes acting on a range of substrates:

• Methane monoxygenase of methanotrophic bacteria: When grown on methane,

methanol, or formate, these aerobic bacteria are able to cometabolize a large array

of organic molecules, including several major pollutants In each instance, methane monooxygenase is the responsible catalyst Other chlorinated aliphatic hydrocar-

bons transformed by one such methanotroph, Methylosinus trichosporium, are and trans-1,2-dichloroethylene, 1,1-dichloroethylene, 1,2-dichloropropane, and

cis-1,3-dichloropropylene 24 Apparently the same enzyme in other bacteria, after growth on methane, will catalyze the oxidation of n-alkanes with two to eight C atoms, n-alkenes with two to six C atoms, and mono- and dichloroalkanes with five or six C atoms, as well as dialkyl ethers and cycloakanes 19

• Toluene dioxygenase of a number of aerobic bacteria: This enzyme incorporates

both atoms of oxygen from O2 (hence, it is a dioxygenase) into toluene as it catalyzes the first step in the degradation of toluene by bacteria grown on that aromatic hydrocarbon ( Figure 2.3a ) However, that same enzyme has very low specificity and also is able to bring about the degradation of TCE, 19,25,26 to convert 2- and 3-nitrotoluene to the corresponding alcohols, and to hydroxylate the ring

of 4-nitrotoluene 19,27

• Toluene monooxygenase of several aerobic bacteria: Differing from the

dioxyge-nase, this enzyme incorporates only one atom of oxygen from O2 into toluene to give o-cresol ( Figure 2.3b ) However, because of this enzyme, bacteria can come- tabolize TCE, convert 3- and 4-nitrotoluenes to the corresponding benzyl alcohols and benzaldehydes, and add hydroxyl groups to other aromatic compounds 19,28,29

• Oxygenase of propane-utilizing bacteria: Aerobes using propane as C and energy

source for growth also have an oxygenase of broad specificity This enzyme cometabolizes TCE, vinyl chloride, and 1,1-di- and trans- and cis-1,2-dichloroet- hylene and has been recently known to degrade MtBE 19,30

• Ammonia monooxygenase of Nitrosomonas europaea: This bacterium, which is a

chemoautotroph whose energy source in nature is NH4+ and whose C source is

CO2, cometabolizes TCE, 1,1-dichloroethylene, various mono- and nated ethanes, and a variety of monocyclic aromaticcompounds and thioethers, as well as methyl fluoride and dimethyl ether 19,31

polyhaloge-• An alkane hydroxylase hydroxylates a number of alkylbenzenes and linear,

branched, and cyclic alkanes 19,23

• An alkane monooxygenase degrades TCE, vinyl chloride, and dichloroethylenes

Figure 2.3a Reactions catalyzed by toluene dioxygenase (adapted from Alexander, 1999).

H H

HC COOH + HCOOH + O

CI-The organism containing these enzymes may be able to use one of several ofthe enzyme’s substrates for growth However, many of the substrates are transformedbut do not support growth The product of the reactions then accumulates.

Because cometabolism generally leads to a slow degradation of the substrate,attention has been given to enhancing its rate.19 The addition of a number of organiccompounds to the contaminated zone promotes the rate of cometabolism of a number

of chlorinated aliphatic and aromatic compounds and chlorinated phenols, but theresponses to such additions are not predictable No relation is known to exist betweenthe metabolic pathways involved in the degradation of the added mineralized sub-strate and the compound that is cometabolized in these circumstances The addedsubstrates were randomly chosen in these trials, and sometimes they do and some-times they do not stimulate cometabolism In instances in which stimulation occurs,the benefit probably results from an unpredicted increase in the biomass of organ-isms, some of which fortuitously cometabolize the compound of interest

An alternative approach is to add mineralizable compounds that are structurallyanalogous to the compound whose cometabolism one wishes to promote Presum-ably, the microorganism that grows on the mineralizable compound containsenzymes transforming the analogous molecule that is cometabolized This largerbiomass thus has more of the degradative enzyme than is present in the unsupple-mented water or soil This method of analogue enrichment has been used to enhancethe cometabolism of PCBs by additions of biphenyl The unchlorinated biphenylwas selected for addition to soil since it is mineralizable, nontoxic, and serves as a

C source for microorganisms that are able to cometabolize PCBs.19

Analogue enrichment is a procedure that is similar to the usual means of isolatingbacteria that can cometabolize a compound The enrichment culture contains a Csource that supports growth, and the pure cultures thus obtained also cometabolizestructurally related compounds that would not support growth For example, bacteriaisolated on diphenylmethane and containing enzymes to degrade it also cometabolizechlorinated diphenylmethanes Many of the latter do not sustain growth.19

2.2.2.2 Kinetics of Biodegradation

Impacts of the Environment: Soil, water, sediment, and wastewater

environ-ments have different microbial populations and different available nutrients whichmay affect considerably the rate of biodegradation For example, wastewater

Figure 2.3b Reactions catalyzed by toluene monooxygenase (adapted from Alexander,

treatment plants may have high levels of nutrients and high microbial populationsthat may have been pre-exposed to a contaminant (acclimated microbial population),but the contact (retention) time is relatively short.

In contrast, marine waters are usually fairly low in nitrogen and phosphorouswhich may limit the biodegradation of chemicals (e.g., oil spills) Sediment oftenhas high levels of organic nutrients, but often is anaerobic, while surface waterstend, by comparison, to have low levels of organic nutrients.18 Digestor sludge fromwastewater treatment plants has high organic nutrients and is anaerobic Surfacesoils have high concentrations of organic nutrients (depending on the type of soil),but this usually decreases with depth

In the past, it was believed that groundwater aquifers were devoid of microbiallife However, a number of studies have demonstrated that microorganisms are quiteplentiful in certain aquifers, and, in some instances, the bacterial concentration andactivity in aquifers may be higher than those in surface waters.19,35 In addition,availability of the chemical to the microbial population can be affected considerably

by the conditions of the microenvironment (e.g., organic concentration or claycontent may bind the chemical tightly)

A contaminant may become less available or essentially unavailable for radation if it enters or is deposited in a micropore that is inaccessible even to themicroorganisms These micropores may be filled entirely with water, as in sediments

biodeg-or groundwater aquifers, and the contaminant would have to move out of a micropbiodeg-ore

by diffusion to be accessible to bacteria for its destruction The tortuous path thecontaminant molecule must traverse before it gets destroyed dramatically affectsbioavailability if the contaminant not only is physically remote from potentiallyactive microorganisms, but also is strongly sorbed to solid surfaces associated withthat remote micropore.19

Some organic compounds that persist in the subsurface often undergo a timedependent decline in bioavailability Since this process is slow and time dependent

it is appropriately called aging This modification in bioavailability to isms as a result of aging is also called sequestration.19 In the initial period, thecompound gradually disappears as a result of biodegradation, and possibly by othermass removal mechanisms, but little or none of the compound is destroyed after ithas resided in the soil or sediment for some time Witness the finding that although80% of hexachlorobenzene deposited in the early 1970s in a lake bottom sedimentwas dechlorinated in the succeeding 20 years, all sediment cores still contained atleast 40 ppb of hexachlorebenzene This time-dependent change in the rate ofdegradation, which has been observed with a number of insecticides, was the firstline of evidence for sequestration.19 Because these aged molecules are solventextractable, albeit by vigorous treatment, they are presumed to be present in anuncomplexed form and thus considered to be contaminants and subject to the reg-ulations and cleanup standards In addition to the above mentioned contaminants,PAHs with three or more rings, such as phenanthrene, anthracene, fluorene, pyrene,chrysene, and others will also undergo sequestration

microorgan-It has been known for some time that it is increasingly difficult to remove stronglyhydrophobic compounds from soil with mild extractants as the residence time of

those compounds in the soil increases This phenomenon is not restricted to soils;

a similar decline in extractability is witnessed also from sediment samples.36The amount of a contaminant that is sequestered increases with time Expressed

in another way, the percentage of the chemical that is bioavailable diminishes withincreasing persistence This presumably occurs because more of the contaminant isdiffusing into inaccessible sites However, after a period of time that varies with thesoil and the compound, sequestration of additional quantities slows and possiblystops The reason for this rate of decline is not presently known.36

Based on the preceding discussion, it can be seen that the rates of biodegradationare likely to vary considerably, depending on the environment to which a contaminant

is released, the type of contaminant(s), and the age of contamination Also, the ratesunder different conditions may vary depending upon the type of chemical structure.For example, nitro aromatic compounds are usually fairly resistant to biodegradationunder aerobic conditions but are reduced rapidly to amines under anaerobic condi-tions In contrast, degradation of benzene takes place significantly faster underaerobic conditions than under anaerobic conditions

Structural Effects on Biodegradation: In addition to contaminant

concentra-tion, chemical structure and physical/chemical properties have considerable impact

on the rate and pathways of biodegradation The chemical structure determines thepossible pathways that a substrate may undergo, generally classified as oxidative,reductive, hydrolytic, or conjugative Figure 2.4 provides some examples of commonmicrobial degradation pathways.37 Recently, a computer program was developed thatwill predict the most probable metabolites, and another computer program was alsodeveloped that simulates the biodegradation of synthetic chemicals through thesequential application of plausible biochemical reactions.38,39

Over the years, structure/biodegradability “rules of thumb” have been oped.40,41 Figures 2.4a and b summarize these Some of these structure/biode-gradibility relationships have some biochemical mechanistic underpinings Forexample, highly branched compounds frequently are resistant to biodegradationbecause increased substitution hinders b-oxidation, the process by which alkylchains and fatty acids usually are biodegraded This structural relationship wasdiscovered in the 1950s when detergent scientists found that alkylbenzene sul-fonate (ABS) detergents passed through wastewater treatment plants causing foam-ing problems in rivers and streams This problem was solved by switching fromthe highly branched ABS detergents to linear alkylbenzene sulfonate (LAS) deter-gents, thus illustrating the importance of understanding the relationship betweenstructure and biodegradability

devel-Few other rules of thumb have such mechanistic bases, but there are some generaltrends Functional groups commonly seen by microorganisms in natural productsusually are degraded easily, probably because the microbes have had eons to developthe required enzyme systems in order to gain carbon and energy from the metabolism.Conversely, functional groups less common in nature or newly synthesized by manusually make a chemical more resistant to biodegradation Aromatic substituentsthat are electron withdrawing (e.g., nitro groups and halogens) increase the persis-tence of a chemical, possibly by making it more difficult for enzymes to attack thearomatic ring, whereas electron donating functionalities (e.g., carboxylic acids,phenols, amines) generally increase biodegradation rates

Physical/chemical properties affect the rate of biodegradation mostly by affectingbioavailability Compounds which are sparingly soluble in water tend to be moreresistant to biodegradation, possibly due to an inability to reach the microbial enzymesite, a reduced rate of availability due to solubilization, or sequestration due toadsorption or trapping in inert material.19,40

Biodegradation Rates: The study of the kinetics of biodegradation in natural

environments is often empirical, reflecting the rudimentary level of knowledge aboutmicrobial populations and activity in these environments An example of an empiricalapproach is the power rate model.19

Figures 2.4a Common microbial degradation pathways (after Boethling and MacKay, 2000).

OH

OH CI

CH3[CH2]x CO2H CH3[CH2]x CO2H

R CH3 R CH2OH

R R

O OH

O

S,OP S O

S,O

S,OP O S

S,O S,OP S S

S,O

S,OP OH S,O R

H2O

H2O RS,O S,OP S,O S,O

R S,O

O

R O H O

R O R

Fatty acids and straight chain

of chain to carboxylic acid - see methyl oxidation) Aromatic and aliphatic methyl groups

Olefins

Aromatic to form phenols and hydrocarbons to alcohols and then ketones

Aromatic amines to nitroaromatic

Nitroaromatics aromatic especially fast under anaerobic conditions Bromoxynil, Dichlobenil Subject to Reaction

Sulfides such as aldicarb

where C is substrate concentration, t is time, k is the rate constant for chemical

disappearance, and n is a fitting parameter This model can be fit to disappearance curves by varying n and k until a good fit is achieved It is evidentfrom this equation that the rate is proportional to a power of the substrate concen-tration The power-rate law provides a basis for comparison of different curves, but

substrate-it gives no insight into the reasons for the shapes Therefore, often substrate-it may have nopredictive ability Moreover, investigators interested in kinetics do not always statewhether the model they are using has a theoretical basis or is simply empirical, andwhether constants in an equation have physical meaning or are only fitting param-eters.19 An appropriate introduction to the kinetics of biodegradation is to consider

Figure 2.4b Relationship between chemical structure and biodegradability (after Boethling

and MacKay, 2000).

≤3 rings ≥3 rings

OH OH CI

CI

R R N

R N R

R R N R O

R

R R H

R

R R H

R R R

R H N H

R H N

CI CI

Br OH OH CI CI CI OH

Br

OH CI CI CI

OH CI CI CI CI CI

OH CI CI CI CI

N

CH 3

N S R N