Natural and Enhanced Remediation Systems - Chapter 2 ppt

©2001 CRC Press LLC

Suthersan, Suthan S. “Contaminant and Environmental Characteristics”

Natural and Enhanced Remediation Systems

Edited by Suthan S. Suthersan
Boca Raton: CRC Press LLC, 2001
©2001 CRC Press LLC

CHAPTER

2
Contaminant and Environmental
Characteristics

CONTENTS

2.1 Introduction
2.2 Contaminant Characteristics
2.2.1 Physical/Chemical Properties
2.2.1.1 Boiling Point
2.2.1.2 Vapor Pressure
2.2.1.3 Henry’s Law Constant
2.2.1.4 Octanol/Water Partition Coefficients
2.2.1.5 Solubility in Water
2.2.1.6 Hydrolysis
2.2.1.7 Photolytic Reactions in Surface Water
2.2.2 Biological Characteristics
2.2.2.1 Cometabolism

2.2.2.2 Kinetics of Biodegradation
2.3 Environmental Characteristics
2.3.1 Sorption Coefficient
2.3.1.1 Soil Sorption Coefficients
2.3.1.2 Factors Affecting Sorption Coefficients
2.3.2 Oxidation-Reduction Capacities of Aquifer Solids
2.3.2.1 pe and pH
2.3.2.2 REDOX Poise
2.3.2.3 REDOX Reaction
References

©2001 CRC Press LLC

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 to
obtain 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 the
site. A substantial ongoing national debate associated with site closure centers on
the definition of “how clean is clean” for contaminated subsurface media. The key
issue 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 the
subsurface into the environment and/or is bioavailable to ecological and human
receptors. The dissemination of a contaminant after its release into the environment
is determined by its partition among the water, soil and sediment, and atmospheric
phases, 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, to
achieve site closure; rather, they are meant to broaden the range of options or tools
available to environmental professionals. This chapter will discuss the basis and
parameters 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 determine
contaminant 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 transport
and 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 sorp-
tion, desorption, precipitation, complexation, biodegradation, biotransformation,
hydrolysis, oxidation-reduction, and dissolution, are critical in determining their fate
and mobility in the subsurface environment. Reaction time scales can vary from micro-
seconds for many ion association reactions microseconds and milliseconds for some
ion exchange and sorption reactions, to days, weeks, or months for some microbially
catalyzed reactions, or years for many mineral solution and crystallization reactions.
Both transport and chemical reaction processes can affect the reaction rates in
the subsurface environment. Transport processes include: (1) transport in the solution
phase, across a liquid film at the particle/liquid interface (film diffusion), and in

©2001 CRC Press LLC

liquid-filled macropores, all of which are nonactivated diffusion processes and occur
in mobile regions; (2) particle diffusion processes, which include diffusion of sorbate
occluded 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 activated
diffusion processes (Figure 2.1).

1

Pore and surface diffusion within the immediate
region can be referred to as intra-aggregate (intraparticle) diffusion and diffusion in
the solid can be called interparticle diffusion. The actual chemical reaction at the
surface, e.g., adsorption, is usually instantaneous. The slowest of the chemical
reaction and transport process is the ratelimiting reaction.
As an introduction to the various organic compounds which end up as contam-
inants once discharged into the environment, Table 2.1 gives the basic structure of
the different compounds.

Figure 2.1

Transport processes in solid-liquid soil reactions (adapted from Sparks, 1998).
Liquid (Groundwater)
Film Solid (Soil Grain)
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
1 2
3
44
5
6 6
C
None
Functional
Group
General
Formula
General
Name

Formula IUPAC Name Common Name
Example
C
CC
Cl
Br
OH
O
NH
NR X
2
2
+
-
C H
2n+2
n
n
2n
C H
n
2n-2
C H
ClR
R
R
R
Br
OH
OR

RNH
-
+
4
R N X
1
Alkane
Alkene
Alkyne
Chloride
Bromide
Alcohol
Ether
Amine
Quaternary
2
ammonium
salt
Eth
ane
Eth
ene
Eth
yne
Chloro
ethane
Bromo
methane
Ethan
ol

Ethoxy
ethane
1-
Amino
propane
Decyltrimethyl-
Ammonium
chloride chloride
Ammonium
Decyltrimethyl-
Propylamine
Diethyl ether
Ethyl alcohol
Methyl bromide
Ethyl chloride
Acetylene
Ethylene
Ethane
3
3
CH CH
2
3
CHH C
2
CHHC
CH CH Cl
22
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
22
+
-
9
3
3
OC
H H
COR
CC
ROC
R
O
C OH
O
CROH

Aldehyde
Ketone
Carboxylic acid
Propan
al
2-Butan
one
Ethan
oic acid
Propionaldehyde
ketone
Methyl ethyl
Acetic acid
CH CH CH O
H
CH
OCH CH C
OHCH C
O
3
2
3
2
3
3
Table 2.1 Some Common Functional Groups.
(Continued)

©2001 CRC Press LLC
Table 2.1 (Cont.)

O
CCH OC H
Acetic acid
Ethyl ethan
oate
Ester
OR'RC
O
OR'
C
O
Acetonitrile
Nitromethane
Methyl mercaptan
Dimethyl disulfideDimethyl
disulfide
Methane
thiol
Nitro
methane
Ethane
nitrile
(sulfide)
Disulfide
Thiol
Nitro
Nitrile
1
R
R

R
SH
NO
Example
Common NameIUPAC NameFormula
General
Name
General
Formula
Functional
Group
2
5
Acetamide
Ethan
amide
2
O
C NH
O
CRNH
Amide
NHCH C
O
3
2
2
3
O
CCH Cl

Acid chloride
ClRC
O
Cl
C
O
Ethan
oyl chloride
Acetyl chloride
3
O
CCH
Acid anhydride
RC
O
C
O
Ethan
oic anhydride
Acetic anhydride
O
O
C
O
O
CR OCHC
O
3
CN
S S

NC
2
2
NO
SH
R
Thioether
Dimethyl
thioether
Dimethyl sulfide
SS
R
R
SS
R
N
3
3
NOCH
2
CH
SH
3
3
CH
S
3
CH
CH
3

S
CH
3
S
O
S
ROH
O
S
OH
Sulfonic acid
OHCH
S
O
3
Methane
sulfonic acid
Methanesulfonic
acid
OO
O
O
S
O
S
R
Sulfoxide
Dimethyl
sulfoxide
Dimethyl sulfoxide

R
3
CHCH
S
O
3
O
S
O
O
R
S
O
Sulfone
O
3
O
S
CH
Dimethyl
sulfone
Dimethyl sulfone
RCH
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
3

23

©2001 CRC Press LLC

©2001 CRC Press LLC

2.2 CONTAMINANT CHARACTERISTICS
2.2.1Physical/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 1
atmosphere (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 some
laboratory 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 for

most chemicals of interest the boiling points are in the range of 300 to 600

∞

C.

2

Having a value for a chemical’s boiling point, whether measured or estimated, is
significant because it defines the uppermost temperatures at which the chemical can
exist 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. The
boiling point is associated with a number of molecular properties and features. Most
important is molecular weight; boiling points generally increase with this parameter.
Next is the strength of the intermolecular bonding because boiling points increase
with increasing bonding strength. This bonding, in turn, is associated with processes
and 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 equilibrium
with its liquid or solid phase.

2

Vapor pressure’s importance in environmental work
results from its effects on the transport and partitioning of chemicals among the
environmental media (air, water, and soil). The vapor pressure expresses and controls
the chemical’s volatility. The volatilization of a chemical from the water surface is
determined 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 chemical
from the soil surface is determined largely by its vapor pressure, although this is
tempered by its sorption to the soil matrix and its Henry’s Law Constant between
the 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 substances
that boil at or below 100

∞

C, the vapor pressure is likely to be known, but, for many
high-boiling substances with low vapor pressure, the value may be unknown or
poorly known. An estimation procedure may be needed to help convert the known
vapor 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 high
boiling compounds, the actual boiling point may also be unknown, since the sub-
stance may decompose before it boils.

©2001 CRC Press LLC

2.2.1.3 Henry’s Law Constant

Along with the octanol-water and octanol-air partition coefficients, the Henry’s
Law Constant determines how a chemical substance will partition among the three
primary media of accumulation in the environment, namely air, water, and organic
matter present in soils, solids, and biota. Volatile organic compounds (VOCs) with
large 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 the
rate of subsequent atmospheric processes. For such chemicals, an accurate value of
this parameter K

AW

is essential. Even a very low value of K

AW

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 of
1000; thus even a low atmospheric concentration can represent a significant quantity
of chemical. Further, the rate of evaporation from soils and water is profoundly
influenced by K

AW

because that process involves diffusion in water and air phases
in series, or in parallel, and the relative concentrations which can be established in
these phases control these diffusion rates.

2,3

Accurate values of K

AW

are thus essential for any assessment of the behavior of
existing chemicals or prediction of the likely behavior of new chemicals. Air-water
partitioning 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 of

concentration 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 partial
pressures, at least for chemicals which are not subject to significant dissociation or
association in either phase. This linearity is expressed as

Henry’s Law

. The Henry’s
Law 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
C

W

of mol /m3 yielding H with dimensions of Pa m

3

/mol.
P (Pa) = H (Pa m

3

/mol) C

W

(mol/m

3

)(2.1)
The partial pressure can be converted into a concentration in the air phase C

A

by
invoking the ideal gas law:
C

A

= n/V = P/RT (2.2)
Where n is mols, V is volume (m

3

), R is the gas constant (8.314 Pa m

3

/mol K) and
T is absolute temperature (K).
C

A

= P/RT = (H/RT) C

W

= K

AW

C

W

(2.3)
The dimensionless air-water partition coefficient K

AW

(which can be the ratio in units
of mol/m

3

or g/m

3

or indeed any quantity/volume combination) is thus (H/RT).
A plot of C

A

vs. C

W

is thus usually linear with a slope of K

AW

as Figure 2.2
illustrates. For organic chemicals which are sparingly soluble in water, these con-
centrations are limited on one axis by the water solubility and on the other by the

©2001 CRC Press LLC

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

2,3

2.2.1.4 Octanol/Water Partition Coefficients

The usefulness of the ratio of the concentration of a solute between water and
octanol as a model for its transport between phases in a physical or biological system
has long been recognized.

2,4,5

It is expressed as

P

OCT

=

C

O

/C

W

=

K

OW

. This ratio is
essentially independent of concentration, and is usually given in logarithmic terms
(log P

OCT

or log K

OW

). The importance of bioconcentration in environmental hazard

assessment and the utility of this hydrophobic parameter in its prediction led to an
intense interest in the measurement of P

OCT

and also its prediction from molecular
structure. (So many calculation methods have been published in the last five years
that it is not possible to examine them all in detail.)

2.2.1.5 Solubility in Water

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 maximum
concentration which an aqueous solution will tolerate before the onset of phase
separation.

Figure 2.2

Description of Henry’s Law Constant.
Concentration in Air C
A
Concentration in Water C
w
(Vapor Pressure/RT)

[Solubility of
Compound]
Slope = K
aw
= H/RT

©2001 CRC Press LLC

Substances which are readily soluble in water, such as lower molecular weight
alcohols, will dissolve freely in water if accidentally spilled and will tend to remain
in aqueous solution until degraded. On the contrary, sparingly soluble substances
dissolve more slowly and, when in solution, have a stronger tendency to partition
out of aqueous solution into other phases. They tend to have larger air–water partition
coefficients or Henry’s Law Constants, and they tend to partition more into solid
and biotic phases such as soils, sediments, and fish. As a result, it is common to
correlate partition coefficients from water to those media with solubility in water.
Solubility normally is measured by bringing an excess amount of a pure chemical
phase into contact with water at a specified temperature, so that equilibrium is
achieved 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 a
contaminant site.
(2.4)
where,
C

i
*

= equilibrium solute concentration for component

i

in the mixture
C

i
0

= equilibrium solute concentration for component

i

as a pure compound
x

i

= mole fraction of compound

i

in the mixture

g

i

= activity coefficient of compound

i

in the mixture.
Possible equilibrium situations may exist, depending on the nature of the chem-
ical phase, each of which requires separate theoretical treatment and leads to different
equations for expressing solubility. These equations form the basis of the correlations
discussed later.

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

C

*

= C

o

x

g

(2.5)
In this case, C

*

is also C

∞

. Thus the product x

g

is 1.0 and x is 1/

g

. Sparingly
soluble substances act in such a way because the value of

g

is large.

2

For example, at 25

∞

C benzene has a solubility in water of 1780 g/m

3

or 22.8
mol/m.

3

Since 1 m

3

of solution contains approximately 10

6

/18 mol water (1m

3

is
10

6

g and 18 g /mol is the molecular mass of water), the mole fraction x is
22.8/(10

6

/18) or 0.00041. The activity coefficient

g

is thus 2440; i.e., a benzene
molecule in aqueous solution behaves as if its concentration were 2440 times
higher.
Substances such as polychlorinated biphenyls (PCBs) can have activity coeffi-

cients exceeding 1 million. Hydrophobicity thus is essentially an indication of the
magnitude of

g

. Some predictive methods focus on estimating

g

, from which solu-
bility can be deduced.
CCx
iiii
*
=
0
g

©2001 CRC Press LLC

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 other
environmental partitioning properties with solubility are then impossible.

2

Solubility is a function of temperature because both vapor pressure and

g

are
temperature dependent. Usually

g

falls with increasing temperature, thus solubility
increases. This implies that the process of dissolution is endothermic. Exceptions
are 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 fulvic
acids frequently increases the apparent solubility. This is the result of sorption of
the chemical to organic matter which is sufficiently low in molecular mass to be
retained permanently in solution. The

true

solubility or concentration in the pure
aqueous phase probably is not increased. The apparent solubility is the sum of the
true or dissolved concentration and the quantity which is sorbed.
The solubility of substances such as carboxylic acids, which dissociate or form
ions 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 pK

a

or pK

b

given, to permit the ratio of ionic to
nonionic forms to be calculated as
Ionic/non-ionic = 10

(pH–pKa)

(2.6)
The total solubility is then that of the parent and ionic forms.

2.2.1.6 Hydrolysis

Hydrolysis is a bond-making, bond-breaking process in which a molecule, RA,
reacts with water, forming a new R–O bond with the oxygen atom from water and
breaking the R–A bond in the original molecule. One possible pathway is the direct
displacement of –A with –OH, as Equation 2.7 shows.
RA + H

2

O

Æ

ROH + HA (2.7)

Hydrolytic processes provide the baseline loss rate for any chemical in an
aqueous environment. Although various hydrolytic pathways account for significant
degradation of certain classes of organic chemicals, other organic structures are
completely inert. Strictly speaking, hydrolysis should involve only the reactant
species water provides — that is, H

+

, OH

–

and H

2

O — but the complete picture
includes analogous reactions and thus the equivalent effects of other chemical species
present 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 biodeg-
radation, as well as more soluble. The likelihood that a halogenated solvent will

©2001 CRC Press LLC

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

©2001 CRC Press LLC

(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, NO

2

and O

3

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),

©2001 CRC Press LLC

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,(I

A

) 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 NO

2

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 = I
A
©2001 CRC Press LLC
2.2.2Biological 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 aquatic
ecosystems. 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. During
biotransformation, on the other hand, only a restricted number of metabolic reactions
is accomplished, and the basic framework of the molecule remains essentially
intact.
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 an
ecosystem 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 miner-
alization. Under anaerobic conditions, methane may be formed in addition to carbon
dioxide during fermentation reactions.
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 any
living organism, available information indicates that, by far, the most significant
biological systems involved in ultimate biodegradation of contaminants are bacteria
and 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.
©2001 CRC Press LLC
The initial concentration of the microbial population and the contaminant some-
what affects the growth and proliferation; a lag period often occurs between the
addition of a chemical and the onset of biodegradation. This lag period, usually
attributed to the need for acclimation,
19,20
could result from enzyme induction, gene
transfer or mutation, predation by protozoa, or growth in the population of respon-
sible organisms.
The initial species present, their relative concentrations, the induction of their
enzymes, and their ability to acclimate once exposed to a chemical are likely to vary
considerably, depending upon environmental parameters such as temperature, salin-
ity, pH, oxygen concentration (aerobic or anaerobic), redox potential, concentration
and nature of various substrates and nutrients, concentration of heavy metals (tox-
icity), and effects (synergistic and antagonistic) of associated microflora.
21
Many of

these parameters affect the biodegradation of contaminants in the environment.
One important parameter is the chemical substrate concentration. A number of
chemicals have biodegradation rates proportional to substrate concentration, but
there 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 in
determining biodegradability in nature. Several studies have demonstrated that,
although a chemical freshly added to soil is biodegraded at a moderate rate, the
biodegradation 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 in
the 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 microorganism
degrades the contaminant from which it derives no carbon or energy; instead, it is
sustained 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 acquired
even 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 or
cooperate), there has been some debate regarding the use of the term cometabolism.

Specifically, some classical microbiologists argue that the term should be applied
only 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.
19
Accord-
ing to this view, the transformation of a substance that is not used as a nutrient or
energy source but which occurs in the absence of a chemical supporting growth
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
©2001 CRC Press LLC
reaction, which is consistent with one explanation for the phenomenon. Fortuitous
metabolism is, indeed, a more attractive term because it suggests an explanation for
cometabolism, but the term will be used here as in the original definition, if for no
other 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 the
presence of a second compound that does support multiplication. Cooxidation has
historical precedence in the debate but since it is restricted to oxidation, the word
does 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 envi-
ronment. In one, the cometabolized compound is transformed only in the presence
of a second substrate, which indeed may be the compound that supports growth.
For heterotrophs, the energy-providing substrate is organic; for autotrophs, it is
inorganic. In the other type, the compound is metabolized even in the absence of a
second substrate.
Important reasons for using the more general definition, and even for maintaining
cometabolism as a term apart from bioconversion or biotransformation, are the
environmental consequences of cometabolism. Cometabolic reactions have impacts

in nature that are different from growth-linked biodegradations, and when the trans-
formations take place, it is usually totally unclear whether the microorganisms do
or do not have a second substrate available on which they are growing.
A large number of chemicals are subject to cometabolism in nature. Among
cometabolic conversions that appear to involve a single enzyme, the reactions may
be hydroxylations, oxidations, denitrations, deaminations, hydrolyses, acylations, or
cleavages of ether linkages; however, many of the conversions are complex and
involve 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 of
growth linked biological transformations that heterotrophic bacteria and fungi are
capable of in nature. An example which has no significant importance in contaminant
removal 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 occurring
merely because an organism cannot be isolated from an environment in which a
chemical is undergoing a biological reaction.
19
The isolation of bacteria acting on
specific substrates is usually performed by enriching the organism in a medium when
the only C source is the test chemical, and the agar medium used to plate the
enrichments contains that single organic supplement. Yet, many bacteria that are
able to grow at the expense of that substrate will not develop in such simple media
because they require amino acids, B vitamins, or other growth factors. These essential
growth factors are not routinely included in such liquid media, and hence bacteria
and fungi needing them fail to proliferate. If the only organisms in the environment
able to metabolize a contaminant need these growth factors, no isolate will be
obtained, and an erroneous conclusion will be reached that the compound is come-
tabolized. If a chemical supports the growth of many species, some will undoubtedly

require no growth factors (these organisms are called prototrophs), and they will be
©2001 CRC Press LLC
enriched and ultimately can be isolated. If the compound is acted on by only one
species, 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 the
sole 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 not
support growth but is converted to products that accumulate. Some of these reasons
have experimental support: (1) the initial enzyme converts the substrate to a product
that is not further transformed by other enzymes in the microorganism to yield the
metabolic 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 basis
for this explanation is the fact that many enzymes act on several structurally related
substrates; thus, an enzyme naturally present in the cell will catalyze reactions and
alter synthetic chemicals that are not typical cellular intermediates. These enzymes
are not absolutely specific for their substrates. Consider a normal metabolic sequence
involving 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 CO
2
energy for biosynthesis
reactions 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 molecule
structurally similar to A, namely, A.

1
The product (B
1
) would differ from B in the
same way that A differs from A
1
. However, if enzyme b is unable to act on B
1
(because the structural features controlling which substrate it modifies differ from
those controlling the substrate specificity of enzyme a), B
1
will accumulate:
19
(2.10)
In addition, CO
2
and energy will not be generated and, because cellular carbon
is not formed, the organisms do not multiply. The formation of B
1
is thus entirely
fortuitous.
19
In instances where the contaminant concentration is high, cometabolism may
result from the conversion of the parent compound to toxic products. In the sequence
just depicted, if the rate of reaction catalyzed by enzyme a is faster than the process
catalyzed 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
The

inhibitor that accumulates may affect a single enzyme important for the further
metabolism of the toxin.
ABCD CO energy cell C
abc
ÆÆÆÆÆÆ++
2
–
Æ
æÆæ
a
AB
11
©2001 CRC Press LLC
In some instances, an organism may not be able to metabolize an organic
compound 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 the
transformation.
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 a
single type of reaction on a variety of somewhat dissimilar molecules. The following
are 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 cis-
and trans-1,2-dichloroethylene, 1,1-dichloroethylene, 1,2-dichloropropane, and

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 O
2
(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 O
2
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 NH
4
+
and whose C source is
CO
2
, cometabolizes TCE, 1,1-dichloroethylene, various mono- and polyhaloge-
nated ethanes, and a variety of monocyclic aromaticcompounds and thioethers, as
well as methyl fluoride and dimethyl ether.
19,31
• 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
and propylenes.
19
• Naphthalene dioxygenase acts on xylene, isomers of nitrotoluene, and ethylben-
zene.
19,33
• Biphenyl dioxygenase transforms several PCB congeners.
19,34
©2001 CRC Press LLC
Figure 2.3a Reactions catalyzed by toluene dioxygenase (adapted from Alexander, 1999).
CH
3
TOLUENE
O

2
2H
OH
OH
H
H
CH
2
OH
CH
2
OH
CH
3
NO
2
NO
2
CH
3
2-NITROTOLUENE
NO
2
NO
2
CH
3
3-NITROTOLUENE
CH
3

4-NITROTOLUENE
CCI2 CICH
TCE
OH
NO
2
NO
2
CH
3
NO
2
OH
OH
+
HC COOH + HCOOH + CI
-
O
©2001 CRC Press LLC
The organism containing these enzymes may be able to use one of several of
the enzyme’s substrates for growth. However, many of the substrates are transformed
but 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 organic
compounds to the contaminated zone promotes the rate of cometabolism of a number
of chlorinated aliphatic and aromatic compounds and chlorinated phenols, but the
responses to such additions are not predictable. No relation is known to exist between
the metabolic pathways involved in the degradation of the added mineralized sub-

strate and the compound that is cometabolized in these circumstances. The added
substrates 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 structurally
analogous to the compound whose cometabolism one wishes to promote. Presum-
ably, the microorganism that grows on the mineralizable compound contains
enzymes transforming the analogous molecule that is cometabolized. This larger
biomass 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 enhance
the cometabolism of PCBs by additions of biphenyl. The unchlorinated biphenyl
was 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 isolating
bacteria that can cometabolize a compound. The enrichment culture contains a C
source that supports growth, and the pure cultures thus obtained also cometabolize
structurally related compounds that would not support growth. For example, bacteria
isolated on diphenylmethane and containing enzymes to degrade it also cometabolize
chlorinated 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 which
may affect considerably the rate of biodegradation. For example, wastewater
Figure 2.3bReactions catalyzed by toluene monooxygenase (adapted from Alexander,
1999).
CH
3

CH
3
TOLUENE o-CRESOL
OH
©2001 CRC Press LLC
treatment plants may have high levels of nutrients and high microbial populations
that 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 phosphorous
which may limit the biodegradation of chemicals (e.g., oil spills). Sediment often
has high levels of organic nutrients, but often is anaerobic, while surface waters
tend, by comparison, to have low levels of organic nutrients.
18
Digestor sludge from
wastewater treatment plants has high organic nutrients and is anaerobic. Surface
soils 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 microbial
life. However, a number of studies have demonstrated that microorganisms are quite
plentiful in certain aquifers, and, in some instances, the bacterial concentration and
activity 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 clay
content may bind the chemical tightly).
A contaminant may become less available or essentially unavailable for biodeg-
radation if it enters or is deposited in a micropore that is inaccessible even to the
microorganisms. These micropores may be filled entirely with water, as in sediments
or groundwater aquifers, and the contaminant would have to move out of a micropore

by diffusion to be accessible to bacteria for its destruction. The tortuous path the
contaminant molecule must traverse before it gets destroyed dramatically affects
bioavailability if the contaminant not only is physically remote from potentially
active microorganisms, but also is strongly sorbed to solid surfaces associated with
that remote micropore.
19
Some organic compounds that persist in the subsurface often undergo a time
dependent decline in bioavailability. Since this process is slow and time dependent
it is appropriately called aging. This modification in bioavailability to microorgan-
isms as a result of aging is also called sequestration.
19
In the initial period, the
compound gradually disappears as a result of biodegradation, and possibly by other
mass removal mechanisms, but little or none of the compound is destroyed after it
has resided in the soil or sediment for some time. Witness the finding that although
80% of hexachlorobenzene deposited in the early 1970s in a lake bottom sediment
was dechlorinated in the succeeding 20 years, all sediment cores still contained at
least 40 ppb of hexachlorebenzene. This time-dependent change in the rate of
degradation, which has been observed with a number of insecticides, was the first
line of evidence for sequestration.
19
Because these aged molecules are solvent
extractable, albeit by vigorous treatment, they are presumed to be present in an
uncomplexed 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.
It has been known for some time that it is increasingly difficult to remove strongly
hydrophobic compounds from soil with mild extractants as the residence time of
©2001 CRC Press LLC

those compounds in the soil increases. This phenomenon is not restricted to soils;
a similar decline in extractability is witnessed also from sediment samples.
36
The amount of a contaminant that is sequestered increases with time. Expressed
in another way, the percentage of the chemical that is bioavailable diminishes with
increasing persistence. This presumably occurs because more of the contaminant is
diffusing into inaccessible sites. However, after a period of time that varies with the
soil and the compound, sequestration of additional quantities slows and possibly
stops. 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 biodegradation
are 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 rates
under different conditions may vary depending upon the type of chemical structure.
For example, nitro aromatic compounds are usually fairly resistant to biodegradation
under aerobic conditions but are reduced rapidly to amines under anaerobic condi-
tions. In contrast, degradation of benzene takes place significantly faster under
aerobic 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 the
possible pathways that a substrate may undergo, generally classified as oxidative,
reductive, hydrolytic, or conjugative. Figure 2.4 provides some examples of common
microbial degradation pathways.
37
Recently, a computer program was developed that
will predict the most probable metabolites, and another computer program was also
developed that simulates the biodegradation of synthetic chemicals through the
sequential application of plausible biochemical reactions.
38,39

Over the years, structure/biodegradability “rules of thumb” have been devel-
oped.
40,41
Figures 2.4a and b summarize these. Some of these structure/biode-
gradibility relationships have some biochemical mechanistic underpinings. For
example, highly branched compounds frequently are resistant to biodegradation
because increased substitution hinders b-oxidation, the process by which alkyl
chains and fatty acids usually are biodegraded. This structural relationship was
discovered 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 from
the highly branched ABS detergents to linear alkylbenzene sulfonate (LAS) deter-
gents, thus illustrating the importance of understanding the relationship between
structure and biodegradability.
Few other rules of thumb have such mechanistic bases, but there are some general
trends. Functional groups commonly seen by microorganisms in natural products
usually are degraded easily, probably because the microbes have had eons to develop
the required enzyme systems in order to gain carbon and energy from the metabolism.
Conversely, functional groups less common in nature or newly synthesized by man
usually make a chemical more resistant to biodegradation. Aromatic substituents
that 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 the
aromatic ring, whereas electron donating functionalities (e.g., carboxylic acids,
phenols, amines) generally increase biodegradation rates.
©2001 CRC Press LLC
Physical/chemical properties affect the rate of biodegradation mostly by affecting
bioavailability. Compounds which are sparingly soluble in water tend to be more
resistant to biodegradation, possibly due to an inability to reach the microbial enzyme
site, a reduced rate of availability due to solubilization, or sequestration due to
adsorption 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 about
microbial populations and activity in these environments. An example of an empirical
approach is the power rate model.
19
–dC/dt = kC
n
(2.11)
Figures 2.4a Common microbial degradation pathways (after Boethling and MacKay, 2000).
OH
OHCI
CH
3
[CH
2
]x CO
2
HCH
3
[CH
2
]x CO
2
H
R CH
3
R CH
2
OH

R
R
R CH
2
HR CHO
R NH
2
R NHOH R NO
2
R N=O
R NO
2
R CN
R S R
R CO
2
H
R N=O R NH
2
R NHOH
R
R
O
OOH
R
R
RR
S
RR
S

R
RR R
O
R CH
2
CI R CH
2
OH
R CCI
3
R CO
2
H
RNH
2
O
O
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

H
2
O
H
2
O
R
S,O
S,OP S,O
S,O
RR
R
S,O
O
RO
H
O
RO
R
β-oxidation
Methyl oxidation
Epoxide formation
Hydroxylation and ketone formation
Type of Reactions (not all steps are given)
Nitrogen oxidation
Nitro reduction
Nitrile/amide metabolism
Sulfur oxidation
Thiophosphate ester oxidation
Dehalogenation

Hydrolysis
Fatty acids and straight chain
hydrocarbons (after oxidation
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
amines (e.g., parathion)
especially fast under
anaerobic conditions
Bromoxynil, Dichlobenil
Example of Chemicals
Subject to Reaction
Sulfides such as aldicarb
Thiophosphate pesticides
Aromatic and aliphatic
halogens
Phosphate and carboxylic
esters
©2001 CRC Press LLC
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 substrate-
disappearance curves by varying n and k until a good fit is achieved. It is evident

from 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
it gives no insight into the reasons for the shapes. Therefore, often it may have no
predictive ability. Moreover, investigators interested in kinetics do not always state
whether the model they are using has a theoretical basis or is simply empirical, and
whether 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
OHOH
CI
CI
RR
N
RR
N
RR
N
R
O
R
RR
H
R
RR
H
RR

R
HR
R
R CH
2
OH
R CI
R CO
2
H
R NH
2
OH
CH
3
CO
2
H
OMe
NH
2
SO
3
H
CF
3
CI
NO
2
Br

R SO
3
R
RR
N
R
RH
N
H
RH
N
R
O
N
CH
2
H
3
OHOH
CI CI
Br
OHOH
CI
CI
CI
OH
Br
OH
CI
CI

CI
OH
CI
CI
CI
CI
CI
OH
CI
CI
CI
CI
N
CH
3
N SR
N
N
CH
3
CH
2
H
3
N
CH
3
N NCI
N
N

CH
3
More Biodegradable
(Less Perisistent)
Less Biodegradable
(More Persistent)
Branching
Aliphatic functional groups
Aliphatic amines
Halophenols
Polycyclic aromatics
Triazines
Aromatic functional groups (benzene, naphthalene, pyridine rings)