Organic Chemicals : An Environmental Perspective - Chapter 4 doc

Neilson, Alasdair H. "Persistence: General Orientation"
Organic Chemicals : An Environmental Perspective
Boca Raton: CRC Press LLC,2000

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4

Persistence: General Orientation

SYNOPSIS

An overview is presented of the factors that determine the per-
sistence of xenobiotics including the role of both abiotic and biotic reactions.
Examples are given of photochemical reactions including those that take
place in the troposphere and transformation products that may subsequently
enter aquatic or terrestrial systems, and of chemical transformations includ-
ing hydrolysis, dehalogenation, oxidation, and reduction. It is pointed out
that combinations of abiotic and biotic processes may be of determinative sig-
nificance, and the significance of these reactions in determining the analytes
that may be included in monitoring programs is emphasized. Biotic reactions
are discussed in detail and the important distinction between biodegradation
and biotransformation is emphasized. Attention is directed to the metabolic
potential of groups of microorganisms that have been less extensively exam-
ined; these include enteric bacteria, ammonia oxidizers, marine and
lithotrophic bacteria, algae, and anaerobic phototrophic bacteria. The signifi-
cance of electron acceptors other than oxygen is noted, and examples are
illustrated with organisms using nitrate and related compounds, and those
growing anaerobically by reduction of Fe(III), Mn(IV), or U(VI). Some impor-
tant reactions mediated by yeasts, fungi, and algae are outlined. The mecha-
nisms whereby oxygen is introduced into xenobiotics are discussed, and brief

accounts of the enzymology are included. Attention is directed to metabolic
interactions where several organisms and a single substrate are present, or
where several substrates and a single organism occur. Examples are given of
metabolic limitations imposed by enzyme regulatory mechanisms and of
metabolic situations where a single readily degraded substrate is present in
addition to a more recalcitrant xenobiotic. Factors that may critically deter-
mine the biodegradability of xenobiotics in natural systems are summarized;
these include temperature, the oxygen concentration, the substrate concen-
tration, the synthesis of natural emulsifying agents, the nature of transport
mechanisms, and the cardinal issue of the bioavailability of the xenobiotic. A
number of incompletely resolved issues are discussed including biodegrada-
tion in pristine environments, natural enrichment in contaminated environ-
ments, estimation of the rates of metabolic reactions both in laboratory and
natural systems, and the significance of toxic metabolites. Brief comments are
given on the role of catabolic plasmids.

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Introduction

Procedures for the analysis of environmental samples have been outlined in
Chapter 2, and the processes that determine the dissemination of xenobiotics
after discharge from point sources have been discussed in Chapter 3. In the
next three chapters, the factors that determine the ultimate fate of xenobiotics
will be discussed. This chapter attempts to present an overview of the factors
that determine the persistence of xenobiotics, while Chapter 5 will be devoted
to experimental procedures for carrying out the relevant investigations, and
Chapter 6 to a detailed examination of the pathways taken for the degradation
and transformation of a wide range of structurally diverse xenobiotics. Atten-
tion will be focused on microorganisms, and in particular on bacteria that are

the most important degradative organisms in virtually all aquatic ecosystems.
A certain degree of overlap between this chapter and Chapter 6 is inevitable,
but an attempt has been made to minimize this by inclusion of cross-references.
It was the persistence of DDT which raised the greatest alarm over its
extensive use during the years 1940 to 1968. Although levels since its banning
have decreased dramatically, those of its metabolite DDE may still be appre-
ciable and serve to sustain the initial concern. Many organic compounds have
become environmentally suspect, but it is especially the highly chlorinated
ones such as the polychlorinated biphenyls, polychlorinated camphenes, and
mirex which have acquired the reputation of being unacceptable due to their
apparent persistence. As a result of these fears, there has emerged a general
concern with all synthetic chlorinated organic compounds (Hileman 1993)
which may possibly have deflected interest from other groups which merit
comparable attention. It should, of course, be appreciated that on the other
hand a number of compounds and products such as modern plastics have
been developed for their stability under a variety of conditions — and are
produced with this end in view.
For these reasons, studies on biodegradation began to occupy a central
position in discussions on the environmental impact of organic chemicals,
and the complexities have been clearly presented (Landner 1989). It should be
appreciated at the outset that the terms

persistent

and

recalcitrant

are relative
rather than absolute since probably most chemical structures can be degraded

or transformed by microorganisms. The crucial issue is the rate at which the
reactions occur, and the area between slowly degradable compounds and
truly persistent ones is often unresolved. For example, in spite of the fact that
degradation of some PCB congeners has been demonstrated under aerobic
conditions, and biotransformation (dechlorination) under anaerobic condi-
tions, these compounds are still recoverable from many environmental sam-
ples; they should therefore be regarded as persistent. The critical questions
are both what reactions take place and the rate at which they occur in the
environment into which the compound is discharged. Both of these should be
addressed in investigations aimed at incorporating environmental relevance.

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Two essentially different processes determine the persistence of an organic
compound in the aquatic environment. The first are abiotic reactions, and for
some groups of compounds these reactions may be dominant in determining
their fate. The second are biotic reactions mediated by a wide range of organ-
isms. Only microorganisms will be discussed in the next three chapters,
although a brief discussion of the metabolism of xenobiotics by higher organ-
isms is given in Chapter 7 (Section 7.5).

4.1 Abiotic Reactions

Virtually any of the plethora of reactions known in organic chemistry may be
exploited for the abiotic degradation of xenobiotics. Hydrolytic reactions may
convert compounds such as esters, amides, or nitriles into the corresponding
carboxylic acids, or ureas and carbamides into the amines. These abiotic reac-
tions may therefore be the first step in the degradation of such compounds;
the transformation products may, however, be resistant to further chemical
transformation so that their ultimate fate is dependent upon subsequent

microbial reactions. For example, for urea herbicides the limiting factor is the
rate of microbial degradation of the chlorinated anilines which are the initial
hydrolysis products. The role of abiotic reactions should, therefore, always be
taken into consideration, and should be carefully evaluated in all laboratory
experiments on biodegradation and biotransformation (Section 5.3). It should
be appreciated that the results of experiments directed to microbial degrada-
tion are probably discarded if they show substantial interference from abiotic
reactions. A good illustration of the complementary roles of abiotic and biotic
processes is offered by the degradation of tributyl tin compounds. Earlier
experiments (Seligman et al. 1986) had demonstrated the transformation of
tributyltin to dibutyltin primarily by microbial processes. It was subsequently
shown, however, that an important abiotic reaction mediated by fine-grained
sediments resulted in the formation also of monobutyltin and inorganic tin
(Stang et al. 1992). It was therefore concluded that both processes were impor-
tant in determining the fate of tributyl tin in the marine environment.
A study of the carbamate biocides, carbaryl and propham, illustrates the
care that should be exercised in determining the relative importance of chem-
ical hydrolysis, photolysis, and bacterial degradation (Figure 4.1) (Wolfe et al.
1978a). For carbaryl, the half-life for hydrolysis increased from 0.15 day at pH
9 to 1500 day at pH 5, while that for photolysis was 6.6 day: biodegradation
was too slow to be significant. On the other hand, the half-lives of propham
for hydrolysis and photolysis were >10

4

and 121 day - so greatly exceeding
the half-life of 2.9 day for biodegradation that abiotic processes would be
considered to be of subordinate significance. Close attention to structural fea-
tures of xenobiotics is therefore clearly imperative before making generaliza-
tions on the relative significance of alternative degradative pathways.

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4.1.1 Photochemical Reactions in Aqueous and Terrestrial Environments

Photochemical reactions may be important especially in areas of high solar
irradiation, or on the surface of soils, or in aquatic systems containing ultravi-
olet (UV) absorbing humic and fulvic acids (Zepp et al. 1981), and they may
be especially relevant for otherwise recalcitrant compounds. It has also been
shown (Zepp and Schlotqhauer 1983) that, although the presence of algae may
enhance photometabolism, this is subservient to direct photolysis at the cell
densities likely to be encountered in rivers and lakes. It should be noted that
different products may be produced in natural river water and in buffered
medium; for example, photolysis of triclopyr (3,5,6-trichloro-2-pyridyloxy-
acetic acid) in sterile medium at pH 7 resulted in hydrolytic replacement of
one chlorine atom, whereas in river water the ring was degraded to form
oxamic acid as the principal product (Woodburn et al. 1993). Particular atten-
tion has understandably therefore been directed to the photolytic degradation
of biocides — including agrochemicals — that are applied to terrestrial sys-
tems. There has been increased interest in the phototoxicity toward a range of
biota (references in Monson et al. 1999), and this may be attributed to some of
the reactions and transformations that are discussed later in this chapter. It
should be emphasized that photochemical reactions may produce molecules
structurally more complex and less susceptible to degradation than their pre-
cursors, even though the deep-seated rearrangements induced in complex
compounds such as the terpene santonin during UV irradiation (Figure



4.2)

are not likely to be encountered in environmental situations.

The Diversity of Photochemical Transformations

In broad terms, the following types of reactions are mediated by the
homolytic fission products of water (formally, hydrogen and hydroxyl radi-
cals) and molecular oxygen or its excited states: hydrolysis, elimination, oxi-
dation, reduction, and cyclization.

The Role of Hydroxyl Radicals

The hydroxyl radical plays two essentially different roles: (1) as a reactant
mediating the transformations of xenobiotics and (2) as a toxicant operating
by damaging DNA. Hydroxyl radicals are important in a number of

FIGURE 4.1

Carbaryl (A) and propham (B).

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environments: (1) in aquatic systems under irradiation, (2) in the troposphere
that is discussed later in this section, and (3) in biological systems that are
noted in the context of superoxide dismutase and the role of Fe in Section
4.6.1.2 and in Sections 5.2.4 and 5.5.5. Hydroxyl radicals in aqueous media
may be generated by (1) photolysis of nitrite and nitrate (Brezonik and Fulk-
erson-Brekken 1998), (2) the Fenton reaction with H

2

O

2

and Fe

2+

in the pres-
ence of light that is noted later, and (3) photolysis of fulvic acids under
anaerobic conditions (Vaughan and Blough 1998), and (d) reaction of Fe(III) or
Cu(II) complexes of humic acids with hydrogen peroxide (Paciolla et al. 1999).
For the sake of completeness, attention is drawn to the following: (1) the
interactive role of hydroxyl radicals, superoxide, and Fe levels in wild and
mutant strains of

Escherichia coli

lacking Fe and Mn superoxide dismutase is
discussed in Sections 5.2.4 and 5.5.5 and (2) the possible role of hydroxyl rad-
icals in mediating the transformations accomplished by the brown-rot fun-
gus

Gleophyllum striatum

which is supported by the overall similarity in the
structures of the fungal metabolites with those produced with Fenton’s
reagent (Wetzstein et al. 1997).
Analytical procedures for hydroxyl radicals noted in Section 2.3 and have
been used to demonstrate the role of the anticancer drug 2,5-bis(1-azacyclo-

propyl)-3,6-bis(carboethoxyamino)benzo-1,4-quinone in mediating the pro-
duction of hydroxyl radicals in JB6 mouse epidermal cells (Li et al. 1997).

Illustrative Examples of Photochemical Transformations
in Aqueous Solutions

1. Atrazine is successively transformed to 2,4,6-trihydroxy-1,3,5-
triazine (Pelizzetti et al. 1990) by dealkylation of the alkylamine

FIGURE 4.2

Photochemical transformation of santonin.

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side chains and hydrolytic displacement of the ring chlorine and
amino groups (Figure 4.3). A comparison has been made between
direct photolysis and nitrate-mediated hydroxyl radical reactions
(Torrents et al. 1997). The rates of the latter were much greater under
the conditions of this experiment, and the major difference in the
products was the absence of ring hydroxylation with loss of chloride.
2. Pentachlorophenol produces a wide variety of transformation
products including chloranilic acid (2,5-dichloro-3,6-dihydroxy-
benzo-1,4-quinone) by hydrolysis and oxidation, a dichlorocyclo-
pentanedione by ring contraction, and dichloromaleic acid by
cleavage of the aromatic ring (Figure 4.4) (Wong and Crosby 1981).
3. The main products of photolysis of 3-trifluoromethyl-4-nitrophe-
nol are 2,5-dihydroxybenzoate produced by hydrolytic loss of the
nitro group and oxidation of the trifluoromethyl group, together
with a compound identified as a condensation product of the orig-

inal compound and the dihydroxybenzoate (Figure 4.5) (Carey and
Cox 1981).

FIGURE 4.3

Photochemical transformation of atrazine.

FIGURE 4.4

Photochemical transformation of pentachlorophenol.

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4. The potential insecticide that is a derivative of tetrahydro-1,3-thi-
azine undergoes a number of reactions resulting in some 43 prod-
ucts of which the dimeric azo compound is the principal one in
aqueous solutions (Figure 4.6) (Kleier et al. 1985).
5. The herbicide trifluralin undergoes a photochemical reaction in
which the

n

-propyl side chain of the amine reacts with the vicinal
nitro group to form the benzopyrazine (Figure 4.7) (Soderquist et
al. 1975).
6. Heptachlor and

cis-

chlordane both of which are chiral form caged

or half-caged structures (Figure 4.8) on irradiation and these prod-
ucts have been identified in biota from the Baltic, from the Arctic,
and from the Antarctic (Buser and Müller 1993).
7. Methylcyclopentadienyl manganese tricarbonyl that has been sug-
gested as a fuel additive is decomposed in aqueous medium pri-
marily by photolysis. This resulted in the formation of
methylcyclopentadiene that may plausibly be presumed to poly-
merize, and a manganese carbonyl that decomposed to Mn
3
O
4
(Garrison et al. 1995).

FIGURE 4.5

Photochemical transformation of 3-trifluoromethyl-4-nitrophenol.

FIGURE 4.6

Photochemical transformation of a tetrahydro-1,3-thiazine.

FIGURE 4.7

Photochemical transformation of trifluralin.

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8. Stilbenes that are used as fluorescent whitening agents are pho-
tolytically degraded by reactions involving

cis-trans

isomerization
followed by hydration of the double bond or oxidative fission of
the double bond to yield aldehydes (Kramer et al. 1996).
9. The photolysis of chloroalkanes and chloroalkenes has received
attention and results in the formation of phosgene as one of the
final products. The photodegradation of 1,1,1-trichloroethane pro-
ceeds by hydrogen abstraction and oxidation to trichloroacetalde-
hyde that is degraded by a complex series of reactions to phosgene
(Nelson et al. 1990; Platz et al. 1995).Tetrachloroethene is degraded
by reaction with chlorine radicals and oxidation to pentachloropro-
panol radical which also forms phosgene (Franklin 1994). Attention
has already been drawn to the significance of these reactions in the
context of environmental analytes (Section 2.5), and the atmo-
spheric dissemination of xenobiotics (Section 3.5.3).
10. Although EDTA is biodegradable under specific laboratory condi-
tions (Belly et al. 1975; Lauff et al. 1990; Nörtemann 1992; Witschel
et al. 1997), the primary mode of degradation in the natural aquatic
environment involves photolysis of the Fe complex (Lockhart and
Blakeley 1975; Kari and Giger 1995). Other metal complexes are
relatively resistant, so that its persistence is critically determined
not only by the degree of insolation but by the concentration of Fe
in the environment. The available evidence suggests that, in con-
trast to NTA that is more readily biodegradable, EDTA is likely to
be persistent except in environments in which concentrations of Fe
greatly exceed those of other cations.
11. The photolytic degradation of the fluoroquinolone antibiotic enro-
floxacin involves a number of reactions that produce 6-fluoro-7-
amino-1-cyclopropylquinolone 2-carboxylic acid that is then

degraded to CO

2

via reactions involving fission of the benzenoid
ring with loss of fluoride, dealkylation, and decarboxylation
(Burhenne et al. 1997a,b) (Figure



4.9).
12. Photolysis of the oxime group in the pyrazole miticide fenpyroxi-
mate resulted in the formation of two principal transformation
products: the nitrile via an elimination reaction and the aldehyde
by hydrolysis (Swanson et al. 1995).

FIGURE 4.8

Photochemical transformation of chlordane.

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13. Photochemical transformation of pyrene in aqueous media pro-
duced the 1,6- and 1,8-quinones as stable end products after initial
formation of 1-hydroxypyrene (Sigman et al. 1998). Irrespective of
mechanism, these reactions are formally comparable to those oper-
ating during the transformation of benzo[

a

]pyrene by

Phanerochaete
chrysosporium

(Chapter 6, Section 6.2.2).
14. The transformation of isoquinoline has been studied both under
photochemical conditions with hydrogen peroxide, and in the dark
with hydroxyl radicals (Beitz et al. 1998). The former resulted in
fission of the pyridine ring with formation of phthalic dialdehyde
and phthalimide whereas the major product from the latter
involved oxidation of the benzene ring with formation of the 5,8-
quinone and a hydroxylated quinone.
15. In the presence of both light and hydrogen peroxide, 2,4-dinitro-
toluene is oxidized to the corresponding carboxylic acid; this is then
decarboxylated to 1,3-dinitrobenzene which is degraded further by
hydroxylation and ring fission (Figure 4.10) (Ho 1986). Comparable
reaction products were formed from 2,4,6-trinitrotoluene and
hydroxylated to various nitrophenols and nitrocatechols before
cleavage of the aromatic rings, and included the dimeric 2,2

′

car-
boxy-3,3

′

,5,5

′

-tetranitroazoxybenzene (Godejohann et al. 1998).

Hydroxyl Radicals in the Destruction of Contaminants

The use of hydroxyl-radical mediated reactions has attracted interest in the
context of destruction of contaminants, and two are provided as illustration.
These reactions should be viewed against those with hydroxyl radicals that
occur in the troposphere that are considered in Section 4.1.2.

FIGURE 4.9

Photochemical degradation of enrofloxin.

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1. Fenton’s reagent — hydrogen peroxide in the presence of Fe

2+

or
Fe

3+

— both in the presence of oxygen and under the influence of
irradiation. The reaction involves hydroxyl radicals and has been
studied particularly intensively for the destruction of chlorinated
phenoxyacetic acid herbicides (Sun and Pignatello 1993). System-

atic investigations have been carried out on the effect of pH, the
molar ratio of H

2

O

2

/substrate, and the possible complications
resulting from the formation of iron complexes. Although this reac-
tion may have limited environmental relevance except under rather
special circumstances, an example of its use in combination with
biological treatment of PAHs is given in Chapter 8, Section 8.2.1.
Attention is drawn to it here since, under conditions where the
concentration of oxidant is limiting, intermediates may be formed
that are stable and that may possibly exert adverse environmental
effects. Some examples that illustrate the formation of intermedi-
ates are given, although it should be emphasized that total destruc-
tion of the relevant xenobiotics under optimal conditions can be
successfully accomplished. The structure of the products that are
produced by the action of Fenton’s reagent on chlorobenzene are
shown in Figure 4.11a (Sedlak and Andren 1991), and those from
2,4-dichlorophenoxyacetate in Figure 4.11b (Sun and Pignatello
1993). Whereas the degradation of azo dyes by Fenton’s reagent
produced water-soluble and CHCl

2

-soluble transformation prod-

ucts including nitrobenzene from Disperse Orange 3 that contains
a nitro group, benzene was tentatively identified among volatile
products from Solvent Yellow 14 (Spadaro et al. 1994).
2. Photolytic degradation on TiO

2

.
a. In the presence of slurries of TiO
2
that served as a photochem-
ical sensitizer, methyl

t

-butyl ether was photochemically

FIGURE 4.10

Photochemical transformation of 2,4-dinitrotoluene.

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decomposed at wavelengths < 290 nm. The mechanism in-
volves photochemical production of a free electron in the con-
duction band (e

cb

-

) and a corresponding hole (h

vb

+

) in the va-
lence band. Both of these produce H

2

O

2

, and thence hydroxyl
radicals, and the products were essentially the same as those
produced by hydroxyl radicals under atmospheric conditions
(Barreto et al., 1995):

t-

butyl formate and

t

-butanol were rapidly
formed and further degraded to formate, acetone, acetate, and
but-2-ene.

b. The degradation of haloalkanes has been extensively studied
and involves the same principles that have been noted earlier.
For these substrates, the initial reaction is abstraction of a hy-
drogen atom, and this is followed by a complex series of reac-
tions. From trichloroethene, a number of products are formed
including tetrachloromethane, hexachloroethane, pentachloroet-
hane, and tetrachlororethene, although the last two were shown
to be degradable in separate experiments (Hung and Marinas
1997). In TiO

2

slurries, the photochemical degradation of chlo-
roform, bromoform, and tetrachloromethane involves initial for-
mation of the trihalomethyl radicals. In the absence of oxygen,
these are further decomposed via dihalocarbenes to CO. Dichlo-
rocarbene was found as an intermediate in the degradation of
trichloroacetate (Choi and Hoffmann 1997).
c. The photocatalytic oxidation of various EDTA complexes has
been examined (Madden et al. 1997). The rates and efficiencies
were strongly dependent on the metal and the reactions are
generally similar to those involved in electrochemical oxidation
(Pakalapati et al. 1996).

FIGURE 4.11

Transformation products from (a) chlorobenzene and (b) 2,4-dichlorophenoxyacetate.

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Other Photochemically Induced Reactions

1. Two groups of reactions are important in the photochemical trans-
formation of PAHs: those with molecular oxygen, and those involv-
ing cyclization. Illustrative examples are provided by the
photooxidation of 7,12-dimethylbenz[

a

]anthracene-3,4-dihy-
drodiol (Lee and Harvey 1986) (Figure 4.12a) and benzo[

a

]pyrene
(Lee-Ruff et al. 1986) (Figure 4.12b), and the cyclization of

cis-

stilbene (Figure 4.12 c).
2. In nonaqueous solutions, two other groups of reactions have been
observed with polycyclic arenes: condensation via free-radical reac-
tions and oxidative ring fission.
a. Irradiation of benz[

a

]anthracene in benzene solutions in the
presence of xanth-9-one or vanillin produced a number of trans-
formation products tentatively identified as the result of oxida-

tion and cleavage of ring A, ring C, ring D, and rings C and D,
and rings B, C, and D (Jang and McDow 1997).
b. 1-Nitropyrene is a widely distributed contaminant produced
in the troposphere by reaction of nitrate radicals with pyrene
that is discussed in Section 4.1.2. A solution in benzene was

FIGURE 4.12

Photooxygenation of (a) 7,12-dimethylbenz[

a

]anthracene, (b) benzo[

a

]pyrene, and (c) photocyl-
ization of

cis

-stilbene.

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photochemically transformed into 9-hydroxy-1-nitropyrene
that is less mutagenic than its precursor (Koizumi et al. 1994).
3. The photochemical transformation of phenanthrene sorbed on sil-
ica gel (Barbas et al. 1996) resulted in a variety of products includ-
ing

cis

-9,10-dihydrodihydroxyphenanthrene and phenanthrene-
9,10-quinone, and a number of ring fission products including
biphenyl-2,2

′

-dicarboxaldehyde, naphthalene-1,2-dicarboxylic
acid, and benzo[

c

]coumarin. This may be compared with the prod-
ucts from the activated solution photooxidation of
benz[

a

]anthracene that have already been noted.
4. The photooxidation of naphthylamines adsorbed on particles of
silica and alumina produced products putatively less toxic than
their precursors (Hasegawa et al. 1993) (Figure



4.13).
5. It has been suggested that photochemically induced reactions may
take place between biocides and biomolecules of plant cuticles:

laboratory experiments have examined addition reactions between
DDT and methyl oleate and have been used to illustrate reactions
which result in the production of “bound” DDT residues (Figure
4.14) (Schwack 1988).

FIGURE 4.13

Photooxidation of sorbed naphthylamines.

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Interactions Between Photochemical and Other Reactions

It has been shown that a combination of photolytic and biotic reactions may
result in enhanced degradation of xenobiotics in municipal treatment systems,
for example, of chlorophenols (Miller et al. 1988a) and benzo[

a

]pyrene (Miller
et al. 1988b). Two examples may be used to illustrate the success of a combina-
tion of microbial and photochemical reactions in accomplishing the degrada-
tion of widely different xenobiotics in natural ecosystems: both of them
involved marine bacteria and it therefore seems plausible to assume that such
processes might be especially important in warm-water marine environments.
1. The degradation of pyridine dicarboxylates (Amador and Taylor
1990).
2. The degradation of 3- and 4-trifluoromethylbenzoate: the microbial
transformation resulted in the formation of catechol intermediates
that were converted into 7,7,7-trifluoro-hepta-2,4-diene-6-one car-

boxylate. This was subsequently degraded photochemically with
the loss of fluoride (Taylor et al. 1993) (Figure 4.15). This degrada-
tion may be compared to the purely photochemical degradation of
3-trifluoromethyl-4-nitrophenol that has already been noted and
contrasted with the resistance to microbial degradation of trifluo-
romethylbenzoates that is noted in Section 6.10.
Collectively, these examples illustrate the diversity of transformations of
xenobiotics that are photochemically induced in aquatic and terrestrial sys-
tems. Photochemical reactions in the troposphere are also extremely important

FIGURE 4.14

Product of reaction between DDT and methyl oleate.

FIGURE 4.15

Microbial followed by photochemical degradation of 3-trifluorobenzoate.

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in determining the fate and persistence not only of xenobiotics but also of nat-
urally occurring compounds. These are discussed more fully with mechanistic
details in Section 4.1.2.

4.1.2 Reactions in the Troposphere

Although chemical transformations in the troposphere may seem peripheral
to this discussion, these reactions should be kept in mind since their prod-
ucts may subsequently enter the aquatic and terrestrial environments. The
persistence and the toxicity of these secondary products are therefore

directly relevant to this discussion. Details of the relevant principles and
details of the methodology are covered in the comprehensive treatise by Fin-
layson-Pitts and Pitts (1986), and reference should be made to reviews on tro-
pospheric air pollution (Finlayson-Pitts and Pitts 1997) and atmospheric
aerosols (Andreae and Crutzen 1997). The reactions are dominated by those
involving free radicals.
There are several important reasons for discussing the reactions of organic
compounds in the troposphere.
1. The partitioning of compounds between the various phases has
been discussed in Chapter 3, and those of sufficient volatility or
associated with particles may be transported over long distances.
This is not a passive process, however, since important transfor-
mations may take place in the troposphere so that attention should
also be directed to their transformation products.
2. Considerable attention has been given to the persistence and fate
of organic compounds in the troposphere, and this has been
increasingly motivated by their possible role in the production of
ozone by reactions involving NO

x

.
3. Concern has been expressed over the destruction of ozone in the
stratosphere brought about by its reactions with chlorine atoms
produced from chlorofluoroalkanes that are persistent in the tro-
posphere and that may contribute to radiatively acting gases other
than CO

2

.
By way of introduction, a few examples are given here.
1. The occurrence of C

8

and C

9

dicarboxylic acids in samples of atmo-
spheric particles and in recent sediments (Stephanou 1992; Steph-
anou and Stratigakis 1993) has been attributed to photochemical
degradation of unsaturated carboxylic acids that are widespread
in almost all biota.
2. The formation of peroxyacetyl nitrate from isoprene (Grosjean et
al. 1993a) and of peroxypropionyl nitrate (Grosjean et al. 1993b)

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from

cis-

3-hexen-1-ol that is derived from higher plants, may be
given as illustration of important contributions to atmospheric deg-
radation (Seefeld and Kerr 1997).
3. Attention has been given to possible adverse effects of incorporat-
ing

t

-butyl methyl ether into automobile fuels, and it has been
shown that photolysis of

t

-butyl formate (that is an established
product of photolysis) in the presence of NO can produce the
relatively stable

t

-butoxyformyl peroxynitrate. This has a stability
comparable to that of peroxyacetyl nitrate and may therefore
increase the potential for disseminating NO

x

(Kirchner et al. 1997).
Reactions in the troposphere are mediated by reactions involving hydroxyl
radicals produced photochemically during daylight, by nitrate radicals that
are significant during the night (Platt et al. 1984), by ozone, and in some cir-
cumstances by O(

3

P).
The overall reactions involved in the production of hydroxyl radicals are
O

3

+ h

ν



→

O

2

+ O (

1

D); O(

1

D) + H

2

O

→

2OH
O(

1

D)

→

O(

3

P); O(

3

P) + O

2



→

O

3

Note that Roman capitals (S, P, D) are used for the states of atoms and
Greek capitals (

Σ, Π, ∆

) for those of molecules, and that the ground state of O

2

is a triplet O

2

(

3

Σ

). The reaction O

3

+ h

ν



→

O(

1

D) + O

2

(

1

∆

) has an energy thresh-
old at 310 nm, and the other possible reaction O

3

+ h

ν



→

O(

1

D) + O

2

(

3

Σ

) is for-
mally forbidden by conservation of spin. Increasing evidence has, however,
accumulated to show that the rate of production of O(

1

D) and therefore
hydroxyl radicals at wavelengths >310 nm is significant, and that, therefore,
in contrast to previous assumptions, the latter reaction makes an important
contribution (Ravishankara et al. 1998).
Nitrate radicals are formed from NO which is produced during combus-
tion processes and are significant only during the night in the absence of pho-
tochemically produced OH radicals. They are formed by the reactions:
NO + O

3

→

NO

2

+ O

2

; NO

2

+ O

3



→

NO

3

+ O

2

The concentrations of all these depend on local conditions, the time of day,

and both altitude and latitude. Values of ~10

6

molecules·cm

–3

for OH, 10

8

to
10

10

molecules.cm

–3

for NO

3

and ~10

11

molecules·cm

–3

for ozone are represen-
tative. Not all of these reactants are equally important, and the rates of reac-
tion of a substrate vary considerably; reactions with hydroxyl radicals are
generally the most important and some illustrative values are given for the
rates of reaction (cm
3
s
–1
molecule
–1
) with hydroxyl radicals, nitrate radicals,
and ozone (Atkinson 1990; summary of PAHs by Arey 1998),
©2000 CRC Press LLC
Survey of Reactions
The major reactions carried out by hydroxyl and nitrate radicals may be rep-
resented for a primary alkane RH or a secondary alkane R
2
CH; in both,
hydrogen abstraction is the initiating reaction.
1. Hydrogen abstraction:
RH + HO → R + H
2
O
RH + NO
3
→

R + HNO
3
2. Formation of alkylperoxy radicals:
R + O
2
→ R·O·O
3. Reactions of alkylperoxy radicals with NO
x
:
R·O·O + NO → R·O + NO
2
R·O·O + NO → R·O·NO
2
4. Reactions of alkyloxy radicals:
R
2
CH·O + O
2
→

R
2
CO + HO
2
R
2
CH·O + NO → R
2
CH·O·NO
2

→ R
2
CO + HNO
The concentration of NO determines the relative importance of reaction 3,
and the formation of NO
2
is particularly significant since this is readily
photolyzed to produce O(
3
P) that reacts with oxygen to produce ozone. This
alkane–NO
x
reaction may produce O
3
at the troposphere/stratosphere
interface:
NO
2
→ NO + O(
3
P); O(
3
P) + O
2
→ O
3
This is the main reaction for the formation of ozone, but under equilibrium
conditions, the concentrations of NO
2
,

NO, and O
3
are interdependent and no
net synthesis of O
3
occurs. When, however, the equilibrium is disturbed and
NO is removed by reactions with alkylperoxy radicals (reactions 1 + 2 + 3),
RH + OH → R + H
2
O; R + O
2
→ RO
2
; RO
2
+ NO → RO + NO
2
Hydroxyl Radicals Nitrate Radicals Ozone
n-Butane 2.54 × 10
–12
6.5 × 10
–17
9.8 × 10
–24
Acetaldehyde 15.8 × 10
–12
2.7 × 10
–15
<10

–20
Naphthalene 23.16 × 10
–11
3.6 × 10
–28
[NO
2
]<2 × 10
–19
©2000 CRC Press LLC
synthesis of O
3
may take place. The extent to which this occurs depends on a
number of factors (Finlayson-Pitts and Pitts 1997), including the reactivity of
the hydrocarbon which is itself a function of many factors. It has been pro-
posed that the possibility of ozone formation is best described by a reactivity
index incremental hydrocarbon reactivity (Carter and Atkinson 1987; 1989)
that combines the rate of formation of O
3
with that of the reduction in the con-
centration of NO. The method has been applied, for example, to oxygenate
additives to automobile fuel (Japar et al. 1991), and both anthropogenic com-
pounds and naturally occurring hydrocarbons may be reactive.
Clearly, whether or not ozone is formed depends also on the rate at which
it is destroyed, for example, by reaction with unsaturated hydrocarbons.
Rates of reactions with alkanes are, as noted above, much slower than for
reaction with OH radicals, and reactions with ozone are of the greatest sig-
nificance with unsaturated aliphatic compounds. The pathways plausibly
follow those involved in chemical ozonization (Hudlicky 1990), and some of
these are noted later.

Details of the kinetics of the various reactions have been explored in detail
using large-volume chambers that can be used to simulate the reactions in the
troposphere, and have frequently used hydroxyl radicals formed by photol-
ysis of methyl (or ethyl) nitrite, with the addition of NO to inhibit photolysis
of NO
2
. This would result in the formation of O(
3
P) atoms, and subsequent
reaction with O
2
to produce ozone and hence NO
3
radicals from NO
2
. Nitrate
radicals are produced by the thermal decomposition of N
2
O
5
, and in experi-
ments with O
3
, a scavenger for hydroxyl radicals is added (Chapter 5, Section
5.1). Details of the different experimental procedures for the measurement of
absolute and relative rates have been summarized, and attention drawn to
the often considerable spread of values for experiments carried out at room
temperature (~298 K) (Atkinson, 1986). It should be emphasized that in the
real troposphere, both the rates — and possibly the products — of transfor-
mation will be determined by seasonal differences both in temperature and

the intensity of solar radiation. These are determined both by latitude and
altitude.
The kinetics of the reactions of many xenobiotics with hydroxyl and
nitrate radicals have been examined under simulated atmospheric condi-
tions and include (1) aliphatic and aromatic hydrocarbons (Tuazon et al.
1986) and substituted monocyclic aromatic compounds (Atkinson et al.
1987c); (2) terpenes (Atkinson et al. 1985a); (3) amines (Atkinson et al.
1987a); (4) heterocyclic compounds (Atkinson et al. 1985b); and (5) chlori-
nated aromatic hydrocarbons (Kwok et al. 1995). For PCBs (Anderson and
Hites 1996), rate constants were highly dependent on the number of chlo-
rine atoms, and calculated atmospheric lifetimes varied from 2 days for 3-
chlorobiphenyl to 34 days for 2,2′,3,5′,6-pentachlorbiphenyl. It was esti-
mated that loss by hydroxylation in the atmosphere was a primary process
for removal of PCBs from the environment. It was later shown that the prod-
ucts were chlorinated benzoic acids produced by initial reaction with a
©2000 CRC Press LLC
hydroxyl radical at the 1-position followed by transannular dioxygenation
at the 2- and 5-positions followed by ring fission (Brubaker and Hites 1998).
Reactions of hydroxyl radicals with polychlorinated dibenzo[1,4]dioxins
and dibenzofurans also play an important role for their removal from the
atmosphere (Brubaker and Hites 1997). The gas phase and the particulate
phase are in equilibrium, and the results show that gas-phase reactions with
hydroxyl radicals are important for the compounds with fewer numbers of
chlorine atoms whereas for those with larger numbers of substituents parti-
cle-phase removal is significant.
Considerable attention has been directed to determining the products from
reactions of aromatic compounds and unsaturated compounds including
biogenic terpenes that exhibit appreciable volatility. These studies have been
conducted both in simulation chambers that have been noted and using nat-
ural sunlight in the presence of NO.

Aromatic Hydrocarbons
Ring fission of aromatic hydrocarbons may take place; for example, o-xylene
forms diacetyl, methylglyoxal, and gloxal (Tuazon et al. 1986) which are also
the products of ozonolysis (Levine and Cole 1932), while naphthalene forms
2-formylcinnamaldehyde (Arey 1998). The photooxidation of alkyl benzenes
that are atmospheric contaminants with high volatility has been studied in
detail and the reaction pathways have been delineated (Yu et al. 1997). Prod-
ucts from alkyl benzenes included both those with the ring intact such as aro-
matic aldehydes and quinones together with a wide range of aliphatic
compounds containing alcohol, ketone, and epoxy functional groups result-
ing from ring fission. The significance of epoxide intermediates (Yu and
Jeffries 1997) is noted in the next section. Attention is drawn later to the
important reactions of arenes that result in the production of nitroarenes.
Biogenic Terpenes
Monoterpenes are appreciably volatile and are produced in substantial quan-
tities by a range of higher plants and trees. Only some summary remarks are
given here.
1. The photochemical reactions of isoprene (references in Grosjean et
al. 1993a);
2. The products from reaction of α-pinene with ozone that produced
a range of cyclobutane carboxylic acids (Kamens et al. 1999);
3. The rapid reactions of linalool with OH radicals, NO
3
radicals and
ozone in which the major products were acetone and 5-ethe-
nyldihydro-5-methyl-2(3H)-furanone (Shu et al. 1997);
4. The plant metabolite cis-hex-3-ene-1-ol that is the precursor of per-
oxypropionyl nitrate (Grosjean et al. 1993b) analogous to peroxy-
acetyl nitrate;
©2000 CRC Press LLC

5. The degradation of many other terpenes has been examined includ-
ing the β-pinene, D-limonene, and trans-caryophyllene (Grosjean et
al. 1993b).
6. The products formed by reaction of NO
3
radicals with α-pinene
have been identified and include pinane epoxide, 2-hydroxypi-
nane-3-nitrate, 3-ketopinan-2-nitrate formed by reactions at the
double bond, and pinonaldehyde that is produced by ring fission
between C2 and C3 (Wängberg et al. 1997). These reactions should
be viewed in the general context of “odd nitrogen” to which alkyl
nitrates belong (Schneider et al. 1998).
7. Gas-phase products from the reactions of ozone with the mono-
terpenes (–)-β-pinene and (+)-sabinene which include the ketones
formed by oxidative fission of the exocyclic C=C bonds as well as
ozonides from the addition of ozone to this bond (Griesbaum et
al. 1998).
Reentry of Tropospheric Transformation Products
Some important illustrative examples are given in which the tropospheric
transformation products enter aquatic or terrestrial ecosystems by deposition
on particles.
1. Halogenated Alkanes and Alkenes
The stability of perchlorofluoroalkanes is due to the absence of hydrogen
atoms that may be abstracted in reaction with hydroxyl radicals. Attention has
therefore been directed to chlorofluoroalkanes containing at least one hydro-
gen atom (Hayman and Derwcut 1997). Considerable effort has been directed
to the reactions of chloroalkanes and chloroalkenes, and this deserves a more-
detailed examination in the light of interest in the products formed.
a. There has been concern over the fate of halogenated aliphatic com-
pounds in the atmosphere, and a single illustration of the diverse

consequences is noted here. The initial reaction of 1,1,1-trichloro-
ethane with hydroxyl radicals produces the Cl
3
C.CH
2
radical by
abstraction of H and then undergoes a complex series of reactions
including the following:
Cl
3
C.CH
2
+ O
2
→ Cl
3
C.CH
2
O
2
2Cl
3
C.CH
2
O
2
→ 2Cl
3
C.CH
2

O
Cl
3
C.CH
2
O + O
2
→ Cl
3
C.CHO
In addition, the alkoxy radical Cl
3
C.CH
2
O produces highly reac-
tive phosgene (COCl
2
) (Platz et al. 1995; Nelson et al. 1990) that
has been identified in atmospheric samples and was attributed to
the transformation of gem-dichloro aliphatic compounds (Grosjean
1991).
©2000 CRC Press LLC
b. The atmospheric degradation of tetrachloroethene produces
trichloroacetyl chloride as the primary intermediate which is
formed by an initial reaction with Cl radicals followed by the
following reactions (Franklin 1994):
Cl
3
CCl
2

+ O
2
→ Cl
3
C.CCl
2
O
2
Cl
3
C.CCl
2
O
2
+ NO → Cl
3
C.CCl
2
O + NO
2
Cl
3
C.CCl
2
O → Cl
3
C.COCl + Cl
→ COCl
2
+ CCl

3
.
CCl
3
+ O
2
+ NO → COCl
2
+ NO
2
+ Cl
An overview of the reactions involving X
3
C. CHYZ, where X, Y, and Z are
halogen atoms, has been given in the context of ozone depletion (Hayman
and Derwent 1997). Interest in the formation of trichloroacetaldehyde formed
from trichloroethane and tetrachloroethene is heightened by the phytotoxic-
ity of trichloroacetic acid (Frank et al. 1994), and by its occurrence in rainwa-
ter which seems to be a major source of this contaminant (Müller et al. 1996).
The situation in Japan seems, however, to underscore the possible signifi-
cance of other sources including chlorinated wastewater (Hashimoto et al.
1998).
Low concentrations of trifluoroacetate have been found in lakes in Califor-
nia and Nevada (Wujcik et al. 1998). It is formed by atmospheric reactions
from 1,1,1,2-tetrafluoroethane, and from the chlorofluorocarbon replacement
compound CF
3
.CH
2
F (HFC-134a) in an estimated yield of 7 to 20% (Walling-

ton et al. 1996); CF
3
OH formed from CF
3
in the stratosphere is apparently a
sink for its oxidation products (Wallington and Schneider 1994).
CF
3
·CH
2
F + OH → CF
3
·CHF
CF
3
·CHF + O
2
→ CF
3
·CHFO
2
CF
3
·CHFO
2
+ NO → CF
3
·CHFO + NO
2
CF

3
·CHFO + O
2
→ CF
3
·COF
CF
3
·CHFO → CF
3
+ H·COF
Although trifluoroacetate is accumulated by a range of biota through incor-
poration into biomolecules (Standley and Bott 1998), unlike trichloroacetate
it is only weakly phytotoxic and there is no evidence for its inhibitory effect
on methanogenesis (Emptage et al. 1997).
2. Arenes and Nitroarenes
The transformation of arenes in the troposphere has been discussed in detail
(Arey 1998). Destruction can be mediated by reaction with hydroxyl radicals,
and from naphthalene, a wide range of compounds is produced, including
1- and 2-naphthols, 2-formylcinnamaldehyde, phthalic anhydride, and, with
less certainty, 1,4-naphthoquinone and 2,3-epoxynaphthoquinone. Both
©2000 CRC Press LLC
1- and 2-nitronaphthalene were formed through the intervention of NO
2
(Bunce et al. 1997). Attention has also been directed to the composition of sec-
ondary organic aerosols from the photooxidation of monocyclic aromatic
hydrocarbons in the presence of NO
x
(Forstner et al. 1997); the main products
from a range of alkylated aromatics were 2,5-furandione and the 3-methyl

and 3-ethyl congeners.
Considerable attention has been directed to the formation of nitroarenes
which may be formed by two different mechanisms: (a) initial reaction with
hydroxyl radicals followed by reactions with nitrate radicals or NO
2
and
(b) direct reaction with nitrate radicals. The first is important for arenes in the
troposphere, whereas the second is a thermal reaction that occurs during
combustion of arenes. The kinetics of formation of nitroarenes by gas-phase
reaction with N
2
O
5
has been examined for naphthalene (Pitts et al. 1985a) and
methylnaphthalenes (Zielinska et al. 1989); biphenyl (Atkinson et al. 1987b);
acephenanthrylene (Zielinska et al. 1988), and for adsorbed pyrene (Pitts et
al. 1985b). Both 1- and 2-nitronaphthalene were formed through OH-radical-
initiated reactions with napthhalene by the intervention of NO
2
(Bunce et al.
1997). From naphthalene the major product from the first group of reactions
is 2-nitronaphthalene, and a number of other nitroarenes have been identi-
fied including nitropyrene and nitrofluoranthenes (Arey 1998). The tentative
identification of hydroxylated nitroarenes in air particulate samples (Nish-
ioka et al. 1988) is consistent with operation of this dual mechanism. Reaction
of methyl arenes with nitrate radicals in the gas phase gives rise to a number
of products. From toluene, the major product was benzaldehyde with lesser
amounts of 2-nitrotoluene > benzyl alcohol nitrate > 4-nitrotoluene > 3-nitro-
toluene (Chlodini et al. 1993). An interesting example is the formation of the
mutagenic 2-nitro- and 6-nitro-6H-dibenzo[b,d]pyran-6-ones (Figure 4.16)

from the oxidation of phenanthrene in the presence of NO
x
and methyl nitrite
as a source of hydroxyl radicals (Helmig et al. 1992a). These compounds have
been identified in samples of ambient air (Helmig et al. 1992b), and analo-
gous compounds from pyrene have been tentatively identified (Sasaki et al.,
1995). These compounds add further examples to the list of mononitroarenes
that already include 2-nitropyrene and 2-nitrofluoranthene, and it appears
plausible to suggest that comparable reactions are involved in the formation
FIGURE 4.16
Product from the photochemical reaction of phenanthrene and NO
x.
©2000 CRC Press LLC
of the 1,6- and 1,8-dinitroarenes that have been identified in diesel exhaust.
3-nitrobenzanthrone that is formally analogous to the dibenzopyrones noted
earlier has also been identified in diesel exhaust and is also highly mutagenic
to Salmonella typhimurium strain TA 98 (Enya et al. 1997).
Many nitroarenes are direct-acting frame-shift mutagens in the Ames test
(Rosenkranz and Mermelstein 1983) and, although the mechanism has not
been finally resolved, it appears to involve metabolic participation of the test
organisms. 2-nitronaphthalene is a potent mutagen. In addition, nitroarenes
may be reduced microbiologically (Chapter 6, Section 6.8.2) in terrestrial and
aquatic systems to the amino compounds that have two undesirable proper-
ties: (a) some, including 2-aminonaphthalene are carcinogenic to mammals
and (b) they react with components of humic and fulvic acids (Section 3.2.4)
which makes them more recalcitrant to degradation and therefore more per-
sistent in ecosystems.
It should also be noted that a wide range of azaarenes are produced during
combustion (Herod 1998) and may enter the troposphere, so that formation
of the corresponding nitro derivatives may occur.

3. Alkylated Arenes
The products from the oxidation of alkylbenzenes under simulated atmo-
spheric conditions have been noted earlier. Both ring epoxides that were
highly functionalized and aliphatic epoxides from ring fission were tenta-
tively identified (Yu and Jeffries 1997), and formation of the latter, many of
which are mutagenic, may cause further concern over transformation prod-
ucts from monocyclic aromatic hydrocarbons in the atmosphere.
4. Sulfides and Disulfides
An example in which formation of a carbon radical is not the initial reaction
is provided by the atmospheric reactions of organic sulfides and disulfides.
They also provide an example in which rates of reaction with nitrate radicals
exceed those with hydroxyl radicals. 2-dimethylthiopropionic acid is pro-
duced by algae and by the marsh grass Spartina alternifolia, and may then be
metabolized in sediment slurries under anoxic conditions to dimethyl sulfide
(Kiene and Taylor 1988), and by aerobic bacteria to methyl sulfide (Taylor and
Gilchrist 1991). It should be added that methyl sulfide can be produced by
biological methylation of sulfide itself (HS
-
) (Section 6.11.4). Dimethyl sulfide
— and possibly also methyl sulfide — is oxidized in the troposphere to sulfur
dioxide and methanesulfonic acids.
CH
3
·SH → CH
3
·SO
3
H
CH
3

·S·S·CH
3
→ CH
3
·SO
3
H + CH
3
·SO
CH
3
·SO → CH
3
+ SO
2
©2000 CRC Press LLC
It has been suggested that these compounds may play a critical role in pro-
moting cloud formation (Charleson et al. 1987) so that the long-term effect of
the biosynthesis of methyl sulfides on climate alteration may be considerable
— and yet at first glance this seems far removed from the production of an
osmolyte by higher plants, its metabolism in aquatic systems, or microbial
methylation. The occurrence of methyl sulfates in atmospheric samples
(Eatough et al. 1986) should be noted although the mechanism of its forma-
tion appears not to have been established fully. These reactions provide a
good example of the long chain of events which may bring about environ-
mental effects through the subtle interaction of biotic and abiotic reactions in
both the aquatic and atmospheric environments.
Appreciation of the interactive processes outlined earlier has been able to
illuminate discussion on mechanisms of problems as diverse as acidification
of water masses, climate alteration, ozone formation, and destruction, and

the possible environmental roles of trichloroacetic acid and nitroarenes. The
analysis and distribution of these—and other—transformation products is
therefore clearly motivated (Sections 2.5 and 3.6).
4.1.3 Chemically Mediated Transformation Reactions
Only a limited number of the plethora of known chemical reactions are
involved in the transformations of xenobiotics. An attempt is made merely to
present some examples of chemical degradation or transformation on the
basis of a classification of the reactions that take place.
4.1.3.1 Hydrolytic Reactions
Organic compounds containing carbonyl groups flanked by alkoxy groups
(esters) or by amino or substituted amino groups (amides, carbamates, and
ureas) may be hydrolyzed by purely abiotic reactions under appropriate con-
ditions of pH; the generally high pH of seawater (~8.2) may be noted so that
chemical hydrolysis may be quite important in this environment. On the
other hand, although very few natural aquatic ecosystems have pH values
sufficiently low for acidic hydrolysis to be of major importance, this may be
important in terrestrial systems. It is therefore important to distinguish
between alkaline or neutral, and acidic hydrolytic mechanisms. It should also
be appreciated that both hydrolytic and photolytic mechanisms may operate
simultaneously and that the products may not necessarily be identical.
Substantial numbers of important agrochemicals contain the carbonyl
groups noted earlier, so that abiotic hydrolysis may be the primary reaction in
their transformation; the example of carbaryl has already been cited (Wolfe et
al. 1978a). The same general principles may be extended to phosphate and
thiophosphate esters, although in these cases, it is important to bear in mind
the stability to hydrolysis of primary and secondary phosphate esters under
neutral or alkaline conditions that prevail in most natural ecosystems. On the