Multimedia Environmental Models - Chapter 3 potx

McKay, Donald. "Environmental Chemicals and Their Properties"
Multimedia Environmental Models
Edited by Donald McKay
Boca Raton: CRC Press LLC,2001

©2001 CRC Press LLC

CHAPTER

3
Environmental Chemicals and
Their Properties

3.1 INTRODUCTION AND DATA SOURCES

In this book, we focus on techniques for building mass balance models of
chemical fate in the environment, rather than on the detailed chemistry that controls
transport and transformation, as well as toxic interactions. For a fuller account of
the basic chemistry, the reader is referred to the excellent texts by Crosby (1988),
Tinsley (1979), Stumm and Morgan (1981), Pankow (1991), Schwarzenbach et al.
(1993), Seinfeld and Pandis (1997), Findlayson-Pitts and Pitts (1986), Thibodeaux
(1996), and Valsaraj (1995).
There is a formidable and growing literature on the nature and properties of
chemicals of environmental concern. Numerous handbooks list relevant physical-
chemical and toxicological properties. Especially extensive are compilations on
pesticides, chemicals of potential occupational exposure, and carcinogens. Govern-
ment agencies such as the U.S. Environmental Protection Agency (EPA), Environ-
ment Canada, scientific organizations such as the Society of Environmental Toxi-
cology and Chemistry (SETAC), industry groups, and individual authors have
published numerous reports and books on specific chemicals or classes of chemicals.
Conferences are regularly held and proceedings published on specific chemicals

such as the “dioxins.” Computer-accessible databases are now widely available for
consultation. Table 3.1 lists some of the more widely used texts and scientific
journals. Most are available in good reference libraries.
Most of the chemicals that we treat in this book are organic, but the mass
balancing principles also apply to metals, organometallic chemicals, gases such as
oxygen and freons, inorganic compounds, and ions containing elements such as
phosphorus and arsenic. Metals and other inorganic compounds tend to require
individual treatment, because they usually possess a unique set of properties. Organic
compounds, on the other hand, tend to fall into certain well defined classes. We are
often able to estimate the properties and behavior of one organic chemical from that

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Table 3.1 Sources of information on chemical properties and estimation methods (See
Chapter 1.5 of Mackay, et al.,

Illustrated Handbooks of Physical Chemical
Properties and Environmental Fate for Organic Chemicals,

cited below, for

more details)

The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals (Annual),

S.
Budavarie, ed. Whitehouse Station, NJ: Merck & Co., 1996.

Handbook of Chemistry and Physics,

D. R. Lide, ed., 81/e. Boca Raton, FL: CRC Press.

Verschueren’s Handbook of Environmental Data on Organic Chemicals.

New York: John Wiley
& Sons, 1997.

Illustrated Handbook of Physical Chemical Properties and Environmental Fate for Organic
Chemicals

(in 5 volumes). D. Mackay, W. Y Shiu, and K. C. Ma. Boca Raton, FL: CRC Press,
1991–1997. Also available as a CD ROM.

Handbook of Environmental Fate and Exposure Data for Organic Chemicals

(several volumes),
P. H. Howard, ed. Boca Raton, FL: Lewis Publications.

Handbook of Environmental Degradation Rates,

P. H. Howard et al. Boca Raton, FL: Lewis
Publications.

Lange’s Handbook of Chemistry

, 15/e, J. A. Dean, ed. New York: McGraw-Hill, 1998.

Dreisbach’s Physical Properties of Chemical Compounds,

Vol I to III. Washington, DC, Amer.

Chem. Soc.
Technical Reports, European Chemical Industry Ecology and Toxicology Centre (ECETOC).
Brussels, Belgium.

Sax’s Dangerous Properties of Industrial Materials,

10/e. R. J. Lewis, ed. New York: John
Wiley & Sons.

Groundwater Chemicals Desk Reference,

J. J. Montgomery. Boca Raton, FL: Lewis
Publications, 1996.

Genium Materials Safety Data Sheets Collection.

Amsterdam, NY: Genium Publishing Corp.

The Properties of Gases and Liquids,

R. C. Reid, J. M. Prausnitz, and B. E. Poling. New York:
McGraw-Hill, 1987.

NIOSH/OSHA Occupational Health Guidelines for Chemical Hazards.

Washington, DC: U.S.
Government Printing Office.

The Pesticide Manual,

12/e. C. D. S. Tomlin, ed. Loughborough, UK: British Crop Protection
Council.

The Agrochemicals Handbook,

H. Kidd and D. R. James, eds. London: Royal Society of
Chemistry.

Agrochemicals Desk Reference,

2/e, J. H. Montgomery. Boca Raton, FL: Lewis Publications.
ARS Pesticide Properties Database, R. Nash, A. Herner, and D. Wauchope. Beltsville, MD:
U.S. Department of Agriculture, www.arsusda.gov/rsml/ppdb.html.

Substitution Constants for Correlation Analysis in Chemistry and Biology,

C. H. Hansch
(currently out of print). New York: Wiley-Interscience.

Handbook of Chemical Property Estimation Methods,

W. J. Lyman, W. F. Reehl, D. H.
Rosenblatt (currently out of print). New York: McGraw-Hill.

Handbook of Property Estimation Methods for Chemicals,

R. S. Boethling and D. Mackay.
Boca Raton, FL: CRC Press, 2000.

Chemical Property Estimation: Theory and Practice,

E. J. Baum. Boca Raton, FL: Lewis
Publications, 1997.
Toolkit for Estimating Physiochemical Properties of Organic Compounds, M. Reinhard and A.
Drefahl. New York: John Wiley & Sons, 1999.
IUPAC Handbook. Research Triangle Park, NC: International Union of Pure and Applied
Chemistry.
Website for database and EPIWIN estimation methods, Syracuse, NY: Syracuse Research
Corporation

().

©2001 CRC Press LLC

of other, somewhat similar or homologous chemicals. An example is the series of
chlorinated benzenes that vary systematically in properties from benzene to
hexachlorobenzene.
It is believed that some 50,000 to 80,000 chemicals are used in commerce. The
number of chemicals of environmental concern runs to a few thousand. There are
now numerous lists of “priority” chemicals of concern, but there is considerable
variation between lists. It is not possible, or even useful, to specify an exact number
of chemicals. Some inorganic chemicals ionize in contact with water and thus lose
their initial identity. Some lists name PCBs (polychlorinated biphenyls) as one
chemical and others as six groups of chemicals whereas, in reality, the PCBs consist
of 209 possible individual congeners. Many chemicals, such as surfactants and
solvents, are complex mixtures that are difficult to identify and analyze. One des-
ignation, such as

naphtha

, may represent 1000 chemicals. There is a multitude of
pesticides, dyes, pigments, polymeric substances, drugs, and silicones that have
valuable social and commercial applications. These are in addition to the numerous
“natural” chemicals, many of which are very toxic.
For legislative purposes, most jurisdictions have compiled lists of chemicals that
are, or may be, encountered in the environment, and from these “raw” lists of
chemicals of potential concern they have established smaller lists of “priority”
chemicals. These chemicals, which are usually observed in the environment, are
known to have the potential to cause adverse ecological and/or biological effects
and are thus believed to be worthy of control and regulation. In practice, a chemical
that fails to reach the “priority” list is usually ignored and receives

no

priority rather
than

less

priority.
These lists should be regarded as dynamic. New chemicals are being added as
enthusiastic analytical chemists detect them in unexpected locations or toxicologists
discover subtle new effects. Examples are brominated flame retardants, chlorinated
alkanes, and certain very stable fluorinated substances (e.g., trifluoroacetic acid) that
have only recently been detected and identified. In recent years, concern has grown
about the presence of endocrine modulating substances such as nonylphenol, which
can disrupt sex ratios and generally interfere with reproductive processes. The
popular book

Our Stolen Future,

by Colborn et al. (1996) brought this issue to public
attention. Some of these have industrial or domestic sources, but there is increasing
concern about the general contamination by drugs used by humans or in agriculture.
Table 3.2 lists about 200 chemicals by class and contains many of the chemicals of
current concern.

3.2 IDENTIFYING PRIORITY CHEMICALS

It is a challenging task to identify from “raw lists” of chemicals a smaller, more
manageable number of “priority” chemicals. Such chemicals receive intense scrutiny,
analytical protocols are developed, properties and toxicity are measured, and reviews
are conducted of sources, fate, and effects. This selection contains an element of
judgement and is approached by different groups in different ways. A common thread
among many of the selection processes is the consideration of six factors: quantity,

©2001 CRC Press LLC

Table 3.2 List of Chemicals Commonly Found on Priority Chemical Lists

Volatile Halogentated Hydrocarbons Monoaromatic Hydrocarbons

Chloromethane Benzene
Methylene chloride Toluene
Chloroform o-Xylene
Carbontetrachloride m-Xylene
Chloroethane p-Xylene
1,1-Dichloroethane Ethylbenzene
1,2-Dichloroethane Styrene
cis-1,2-Dichloroethene

trans-1,2-Dichloroethene

Polycyclic Aromatic Hydrocarbons

Vinyl chloride Naphthalene
1,1,1-Trichloroethane 1-Methylnaphthalene
1,1,2-Trichloroethane 2-Methylnaphthalene
Tr ichloroethylene Trimethylnaphthalene
Tetrachloroethylene Biphenyl
Hexachloroethane Acenaphthene
1,2-Dichloropropane Acenaphthylene
1,3-Dichloropropane Fluorene
cis-1,3-Dichloropropylene Anthracene
trans-1,3-Dichloropropylene Fluoranthene
Chloroprene Phenanthrene
Bromomethane Pyrene
Bromoform Chrysene
Ethylenedibromide Benzo(a)anthracene
Chlorodibromomethane Dibenz(a,h)anthracene
Dichlorobromomethane Benzo(b)fluoranthene
Dichlorodibromomethane Benzo(k)fluoranthene
Freons (chlorofluoro-hydrocarbons) Benzo(a)pyrene
Dichlorodifluoromethane Perylene
Tr ichlorofluoromethane Benzo(g,h,i)perylene
Indeno(1,2,3)pyrene

Halogenated Monoaromatics

Chlorobenzene
1,2-Dichlorobenzene

Dienes

1,3-Dichlorobenzene 1,3-Butadiene
1,4-Dichlorobenzene Cyclopentadiene
1,2,3-Trichlorobenzene Hexachlorobutadiene
1,2,4-Trichlorobenzene Hexachlorocyclopentadiene
1,2,3,4-Tetrachlorobenzene
1,2,3,5-Tetrachlorobenzene

Alcohols and Phenols

Benzyl alcohol
Phenol
o-Cresol
m-Cresol
p-Cresol
2-Hydroxybiphenyl
4-Hydroxybiphenyl
Eugenol

©2001 CRC Press LLC

1,2,4,5-Tetrachlorobenzene

Halogenated Phenols

Pentachlorobenzene 2-Chlorophenol
Hexachlorobenzene 2,4-Dichlorophenol
2,4,5-Trichlorotoluene 2,6-Dichlorophenol

Octachlorostyrene 2,3,4-Trichlorophenol
2,3,5-Trichlorophenol

Halogenated Biphenyls and Naphthalenes

2,4,5-Trichlorophenol
Polychlorinated Biphenyls (PCBs) 2,4,6-Trichlorophenol
Polybrominated Biphenyls (PBBs) 2,3,4,5-Tetrachlorophenol
1-Chloronaphthalene 2,3,4,6-Tetrachlorophenol
2-Chloronaphthalene 2,3,5,6-Tetrachlorophenol
Polychlorinated Naphthalenes (PCNs) Pentachlorophenol
4-Chloro-3-methylphenol

Aroclor Mixtures (PCBs)

2,4-Dimethylphenol
Aroclor 1016 2,6-Di-t-butyl-4-methylphenol
Aroclor 1221 Tetrachloroguaiacol
Aroclor 1232
Aroclor 1242

Nitrophenols, Nitrotoluenes

Aroclor 1248

and Related Compounds

Aroclor 1254 2-Nitrophenol
Aroclor 1260 4-Nitrophenol
2,4-Dinitrophenol

Chlorinated Dibenzo-p-dioxins

4,6-Dinitro-o-cresol
2,3,7,8-Tetrachlorodibenzo-p-dioxin Nitrobenzene
Tetrachlorinated dibenzo-p-dioxins 2,4-Dinitrotoluene
Pentachlorinated dibenzo-p-dioxins 2,6-Dinitrotoluene
Hexachlorinated dibenzo-p-dioxins
Heptachlorinated dibenzo-p-dioxins 1-Nitronaphthalene
Octachlorinated dibenzo-p-dioxin 2-Nitronaphthalene
Brominated dibenzo-p-dioxins 5-Nitroacenaphthalene

Chlorinated Dibenzofurans Fluorinated Compounds

Tetrachlorinated dibenzofurans Polyfluorinated alkanes
Pentachlorinated dibenzofurans Trifluoroacetic acid
Hexachlorinated dibenzofurans Fluoro-chloro acids
Heptachlorinated dibenzofurans Polyfluorinated chemicals
Octachlorodibenzofuran

Phthalate Esters
Nitrosamines and Other Nitrogen Compounds

Dimethylphthalate
N-Nitrosodimethylamine Diethylphthalate
N-Nitrosodiethylamine Di-n-butylphthalate
N-Nitrosodiphenylamine Di-n-octylphthalate
N-Nitrosodi-n-propylamine Di(2-ethylhexyl) phthalate
Diphenylamine Benzylbutylphthalate
Indole

4-aminoazobenzene

Chlorinated longer chain alkanes
Pesticides, including biocides, fungicides, rodenticides, insecticides and herbicides

Table 3.2 List of Chemicals Commonly Found on Priority Chemical Lists

©2001 CRC Press LLC

persistence, bioaccumulation, potential for transport to distant locations, toxicity,
and a miscellaneous group of other adverse effects.

3.2.1 Quantity

The first factor is the quantity produced, used, formed or transported, including
consideration of the fraction of the chemical that may be discharged to the environ-
ment during use. Some chemicals, such as benzene, are used in very large quantities
in fuels, but only a small fraction (possibly less than a fraction of a percent) is
emitted into the environment through incomplete combustion or leakage during
storage. Other chemicals, such as pesticides, are used in much smaller quantities
but are discharged completely and directly into the environment; i.e., 100% is
emitted. At the other extreme, there are chemical intermediates that may be produced
in large quantities but are emitted in only minuscule amounts (except during an
industrial accident). It is difficult to compare the amounts emitted from these various
categories, because they are highly variable and episodic. It is essential, however,
to consider this factor; many toxic chemicals have no significant adverse impact,
because they enter the environment in negligible quantities.
Central to the importance of quantity is the adage first stated by Paracelsus,
nearly five centuries ago, that the dose makes the poison. This can be restated in
the form that all chemicals are toxic if administered to the victim in sufficient

quantities. A corollary is that, in sufficiently small doses, all chemicals are safe.
Indeed, certain metals and vitamins are essential to survival. The general objective
of environmental regulation or “management” must therefore be to ensure that the
quantity of a specific substance entering the environment is not excessive. It need
not be zero; indeed, it is impossible to achieve zero. Apart from cleaning up past
mistakes, the most useful regulatory action is to reduce emissions to acceptable
levels and thus ensure that concentrations and exposures are tolerable. Not even the
EPA can reduce the toxicity of benzene. It can only reduce emissions. This implies
knowing what the emissions are and where they come from. This is the focus of
programs such as the Toxics Release Inventory (TRI) in the U.S.A. or the National
Pollutant Release Inventory (NPRI) system in Canada. There are similar programs
in Europe, Australia, and Japan. Regrettably, the data are often incomplete. A major
purpose of this book is to give the reader the ability to translate emission rates into
environmental concentrations so that the risk resulting from exposure to these con-
centrations can be assessed. When this can be done, it provides an incentive to
improve release inventories.

3.2.2 Persistence

The second factor is the chemical’s environmental persistence, which may also
be expressed as a

lifetime, half-life,

or

residence time

. Some chemicals, such as DDT
or the PCBs, may persist in the environment for several years by virtue of their

resistance to transformation by degrading processes of biological and physical origin.
They may have the opportunity to migrate widely throughout the environment and
reach vulnerable organisms. Their persistence results in the possibility of establishing

©2001 CRC Press LLC

relatively high concentrations. This arises because, in principle, the amount in the
environment (kilograms) can be expressed as the product of the emission rate into
the environment (kilograms per year) and the residence time of the chemical in the
environment (years). Persistence also retards removal from the environment once
emissions are stopped. A legacy of “in place” contamination remains.
This is the same equation that controls a human population. For example, the
number of Canadians (about 30 million) is determined by the product or the rate at
which Canadians are born (about 0.4 million per year) and the lifetime of Canadians
(about 75 years). If Canadians were less persistent and lived for only 30 years, the
population would drop to 12 million.
Intuitively, the amount (and hence the concentration) of a chemical in the
environment must control the exposure and effects of that chemical on ecosystems,
because toxic and other adverse effects, such as ozone depletion, are generally a
response to concentration. Unfortunately, it is difficult to estimate the environmen-
tal persistence of a chemical. This is because the rate at which chemicals degrade
depends on which environmental media they reside in, on temperature (which
varies diurnally and seasonally), on incidence of sunlight (which varies similarly),
on the nature and number of degrading microorganisms that may be present, and
on other factors such as acidity and the presence of reactants and catalysts. This
variable persistence contrasts with radioisotopes, which have a half-life that is
fixed and unaffected by the media in which they reside. In reality, a substance
experiences a distribution of half-lives, not a single value, and this distribution
varies spatially and temporally. Obviously, long-lived chemicals, such as PCBs,
are of much greater concern than those, such as phenol, that may persist in the

aquatic environment for only a few days as a result of susceptibility to biodegra-
dation. Some estimate of persistence or residence time is thus necessary for priority
setting purposes. Organo-halogen chemicals tend to be persistent and are therefore
frequently found on priority lists. Later in this book, we develop methods of
calculating persistence.

3.2.3 Bioaccumulation

The third factor is potential for bioaccumulation (i.e., uptake of the chemical by
organisms). This is a phenomenon, not an effect; thus bioaccumulation

per se

is not
necessarily of concern. It is of concern that bioaccumulation may cause toxicity to
the affected organism or to a predator or consumer of that organism. Historically, it
was the observation of pesticide bioaccumulation in birds that prompted Rachel
Carson to write

Silent Spring

in 1962, thus greatly increasing public awareness of
environmental contamination.
As we discuss later, some chemicals, notably the hydrophobic or “water-hating”
organic chemicals, partition appreciably into organic media and establish high con-
centrations in fatty tissue. PCBs may achieve concentrations (i.e., they bioconcen-
trate) in fish at factors of 100,000 times the concentrations that exist in the water in
which the fish dwell. For some chemicals (notably PCBs, mercury, and DDT), there
is also a food chain effect. Small fish are consumed by larger fish, at higher trophic
levels, and by other animals such as gulls, otters, mink, and humans. These chemicals

©2001 CRC Press LLC

may be transmitted up the food chain, and this may result in a further increase in
concentration such that they are biomagnified.
Bioaccumulation tendency is normally estimated using an organic phase-water
partition coefficient and, more specifically, the octanol-water partition coefficient.
This, in turn, can be related to the solubility of the chemical in the water. Clearly,
chemicals that bioaccumulate, bioconcentrate, and biomagnify have the potential to
travel down unexpected pathways, and they can exert severe toxic effects, especially
on organisms at higher trophic levels.
The importance of bioaccumulation may be illustrated by noting that, in water
containing 1 ng/L of PCB, the fish may contain 10

5

ng/kg. A human may consume
1000 L of water annually (containing 1000 ng of PCB) and 10 kg of fish (containing
10

6

ng of PCB), thus exposure from fish consumption is 1000 times greater than
that from water. Particularly vulnerable are organisms such as certain birds and
mammals that rely heavily on fish as a food source.

3.2.4 Toxicity

The fourth factor is the toxicity of the chemical. The simplest manifestation of
toxicity is acute toxicity. This is most easily measured as a concentration that will

kill 50% of a population of an aquatic organism, such as fish or an invertebrate (e.g.,

Daphnia magna

), in a period of 24–96 hours, depending on test conditions. When
the concentration that kills (or is lethal to) 50% (the LC50) is small, this corresponds
to high toxicity. The toxic chemical may also be administered to laboratory animals
such as mice or rats, orally or dermally. The results are then expressed as a lethal
dose to kill 50% (LD50) in units of mg chemical/kg body weight of the animal.
Again, a low LD50 corresponds to high toxicity.
More difficult, expensive, and contentious are

chronic

, or

sublethal

, tests that
assess the susceptibility of the organism to adverse effects from concentrations or
doses of chemicals that do not cause immediate death but ultimately may lead to
death. For example, the animal may cease to feed, grow more slowly, be unable to
reproduce, become more susceptible to predation, or display some abnormal behav-
ior that ultimately affects its life span or performance. The concentrations or doses
at which these effects occur are often about 1/10th to 1/100th of those that cause
acute effects. Ironically, in many cases, the toxic agent is also an essential nutrient,
so too much or too little may cause adverse effects.
Although most toxicology is applied to animals, there is also a body of knowledge
on phytotoxicity, i.e., toxicity to plants. Plants are much easier to manage, and killing
them is less controversial. Tests also exist for assessing toxicity to microorganisms.

It is important to emphasise that toxicity alone is not a sufficient cause for concern
about a chemical. Arsenic in a bottle is harmless. Disinfectants, biocides, and pesti-
cides are inherently useful because they are toxic. The extent to which the organism
is injured depends on the inherent properties of the chemical, the condition of the
organism, and the dose or amount that the organism experiences. It is thus misleading
to classify or prioritize chemicals solely on the basis of their inherent toxicity, or on
the basis of the concentrations in the environment or exposures. Both must be
considered. A major task of this book is to estimate exposure. A healthy tension often

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exists between toxicologists and chemists about the relative importance of toxicity
and exposure, but fundamentally this argument is about as purposeful as squabbling
over whether tea leaves or water are the more important constituents of tea.
Most difficult is the issue of genotoxicity, including carcinogenicity, and terato-
genicity. In recent years, a battery of tests has been developed in which organisms
ranging from microorganisms to mammals are exposed to chemicals in an attempt
to determine if they can influence genetic structure or cause cancer. A major difficulty
is that these effects may have long latent periods, perhaps 20 to 30 years in humans.
The adverse effect may be a result of a series of biochemical events in which the
toxic chemical plays only one role. It is difficult to use the results of short-term
laboratory experiments to deduce reliably the presence and magnitude of hazard to
humans. There may be suspicions that a chemical is producing cancer in perhaps
0.1% of a large human population over a period of perhaps 30 years, an effect that
is very difficult (or probably impossible) to detect in epidemiological studies. But
this 0.1% translates into the premature death of 30,000 Canadians per year from
such a cancer, and is cause for considerable concern. Another difficulty is that
humans are voluntarily and involuntarily exposed to many toxic chemicals, including
those derived from smoking, legal and illicit drugs, domestic and occupational
exposure, as well as environmental exposure. Although research indicates that mul-

tiple toxicants act additively when they have similar modes of action, there are cases
of synergism and antagonism. Despite these difficulties, a considerable number of
chemicals have been assessed as being carcinogenic, mutagenic, or teratogenic, and
it is even possible to assign some degree of potency to each chemical. Such chemicals
usually rank high on priority lists. As was discussed earlier, endocrine modulating
substances are of more recent concern. It seems likely that ingenious toxicologists
will find other subtle toxic effects in the future.

3.2.5 Long-Range Transport

As lakes go, Lake Superior is fairly pristine, since there is relatively little industry
on its shores. In the U.S. part of this lake is an island, Isle Royale, which is a
protected park and is thus even more pristine. In this island is a lake, Siskiwit Lake,
which cannot conceivably be contaminated. No responsible funding agency would
waste money on the analysis of fish from that lake for substances such as PCBs.
Remarkably, perceptive researchers detected substantial concentrations of PCBs.
Similarly, surprisingly high concentrations have been detected in wildlife in the
Arctic and Antarctic. Clearly, certain contaminants can travel long distances through
the atmosphere and oceans and are deposited in remote regions.
This potential for long-range transport (LRT) is of concern for several reasons.
There is an ethical issue when the use of a chemical in one nation (which presumably
enjoys social or economic benefit from it) results in exposure in other downwind
nations that derive no benefit, only adverse effects. This transboundary pollution
issue also applies to gases such as SO

2

, which can cause acidification of poorly
buffered lakes at distant locations. A regulatory agency may then be in the position
of having little or no control over exposures experienced by its public. The political

implications are obvious.

©2001 CRC Press LLC

There is therefore a compelling incentive to identify those chemicals that can
undertake long-range transport and implement international agreements to control
them. A start on this process has been made recently by the United Nations Envi-
ronment Program (UNEP), which has identified 12 substances or groups for inter-
national regulations or bans. These substances, listed in Table 3.3, are also identified
as persistent, bioaccumulative, and toxic. Others are scheduled for restriction or
reduction. They may represent merely the first group of chemicals that will be subject
to international controls. Most contentious of the 12 is DDT, which is still widely
and beneficially used for malaria control.

3.2.6 Other Effects

Finally, there is a variety of other adverse effects that are of concern, including

• the ability to influence atmospheric chemistry (e.g., freons)
• alteration in pH (e.g., oxides of sulfur and nitrogen causing acid rain)
• unusual chemical properties such as chelating capacity, which alters the availability
of other chemicals in the environment
• interference with visibility
• odor (e.g., from organo-sulfur compounds)
• color (e.g., from dyes)
• the ability to cause foaming in rivers (e.g., detergents or surfactants)
• formation of toxic metabolites or degradation products

3.2.7 Selection Procedures

A common selection procedure involves scoring these factors on some numeric
hazard scale. The factors then may be combined to give an overall factor and

Table 3.3 Substances Scheduled for Elimination, Restriction, or Reduction by UNEP
Scheduled for Elimination Scheduled for Restriction
Scheduled for
Reduction

Aldrin DDT PAHs
Chlordane Hexachlorocyclohexanes Dioxins/furans
DDT Polychlorinated biphenyls Hexachlorobenzene
Dieldrin
Endrin
Heptachlor
Hexabromobiphenyl
Hexachlorobenzene
Mirex
Polychlorinated biphenyls
Toxaphene

©2001 CRC Press LLC

determine priority. This is a subjective process, and it becomes difficult for two
major reasons.
First, chemicals that are subject to quite different patterns of use are difficult to
compare. For example, chemical X may be produced in very large quantities, emitted
into the environment, and found in substantial concentrations in the environment,
but it may not be believed to be particularly toxic. Examples are solvents such as
trichloroethylene or plasticizers such as the phthalate esters. On the other hand,
chemical Y may be produced in minuscule amounts but be very toxic, an example

being the “dioxins.” Which deserves the higher priority?
Second, it appears that the adverse effects suffered by aquatic organisms and
other animals, including humans, are the result of exposure to a large number of
chemicals, not just to one or two chemicals. Thus, assessing chemicals on a case-
by-case basis may obscure the cumulative effect of a large number of chemicals.
For example, if an organism is exposed to 150 chemicals, each at a concentration
that is only 1% of the level that will cause death, then death will very likely occur,
but it cannot be attributed to any one of these chemicals. It is the cumulative effect
that causes death. The obvious prudent approach is to reduce exposure to all chem-
icals to the maximum extent possible. The issue is further complicated by the
possibility that some chemicals will act synergistically, i.e., they produce an effect
that is greater than additive; or they may act antagonistically, i.e., the combined
effect is less than additive. As a result, there will be cases in which we are unable
to prove that a specific chemical causes a toxic effect but, in reality, it does contribute
to an overall toxic effect. Indeed, some believe that this situation will be the rule
rather than the exception.
A compelling case can be made that the prudent course of action is for society
to cast a fairly wide net of suspicion (i.e., assemble a fairly large list of chemicals)
then work to elucidate sources, fate, and effects with the aim of reducing overall
exposure of humans, and our companion organisms, to a level at which there is
assurance that no significant toxic effects can exist from these chemicals. The risk
from these chemicals then becomes small as compared to other risks such as acci-
dents, disease, and exposure to natural toxic substances. This approach has been
extended and articulated as the “Precautionary Principle,” the “Substitution Princi-
ple,” and the “Principle of Prudent Avoidance.”
One preferred approach is to undertake a risk assessment for each chemical.
Formal procedures for conducting such assessments have been published, notably
by the U.S. Environmental Protection Agency (EPA). The process involves identi-
fying the chemical, its sources, the environment in which it is present, and the
organisms that may be affected. The toxicity of the substance is evaluated and routes

of exposure quantified. Ultimately, the prevailing concentrations or doses are mea-
sured or estimated and compared with levels that are known to cause effects, and
conclusions are drawn regarding the proximity to levels at which there is a risk of
effect. This necessarily involves consideration of the chemical’s behavior in an actual
environment. Risk is thus assessed only for that environment. Risk or toxic effects
are thus not inherent properties of a chemical; they depend on the extent to which
the chemical reaches the organism.

©2001 CRC Press LLC

3.3 KEY CHEMICAL PROPERTIES AND CLASSES
3.3.1 Key Properties

In Chapter 5, we discuss physicochemical properties in more detail and, in
Chapter 6, we examine reactivities. It is useful at this stage to introduce some of
these properties and identify how they apply to different classes of chemicals.
It transpires that we can learn a great deal about how a chemical partitions in
the environment from its behavior in an air-water-octanol (strictly 1-octanol) system
as shown later in Figure 3.2. There are three partition coefficients, K

AW

, K

OW

, and
K

OA

, only two of which are independent, since K

OA

must equal K

OW

/K

AW

. These can
be measured directly or estimated from vapor pressure, solubility in water, and
solubility in octanol, but not all chemicals have measurable solubilities because of
miscibility. Octanol is an excellent surrogate for natural organic matter in soils and
sediments, lipids, or fats, and even plant waxes. It has approximately the same C:H:O
ratio as lipids. Correlations are thus developed between soil-water and octanol-water
partition coefficients, as discussed in more detail later.
An important attribute of organic chemicals is the degree to which they are

hydrophobic

. This implies that the chemical is sparingly soluble in, or “hates,” water
and prefers to partition into lipid, organic, or fat phases. A convenient descriptor of
this hydrophobic tendency is K

OW

. A high value of perhaps one million, as applies to
DDT, implies that the chemical will achieve a concentration in an organic medium
approximately a million times that of water with which it is in contact. In reality,
most organic chemicals are approximately equally soluble in lipid or fat phases, but
they vary greatly in their solubility in water. Thus, differences in hydrophobicity are
largely due to differences of behavior in, or affinity for, the water phase, not differences
in solubility in lipids. The word

lipophilic

is thus unfortunate and is best avoided.
The chemical’s tendency to evaporate or partition into the atmosphere is primarily
controlled by its vapor pressure, which is essentially the maximum pressure that a
pure chemical can exert in the gas phase or atmosphere. It can be viewed as the

solubility

of the chemical in the gas phase. Indeed, if the vapor pressure in units of
Pa is divided by the gas constant, temperature group RT, where R is the gas constant
(8.314 Pa m

3

/mol K), and T is absolute temperature (K), then vapor pressure can
be converted into a solubility with units of mol/m

3

. Organic chemicals vary enor-
mously in their vapor pressure and correspondingly in their boiling point. Some

(e.g., the lower alkanes) that are present in gasoline are very volatile, whereas others
(e.g., DDT) have exceedingly low vapor pressures.
Partitioning from a pure chemical phase to the atmosphere is controlled by vapor
pressure. Partitioning from aqueous solution to the atmosphere is controlled by K

AW

,
a joint function of vapor pressure and solubility in water. A substance may have a
high K

AW

, because its solubility in water is low. Partitioning from soils and other
organic media to the atmosphere is controlled by K

AO

(air/octanol), which is con-
ventionally reported as its reciprocal, K

OA

. Partitioning from water to organic media,
including fish, is controlled by K

OW

. Substances that display a significant tendency
to partition into the air phase over other phases are termed

volatile organic chemicals

or VOCs. They have high vapor pressures.

©2001 CRC Press LLC

Another important classification of organic chemicals is according to their dis-
sociating tendencies in water solution. Some organic acids, notably the phenols, will
form ionic species (phenolates) at high pH. The tendency to ionize is characterized
by the acid dissociation constant K

A

, often expressed as pK

A

, its negative base ten
logarithm.
In concert with partitioning characteristics, the other set of properties that deter-
mine environmental behavior is reactivity or persistence, usually expressed as a half-
life. It is misleading to assign a single number to a half-life, because it depends on
the intrinsic properties of the chemical and on the nature of the environment. Factors
such as sunlight intensity, hydroxyl radical concentration, the nature of the microbial
community, as well as temperature vary considerably from place to place and time
to time. Here, we use a semiquantitative classification of half-lives into classes,
assuming that average environmental conditions apply. Different classes are defined
for air, water, soils, and sediments. The classification is that used in a series of
“Illustrated Handbooks” by Mackay, Shiu, and Ma is shown below in Table 3.4.

The half-lives are on a logarithmic scale with a factor of approximately 3 between
adjacent classes. It is probably misleading to divide the classes into finer groupings;
indeed, a single chemical may experience half-lives ranging over three classes,
depending on environmental conditions such as season.
We examine, in the following sections, a number of classes of compounds that
are of concern environmentally. In doing so, we note their partitioning and persis-
tence properties. The structures of many of these chemicals are given in Figure 3.1.
Table 3.5 gives suggested values of these properties for selected chemicals.
Figure 3.2 is a plot of log K

AW

versus log K

OW

for the chemicals in Table 3.5
on which lines of constant K

OA

lie on the 45° diagonal. This graph shows the wide
variation in properties. Volatile compounds tend to lie to the upper left, water-soluble
compounds to the lower left, and hydrophobic compounds to the lower right. Assum-
ing reasonable relative volumes of air (650,000), water (1300), and octanol (1), the
percentages in each phase at equilibrium can be calculated. The lines of constant
percentages are also shown. Lee and Mackay (1995) have used equilateral triangular
diagrams to display the variation in partitioning properties in a format similar to
that of Figure 3.2.

Table 3.4 Classes of Chemical Half-Life or Persistence, Adapted from

the Handbooks of Mackay et al., 2000
Class Mean half–life (hours) Range (hours)

15<10
2 17 (~ 1 day) 10–30
3 55 (~ 2 day) 30–100
4 170 (~1 week) 100–300
5 550 (~3 weeks) 300–1000
6 1700 (~2 months) 1000–3000
7 5500 (~8 months) 3000–10,000
8 17000 (~2 years) 10,000–30,000
9 55000 (~6 years) >30,000

©2001 CRC Press LLC
Figure 3.1 Structures of selected chemicals of environmental interest (continues).

©2001 CRC Press LLC
Figure 3.1 (continued)

Table 3.5 Physical Chemical properties of Selected Organic Chemicals at 25°C Including Estimated Half-Lives Classified as in Table 3.4 and
Toxicity Expressed as Oral LD50 to the Rat. These data have been selected from a number of sources, including Mackay et al. (2000),

RTECS (2000), and the Hazardous Substances Data Bank (2000).
Degradation Half-lives (h)
Chemical Name
Molar
mass (g/mol)
Vapor

pressure (Pa)
Aqueous
solubility (g/m

3

) Log K

OW

Melting
point ((C) Air Water Soil Sediment
Rat oral
LD50
(mg/kg)

benzene 78.11 12700 1780 2.13 5.53 17 170 550 1700 930
1,2,4-trimethylbenzene 120.2 270 57 3.6 –43.8 17 550 1700 5500 3550
ethylbenzene 106.2 1270 152 3.13 –95 17 550 1700 5500 5460
n-propylbenzene 120.2 450 52 3.69 –101.6 17 550 1700 5500 6040
styrene 104.14 880 300 3.05 –30.6 5 170 550 1700 2650
toluene 92.13 3800 515 2.69 –95 17 550 1700 5500 5000
nitrobenzene 123.11 20 1900 1.85 5.6 5 1700 1700 5500 349
2-nitrotoluene 137.14 17.9 651.42 2.3 –3.85 17 55 1700 5500 891
4-nitrotoluene 137.14 0.653 254.4 2.37 51.7 17 55 1700 5500 1960
2,4-dinitrotoluene 182.14 0.133 270 2.01 70 17 55 1700 5500 268
chlorobenzene 112.6 1580 484 2.8 –45.6 170 1700 5500 17000 1110
1,4-dichlorobenzene 147.01 130 83 3.4 53.1 550 1700 5500 17000 500
1,2,3-trichlorobenzene 181.45 28 21 4.1 53 550 1700 5500 17000 756
1,2,3,4-tetrachlorobenzene 215.9 4 7.8 4.5 47.5 1700 5500 5500 17000 1470

pentachlorobenzene 250.3 0.22 0.65 5 86 5500 17000 17000 17000 11000
hexachlorobenzene 284.8 0.0023 0.005 5.5 230 7350 55000 55000 55000 3500
fluorobenzene 96.104 10480 1430 2.27 –42.21 17 170 550 1700 4399
bromobenzene 157.02 552 410 2.99 –30.8 170 1700 5500 17000 2383
iodobenzene 204.01 130 340 3.28 –31.35 170 1700 5500 17000 1749
n-pentane 72.15 68400 38.5 3.45 –129.7 17 550 1700 5500 90000
n-hexane 86.17 20200 9.5 4.11 –95 17 550 1700 5500 30000
1,3-butadiene 54.09 281000 735 1.99 –108.9 5 170 550 1700 5480
1,4-cyclohexadiene 80.14 9010 700 2.3 –49.2 5 170 550 1700 130

©2001 CRC Press LLC

Table 3.5 (continued)

dichloromethane 84.94 26222 13200 1.25 –95 1700 1700 5500 17000 1600
trichloromethane 119.38 26244 8200 1.97 –63.5 1700 1700 5500 17000 1000
carbon tetrachloride 153.82 15250 800 2.64 –22.9 17000 1700 5500 17000 2350
tribromomethane 252.75 727 3100 2.38 –8.3 1700 1700 5500 17000 933
bromochloromethane 129.384 19600 14778 1.41 –87.95 550 550 1700 5500 5000
bromodichloromethane 163.8 6670 4500 2.1 –57.1 550 550 1700 5500 430
1,2-dichloroethane 98.96 10540 8606 1.48 –35.36 1700 1700 5500 17000 750
1,1,2,2-tetrachloroethane 167.85 793 2962 2.39 –36 17000 1700 5500 17000 200
pentachloroethane 202.3 625 500 2.89 –29 17000 1700 5500 17000 920
hexachloroethane 236.74 50 50 3.93 186.1 17000 1700 5500 17000 5000
1,2-dichloropropane 112.99 6620 2800 2 –100.4 550 5500 5500 17000 1947
1,2,3-trichloropropane 147.43 492 1896 2.63 –14.7 550 5500 5500 17000 505
chloroethene (vinyl chloride) 62.5 354600 2763 1.38 –153.8 55 550 1700 5500 500
trichloroethylene 131.39 9900 1100 2.53 –73 170 5500 1700 5500 4920
tetrachloroethylene 165.83 2415 150 2.88 –19 550 5500 1700 5500 2629
methoxybenzene 108.15 472 2030 2.11 –37.5 17 550 550 1700 3700

bis(2-chloroethyl)ether 143.02 206 10200 1.12 –46.8 17 550 550 1700 75
bis(2-chloroisopropyl)ether 171.07 104 1700 2.58 –97 17 550 550 1700 240
2-chloroethyl vinyl ether 106.55 3566 15000 1.28 –69.7 17 550 550 1700 210
bis(2-chloroethoxy)methane 173.1 21.6 8100 1.26 0 17 550 550 1700 65
1-pentanol 88.149 300 22000 1.5 –78.2 55 55 55 170 3030
1-hexanol 102.176 110 6000 2.03 –44.6 55 55 55 170 720
benzyl alcohol 108.14 12 80 1.1 –15.3 55 55 55 170 1230
cyclohexanol 100.16 85 38000 1.23 25.15 55 55 55 170 1400
benzaldehyde 106.12 174 3000 1.48 –55.6 5 55 55 170 1300
3-pentanone 86.135 4700 34000 0.82 –38.97 55 170 170 550 2410
2-heptanone 114.18 500 4300 2.08 –35 55 170 170 550 1670
cyclohexanone 98.144 620 23000 0.81 –32.1 55 170 170 550 1540
acetophenone 120.15 45 5500 1.63 19.62 550 170 170 550 815

©2001 CRC Press LLC

Table 3.5 (continued)

vinyl acetate 86.09 14100 20000 0.73 –92.8 55 55 170 550 2900
propyl acetate 102.13 4500 21000 1.24 –95 55 55 170 550 9370
methyl methacrylate 100.12 5100 15600 1.38 –42.8 17 55 55 170 7872
diphenylamine 169.23 0.0612 300 3.45 52.8 5 55 170 550 2000
aniline 93.12 65.19 36070 0.9 –6.3 5 170 170 1700 250
quinoline 129.16 1.21 6110 2.06 –14.85 55 170 550 1700 331
thiophene 84.14 10620 3015 1.81 –38 55 55 1700 5500 1400
benzoic acid 122.13 0.11 3400 1.89 122.4 55 55 170 550 1700
hexanoic acid 116.1 5 958 1.92 –3.44 55 55 170 550 6400
phenylacetic acid 136.15 0.83 16600 1.41 77 55 55 170 550 2250
salicylic acid 138.12 0.0208 2300 2.2 159 55 55 170 550 891
anthracene 178.2 0.001 0.045 4.54 216.2 55 550 5500 17000 8000

benzo[a]pyrene 252.3 7 x 10

–7

0.0038 6.04 175 170 1700 17000 55000 n/a
chyrsene 228.3 5.7 x
10

–7

0.002 5.61 255 170 1700 17000 55000 n/a
naphthalene 128.19 10.4 31 3.37 80.5 17 170 1700 5500 2400
phenanthrene 178.2 0.02 1.1 4.57 101 55 550 5500 17000 n/a
p-xylene 106.2 1170 214.9488 3.18 13.2 17 550 1700 5500 4300
pyrene 202.3 0.0006 0.132 5.18 156 170 1700 17000 55000 n/a
benzo(b)thiophene 134.19 26.66 130 3.12 30.85 170 550 1700 5500 2200
1-methylnaphthalene 142.2 8.84 28 3.87 –22 17 170 1700 5500 1840
biphenyl 154.2 1.3 7 3.9 71 55 170 550 1700 3280
PCB-7 223.1 0.254 1.25 5 24.4 170 5500 17000 17000 n/a
PCB-15 223.1 0.0048 0.06 5.3 149 170 5500 17000 17000 n/a
PCB-29 257.5 0.0132 0.14 5.6 78 550 17000 55000 55000 n/a
PCB-52 292 0.0049 0.03 6.1 87 1700 55000 55000 55000 n/a
PCB-101 326.4 0.00109 0.01 6.4 76.5 1700 55000 55000 55000 n/a
PCB-153 360.9 0.000119 0.001 6.9 103 5500 55000 55000 55000 n/a
PCB-209 498.7 5.02x10

–8

10

–6

8.26 305.9 55000 55000 55000 55000 n/a

©2001 CRC Press LLC

Table 3.5 (continued)

total PCB 326 0.0009 0.024 6.6 0 5500 55000 500000 500000 1900
dibenzo-p-dioxin 184 0.055 0.865 4.3 123 55 55 1700 5500 1220
2,3,7,8-tetraCDD 322 0.0000002 1.93x10

–5

6.8 305 170 550 17000 55000 0.02
1,2,3,4,7,8-hexaCDD 391 5.1x10

–9

4.42x10

–6

7.8 273 550 1700 55000 55000 0.8
1,2,3,4,6,7,8-heptaCDD 425.2 7.5x10

–10

2.4x10

–6

8 265 550 1700 55000 55000 6.325
OCDD 460 1.1x10

–10

7.4x10

–8

8.2 322 550 5500 55000 55000 1
dibenzofuran 168.2 0.3 4.75 4.31 86.5 55 170 1700 5500 n/a
2,8-dichlorodibenzofuran 237.1 0.00039 0.0145 5.44 184 170 550 5500 17000 n/a
2,3,7,8-tetrachlorodibenzofuran 306 2x10

–6

4.19x10

–4

6.1 227 170 550 17000 55000 n/a
octachlorodibenzofuran 443.8 5x10

–10

1.16x10

–6

8 258 550 5500 55000 55000 n/a
4-chlorophenol 128.56 20 27000 2.4 43 55 550 550 1700 500
2,4-dichlorophenol 163 12 4500 3.2 44 55 550 550 1700 2830
2,3,4-trichlorophenol 197.45 1 500 3.8 79 170 170 1700 5500 2800
2,4,6-trichlorophenol 197.45 1.25 434 3.69 69.5 170 170 1700 5500 2800
2,3,4,6-tetrachlorophenol 231.89 0.28 183 4.45 70 550 550 1700 5500 140
pentachlorophenol 266.34 0.00415 14 5.05 190 550 550 1700 5500 210
2,4-dimethylphenol 122.17 13.02 8795 2.35 26 17 55 170 550 2300
p-cresol 108.13 13 20000 1.96 34.8 5 17 55 170 207
dimethylphthalate (DMP) 194.2 0.22 4000 2.12 5 170 170 550 1700 2400
diethylphthalate (DEP) 222.26 0.22 1080 2.47 –40.5 170 170 550 1700 8600
dibutylphthalate (DBP) 278.34 0.00187 11.2 4.72 –35 55 170 550 1700 8000
butyl benzyl phthalate 312.39 0.00115 2.69 4.68 –35 55 170 550 1700 13500
di-(2-ethylhexyl)-phthalate
(DEHP)
390.54 1.33x10

–5

0.285 5.11 –50 55 170 550 1700 25000
aldicarb 190.25 0.004 6000 1.1 99 5 550 1700 17000 0.5
aldrin 364.93 0.005 0.02 6.50 104 55 5500 17000 55000 39
carbaryl 201.22 0.0000267 120 2.36 142 55 170 550 1700 230
carbofuran 221.3 0.00008 351 2.32 151 5 170 550 1700 5
chloropyrifos 350.6 0.00227 0.73 4.92 41 17 170 170 1700 82

©2001 CRC Press LLC

Table 3.5 (continued)

cis-chlordane 409.8 0.0004 0.056 6 103 55 17000 17000 55000 500
p,p’-DDE 319 0.000866 0.04 5.7 88 170 55000 55000 55000 880
p,p’-DDT 354.5 0.00002 0.0055 6.19 108.5 170 5500 17000 55000 87
dieldrin 380.93 0.0005 0.17 5.2 176 55 17000 17000 55000 38.3
diazinon 304.36 0.008 60 3.3 0 550 1700 1700 5500 66

g

–HCH (lindane)

290.85 0.00374 7.3 3.7 112 1040 17000 17000 55000 76

a

–HCH

290.85 0.003 1 3.81 157 1420 3364 1687 55000 177
heptachlor 373.4 0.053 0.056 5.27 95 55 550 1700 5500 40
malathion 330.36 0.001 145 2.8 2.9 17 55 55 550 290
methoxychlor 345.7 0.00013 0.045 5.08 86 17 170 1700 5500 1855
mirex 545.59 0.0001 0.000065 6.9 485 170 170 55000 55000 235
parathion 291.27 0.0006 12.4 3.8 6 17 550 550 1700 2
methyl parathion 263.5 0.002 25 3 37 17 550 550 1700 6.01
atrazine 215.68 0.00004 30 2.75 174 5 550 1700 1700 672
2-(2,4-dichlorophenoxy)
acetic acid
221.04 0.00008 400 2.81 140.5 17 55 550 1700 375
dicamba 221.04 0.0045 4500 2.21 114 55 550 550 1700 1039
mecoprop 214.6 0.00031 620 3.94 94 17 170 170 1700 650

metolachlor 283.8 0.0042 430 3.13 0 170 1700 1700 5500 2200
simazine 201.7 8.5x10

–6

5 2.18 225 55 550 1700 5500 971
trifluralin 335.5 0.015 0.5 5.34 48.5 170 1700 1700 5500 1930
thiram 240.4 0.00133 30 1.73 145 170 170 550 1700 560

©2001 CRC Press LLC

©2001 CRC Press LLC
Figure 3.2 Plot of log K
AW
vs. log K
OW
for the chemicals in Table 3.5 on which dotted lines of
constant K
OA
line on the 45° diagonal. This graph shows the wide variation in
properties. Volatile compounds tend to lie to the upper left, water-soluble com-
pounds to the lower left, and hydrophobic compounds to the lower right. The thicker
lines represent constant percentages present at equilibrium in air, water, and
octanol phases, assuming a volume ratio of 656,000:1300:1, respectively. Modified
from Gouin et al. (2000).

©2001 CRC Press LLC

3.3.2 Chemical Classes (see Fig. 3.1 for structures and Table 3.5 for
properties)

3.3.2.1 Hydrocarbons

Hydrocarbons are naturally occurring chemicals present in crude oil and natural
gas. Some are formed by biogenic processes in vegetation, but most contamination
comes from oil spills, effluents from petroleum and petrochemical refineries, and
the use of fuels for transportation purposes.
The alkanes can be separated into classes of normal, branched (or iso) species
and cyclic alkanes, which range in molar mass from methane or natural gas to waxes.
They are usually sparingly soluble in water. For example, hexane has a solubility
of approximately 10 g/m

3

. This solubility falls by a factor of about 3 or 4 for every
carbon added. The branched and cyclic alkanes tend to be more soluble in water,
apparently because they have smaller molecular areas and volumes.
Highly branched or cyclic alkanes such as terpenes are produced by vegetation.
They are often sweet smelling and tend to be very resistant to biodegradation.
The alkenes or olefins are not naturally occurring to any significant extent. They
are mainly used as petrochemical intermediates. The alkynes, of which ethyne or
acetylene is the first member, are also chemical intermediates that are rarely found
in the environment. These unsaturated hydrocarbons tend to be fairly reactive and
short-lived in the environment, whereas the alkanes are more stable and persistent.
Of particular environmental interest are the aromatics, the simplest of which is
benzene. The aromatics are relatively soluble in water, for example, benzene has a
solubility of 1780 g/m

3

. They are regarded as fairly toxic and often troublesome
compounds. A variety of substituted aromatics can be obtained by substituting
various alkyl groups. For example, methyl benzene is toluene.
When two benzene rings are fused, the result is naphthalene, which is also a
chemical of considerable environmental interest. Subsequent fusing of benzene rings
to naphthalene leads to a variety of chemicals referred to as the polycyclic aromatic
hydrocarbons or polynuclear aromatic hydrocarbons (PAHs). These compounds tend
to be formed when a fuel is burned with insufficient oxygen. They are thus present
in exhaust from engines and are of interest because many are carcinogenic.
Biphenyl is a hydrocarbon that is not of much importance as such, but it forms
an interesting series of chlorinated compounds, the PCBs or polychlorinated biphe-
nyls, which are discussed later.

3.3.2.2 Halogenated Hydrocarbons

If the hydrogen in a hydrocarbon is substituted by chlorine (or less frequently
by bromine, fluorine, or iodine), the resulting compound tends to be less flammable,
more stable, more hydrophobic, and more environmentally troublesome. Replacing
a hydrogen with a chlorine usually causes an increase in molar volume and area and
a corresponding decrease in solubility by a factor of about 3.
The stability of many of these compounds makes them invaluable as solvents,
examples being methylene chloride and tetrachloroethylene. The fluorinated and

©2001 CRC Press LLC

chlorofluoro compounds are very stable and are used as refrigerants. Because these
molecules are quite small, they are fairly soluble in water and are therefore able to
penetrate the tissues of organisms quite readily. They are thus used as anaesthetics
and narcotic agents.
The chlorinated aromatics are a particularly interesting group of chemicals. The

chlorobenzenes are biologically active. 1,4 or paradichlorobenzene is widely used
as a deodorant and disinfectant. The polychlorinated biphenyls, or PCBs, and their
brominated cousins, the PBBs, are notorious environmental contaminants, as are
chlorinated terpenes such as toxaphene, which is a very potent and long-lived
insecticide. Many of the early pesticides, such as DDT, mirex, and chlordane, are
chlorinated hydrocarbons. They possess the desirable properties of stability and a
high tendency to partition out of air and water into the target organisms. Thus,
application of a pesticide results in protection for a prolonged time. As Rachel Carson
demonstrated in

Silent Spring,

the problem is that these chemicals persist long
enough to affect non-target organisms and to drift throughout the environment,
causing widespread contamination.
Fluorinated chemicals also possess considerable stability and, because the fluo-
rine atom is lighter than chlorine, they are generally more volatile. Polyfluorinated
substances are very stable in the environment as a result of the strong C-F bond.
Brominated chemicals are also stable, but with reduced volatility. A major use of
brominated substances is in fire retardants, specifically polybrominated diphenyl
ethers.

3.3.2.3 Oxygenated Compounds

The most common oxygenated organic compounds are the alcohols, ethanol
being among the most widely used. Others are octanol, which is a convenient
analytical surrogate for fat, and glycerol is of interest because it forms the backbone
of fat molecules by esterification with fatty acids to form glycerides.
The phenols consist of an aromatic molecule in which a hydrogen is replaced
by an OH group. They are acidic and tend to be biologically disruptive. Phenol, or

carbolic acid, was the first disinfectant. Substituting chlorines on phenol tends to
increase the toxic potency of the substance and its tendency to ionize, i.e., its pKa
is reduced. Pentachlorophenol (PCP) is a particularly toxic chemical and has been
widely used for wood preservation.
The ketones such as acetone, and aldehydes such as formaldehyde, are fairly
reactive in the environment and can be of concern as atmospheric contaminants in
regions close to sources of emission. Much of the smog problem is attributable to
aldehydes formed in combustion processes.
Organic acids such as acetic acid are also fairly reactive. They are not usually
regarded as an environmental problem, but trifluoroacetic acid, which is formed by
combustion of freons and from some pesticides, is very persistent. Some chlorinated
organic acids, e.g., 2,4-D, are potent herbicides. Longer-chain acids, such as stearic
acid, are mainly of interest because they esterify with glycerol to form fats. Humic
and fulvic acids are of considerable environmental importance. These are substances

©2001 CRC Press LLC

of complex and variable structure that are naturally present in soils, water, and
sediments. They are the remnants of living organic materials, such as wood, that
has been subjected to prolonged microbial conversion. These acids are sparingly
soluble in water, but the solubility can be increased at high pH.
The esters or “salts” or organic acids and alcohols tend to be relatively innocuous
and short-lived in most cases. A notable exception is the phthalate esters, which are
very stable oily substances and are invaluable additives (plasticizers) for plastics,
rendering them more flexible. Notable among the phthalate esters is diethylhexy-
lphthalate (DEHP), the ester with two molecules of 2 ethylhexanol. The other esters
of interest are the glycerides—for example, glyceryl trioleate, the ester of glycerine
and oleic acid. This chemical has similar properties to fat and has been suggested
as a convenient surrogate for measuring fat to water partitioning.

The “dioxins” and “furans” are two series of organic compounds

that

have
become environmentally notorious. The chlorinated dibenzo-p-dioxins were never
produced intentionally but are formed under combustion conditions when chlorine
is present. They form a series of very toxic chemicals, the most celebrated of which
is 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD). TCDD is possibly the most toxic
chemical to mammals. A dose of 2

m

g of TCDD per kg of body weight is sufficient
to kill small rodents.

A related series of chemicals is the dibenzofurans, which are similar in properties
to the dioxins. It appears that molecules that are long and flat, with chlorine atoms
strategically located at the ends, are particularly toxic. Examples are the chloronaph-
thalenes, DDT, the PCBs, and chlorinated dibenzo-p-dioxins and dibenzofurans.
Other oxygenated compounds of interest include carbohydrates, cellulose, and
lignins, which occur naturally.

3.3.2.4 Nitrogen Compounds

Nitrogen compounds of environmental interest include amines, amides,
pyridines, quinolines, and amino acids, and various nitro compounds including nitro
polycyclicaromatics and nitroso compounds. Many of these compounds occur nat-
urally, are quite toxic, and are difficult to analyze.

3.3.2.5 Sulfur Compounds

Sulfur compounds, including thiols, thiophenes, and mercaptans, are well known
because of their strong odor. One of the most prevalent classes of synthetic organic
chemicals is the alkyl benzene sulfonates, which are widely used in detergents.

3.3.2.6 Phosphorus Compounds
Phosphorus compounds play a key role in energy transfer in organisms. Organo-
phosphate compounds have been developed as pesticides (e.g., chloropyrifos), which
have the very desirable properties of high biological activity but relatively short
environmental persistence. They have therefore largely replaced organo-chlorine
compounds in agriculture.