Major Classes of Contaminants
Introduction
What contaminants presently concern us the most? Many have been
recognized and some have received close attention for decades, whereas
others have only recently emerged as concerns. For the sake of space, only
the most prominent conventional or emerging contaminants are introduced
here.
Following convention, chemical contaminants are divided grossly into
organic and inorganic. The terms organic and inorganic were applied
originally to indicate whether the chemical came from living organisms
(organic) or mineral sources (inorganic).
Organic contaminants include intentional poisons such as insecticides,
herbicides, fungicides, and wood preservatives. They become a problem only if
nontarget species come into contact with sufficiently high concentrations of
them.
Other organic contaminants are unintentional poisons, for example, degreasers,
solvents, and various industrial byproducts. Some organic compounds, such as
those in personal care products (e.g., detergents and musks) or pharmaceuticals
(e.g., drugs, antibiotics, and birth control substances), are designed to be
directly beneficial to an individual's well-being yet still cause problems after
release into the environment.
Similarly, inorganic contaminants are composed of intentional and unintentional
poisons. Some are released for a very specific purpose (for example, sodium
arsenate as a pesticide), but others are released by a wide range of human
activities, such as the lead used in batteries, plumbing, gas additives, and many
other products. Inorganic contaminants, such as nitrite in drinking waters,
become a problem only if our activities elevate their concentrations to
abnormal levels. Indeed, several metals are essential to life but, above certain
concentrations, become harmful.
A mammalian toxicologist would not consider all of the chemicals discussed in
this chapter to be toxicants because they do not directly poison individuals.
However, contaminants such as excessive amounts of phosphorus or nitrogen
species in lakes can have harmful consequences to the associated ecological
community.
Similarly, global changes in atmospheric gases such as carbon dioxide (CO2) and
methane have a pronounced influence on the Earth's ecosystems at
concentrations that are not directly toxic to humans.
The term contaminant simply refers to a substance released by human
activities. No harmful effect is necessarily implied in its release. However, the
term pollutant refers to a contaminant that has or likely will have a deleterious
effect on humans or ecological entities.
Ecotoxicants any agent that has an adverse effect on any biological entity
within the biochemical to biospheric scale. Adverse effects include
conventional lethal and sublethal effects of toxicants. They also include
unconventional, adverse modifications of essential habitat, intraspecies or
interspecies interactions, metapopulation or metacommunity dynamics,
material cycling, or energy flow.
Inorganic Chemicals
o
Contaminants that contain elements other than
carbon
•
o
o
Do not degrade easily
Lead
•
o
Examples: acids, salts, and heavy metals
Found in old paint, industrial pollutants, leaded
gasoline
Mercury
•
Mercury bioaccumulates in the muscles of top
predators of the open ocean
Organic Compounds
o
Chemicals that contain carbon atoms
•
•
Natural examples: sugars, amino acids, and oils
Human-made examples: pesticides, solvents,
industrial chemicals, and plastics
Radioactive Substances
o
o
Contain atoms of unstable isotopes that
spontaneously emit radiation
Sources
•
•
•
•
Mining
Processing radioactive materials
Nuclear power plants
Natural sources
II. MAJOR CLASSES OF CONTAMINANTS
A. Inorganic Contaminants
The periodic table is a reasonable tool for framing our discussion of different
inorganic contaminants. Elements in each of its 18 groupings have similar
outer orbital configurations, resulting in common themes in their
environmental chemistries and toxicologies. The elements in this table can
be divided generally into metals, nonmetals, or metalloids (semi-metals).
Most elements are metals. Pure metals are characteristically solid at room
temperature, although mercury is an exception. They are described as good
electrical and thermal conductors with high luster and malleability. They
emit electrons readily on heating.
Because most of the relevant metals in our environment do not exist
prominently in the pure elemental state, the effects of metal compounds,
complexes, and ions are often much more significant to the ecotoxicologist.
Normal biological processes can be negatively affected by uptake of metals or
metal species from the environment.
Ecotoxicologists must recognize the characteristics that influence changes in
state (e.g., their oxidation, ionization, and incorporation into organic form) in
the environment and subsequent movements and interactions of the metals.
Non-metals are a minority cluster of elements in the upper right side of the
periodic table (plus hydrogen). They include the noble gases that, by
definition, do not readily react with other elements. With the exception of
radon, the noble gases will not hold our attention beyond this brief mention.
Although a minority in the table, the remaining nonmetals constitute the
majority of the bulk elements making up living organisms. They, including the
halogens in period 7A, tend to be very electronegative.
In contrast to the metals, they have a strong tendency to attract electrons
during chemical interactions. The strength of the attraction depends on
several factors, including their outer orbital electron configuration, which, in
turn, partially determines how polar their covalent bonds might be with
other elements such as metals. Generally, the larger the difference in
electronegativity between two interacting elements, the more the bond
between them will tend toward ionic and away from covalent.
Metalloids, the remaining elements in the periodic table, are intermediate in
their properties, having some qualities of metals. For example, they can be
semiconductors with their electrical conductance increasing with
temperature. They can also form strong covalent bonds with some other
elements.
Metals and Metalloids
The top panel of Figure 2.2 is a complexation field diagram (based largely on
HSAB theory) that describes how the stability of bonds changes among
dissolved metals in freshwater and sea-water systems. Progressing across the
diagram from class a (left) to b (right) metals, the stability of metal covalent
bonds with soft ligands increases. Relevant soft ligands in the aquatic
environments are Ch S2-, and HS~. As an example relevant to biomolecules,
the sulfur atoms in the amino acids methionine and cysteine are soft donor
atoms.
Dissolved metal might speciate in natural waters or interact with biomolecules.
For example Na+ in natural waters does not form strong covalent or ionic
bonds with dissolved ligands. Conversely, dissolved cadmium speciation in
seawater is dominated by complexes with Cl-, such as CdCl-.
Another quality to understand is whether an element is essential to life.
Essential elements include bulk and trace components of biological entities.
The bulk essential elements include only four class a metals. Many more trace
essential elements are metals. Too low as well as too high concentrations of
essential elements in organisms can be harmful. If another element is
chemically very similar in its chemistry to an essential element, that essential
element analog can enter into biochemical processes in place of the essential
element and interfere with normal biological functioning if present at too high
a concentration relative to that of the essential element.
Metalloids are also important to understand relative to their binding
chemistries, and this same scheme can be applied to understand their fate
and effects.
Probably the most environmentally relevant is arsenic, which in many ways
acts like its neighboring nonmetal on the periodic table, selenium. Both
arsenic and selenium form oxyanions that often influence their movements
in the environment, biochemical transformations, and effects on biota.
In the case of arsenic, both arsenite (AsO2-) and arsenate (AsO34 -) are
common anionic forms. The biochemical similarity of arsenate to the
phosphates that are involved in essential biochemical processes and
molecules contributes to arsenate's poisonous tendencies. As an oxyanion,
arsenic can also form compounds with toxic metals.
Organometallic Compounds
Some metals combine with organic structures to produce useful products.
Inorganic Gasses
Nitrogen oxides (NOx) and sulfur dioxide (SO2) are also produced at high
volumes by combustion in stationary (e.g., coal power plants) and mobile
(e.g., motor vehicles) sources. NOx and SO2 react in the atmosphere to
produce low pH precipitation or “acid rain”. There is also epidemiological
evidence of adverse health effects of these gases, such as linkage to
various human lung and cardiovascular diseases, and even death. High
levels of SO2 can cause plant leaf necrosis.
4. Anionic Contaminants Including Nutrients
An excess of nitrogen and phosphorus nutrients in aquatic and terrestrial
systems change the structure and functioning of associated ecological
communities. Cultural (accelerated) eutrophication is the classic example of
such an adverse effect.
The combination of high nutrient concentrations in the river water plus high
light conditions in the shallow reservoir caused so much growth that the
demarcation was very clear between the newly arrived river water mass and
the warmer lake water.
Nitrogen species can cause other adverse effects if present at sufficiently high
concentrations. Nitrate can enter water bodies from runoff or sewage
discharge. High concentrations in drinking water causes methemoglobinemia
("blue-baby syndrome" resulting from the reaction of nitrite with hemoglobin
that converts it to methemoglobin, which is incapable of normal transport of
oxygen) in newborn infants. The nitrite is produced from nitrate in the baby's
stomach. Nitrosamines, potent carcinogens, can form from nitrogen
compounds in drinking waters.
Nitrite is toxic to aquatic biota and can also cause methemoglobinemia
directly. Both nitrate and nitrite can also decrease oxygen affinity of
hemocyanin in the hemolymph of aquatic invertebrates.
Sulfate can become problematic as when SO2 released into the atmosphere
from combustion sources reacts with moisture to produce sulfuric acid.
Exposure of sulfide ores from metal mines to oxygen and chemoautotrophic
bacteria such as Thiobacillus ferroxidans generate low pH drainage with very
high sulfate concentrations.
B. Organic Contaminants
Organic compounds are crudely divided into aliphatic and aromatic compounds,
with some compounds having parts that are aliphatic and others that are
aromatic in nature. Aromatic compounds include benzene or similar
compounds in their structure. Aliphatic compounds are composed of carbon
chains with various bonds and other nonaromatic structures.
1. Hydrochlorofluorocarbons and Chlorofluorocarbons
This general grouping of compounds includes the CFCs, which are composed of
C, F, and Cl atoms, and the hydrochlorofluorocarbons (HCFCs), which contain H
in addition.
These compounds were used widely for refrigeration, air conditioning,
firefighting, foam blowing in Styrofoam and polyurethane production, various
solvent applications such as electronics cleansing, propellants such as those in
aerosol cans, and in the case of l-chloro-l,l-difluoroethane. as intermediates in
fluoropolymer production.
Generally, they are nontoxic, odorless gases at room temperature that can be
made liquid easily with mild compression. This last quality makes them ideal for
the uses just listed. They are moderately soluble in water and have low
lipophilicity. The hydrochlorofluorocarbons were introduced as substitutes for
Chlorofluorocarbons because they have much less capacity to deplete the
ozone layer of the atmosphere. This is largely a consequence of the hydrogen
atom that makes the HCFCs more reactive in the troposphere than the very
inert CFCs For example, the potential of HCFC-141b to deplete stratospheric
ozone (ozone depletion potential [ODP] = 0.1) is only 1/10 that of CFC-11, which
has an ODP of 1 by convention.
2. Organochlorine Alkenes
3. Polycyclic Aromatic Hydrocarbons
4. Polyhalogenated Benzenes, Phenols, and Biphenyls
5. Polychlorinated Naphthalenes
6. Polychlorinated Dibenzodioxins and Dibenzofurans
7. Pesticides
Most OC pesticides and many of the organic chemicals already discussed
(HCB, PCBs, PCDFs, and PCDDs) are classified as POPs because they are
resistant to degradation in the environment and tend to increase in
concentration with movement up through food webs. They can persist at
alarming concentrations in biota, and many can disperse globally as will be
discussed later.
Sometimes, POPs are called persistent toxicants that bioaccumulate or
persistent bioaccumulative toxic chemicals. The most widely recognized OC
pesticide members of the POPs are aldrin, chlordane, ODD, DDE, DDT,
dieldrin/endrin, heptachlor, mirex, and toxaphene. Chlordecone (kepone) is
also a POP that can cause significant contamination associated with
pesticide manufacturing. Of course, being so structurally similar to PCBs, the
PBBs will behave as POPs.
Numerous national laws ban production of members of the POPs. Recognizing
the global context of the problem posed by POPs, the United Nations Stockholm
Convention on Persistent Organic Pollutants was established in 2001, was
eventually signed by 50 countries, and became binding in 2004.
Its aim was to eliminate 12 POPs; however, some use of DDT in developing
countries that are still combating endemic malaria was permitted until a suitable
replacement can be found.
a. Organochlorine
The OC pesticides degrade very slowly and tend to be very soluble in lipids
such as those of organisms. This results in bioaccumulation and possible
increase in concentrations with passage through food webs.
The DDT was an extraordinarily valuable insecticide that was banned in the
1960s when its adverse effects to wildlife became apparent. In addition to
being a degradation product of DDT, DDD was used widely as a pesticide and
was banned along with DDT after lethal and sublethal poisonings of wildlife
occurred. DDE can be produced from DDT in the environment or as a DDT
metabolite in organisms.
All (DDT, DDD, and DDE) are present as mixtures of isomers in the
environment. Often, concentrations of DDT, DDD, and DDE in samples are
summed and reported as ZDDT, total DDT, or tDDT.
Other chlorinated pesticides share with DDT an acute toxicity mode of action
involving disruption of neural transmission. They inhibit ATPases that are
crucial for membrane ion transport in nervous tissues.
b. Organophosphorus
These pesticides degrade faster in the environment than OC pesticides.
However, they tend to be more acutely toxic to mammals than OC pesticides
and are often involved in human pesticide poisonings. They can also
accumulate in fats and oils of organisms.
The mode of action for these compounds is the inhibition of acetylcholine
esterase activity.
So, like the OC pesticides, organophosphorus pesticides are neurotoxicants.
However, they act by blocking acetyl-cholinesterase, an enzyme essential in
breaking down the neurotransmitter acetylcholine. Important examples are
shown in Figure 2.15. Others include fenitrothion, fenthion, fonofos, and
parathion.
c. Carbamate
Like organophosphate insecticides, carbamate insecticides degrade rapidly in the
environment and cause neural dysfunction by inhibiting acetylcholinesterase.
Carbamates have high acute toxicity to mammals, but their toxicities tend to be
lower than those of the organophosphate pesticides.
Examples include those shown in Figure 2.16. They are derived from carbamic
acid (H2NCOOH), having the general structure of R,-NH-COO-R2.
d. Pyrethrin and Pyrethroid
Along with rotenone, toxaphene, and the neonicotinoid insecticides, these
toxicants are themselves, or are similar to, natural pesticides produced by
plants.
The neurotoxic effects of pyrethrum and pyrethroid pesticides result from their
interference with normal sodium channel functioning in cells of nervous and
other excitable tissues.
All are similar to pyrethrum produced by the plants Chrysanthemum cineum
and cinerari-aefolium. Some are natural products, although they might now be
produced synthetically. Others are analogs of or similar to the natural
compounds. The natural compounds are called pyrethrins, and their synthetic
analogs or derivatives are called pyrethroids. Pyrethrin 1 in Figure 2.17 is a
pyrethrin. Allethrin and permethrin are analogs of natural compounds (i.e., are
pyrethroids) that do not incorporate a cyano group (triple-bonded C and N) in
their structures.
Acute poisoning symptoms manifest differently for Type I and II pyrethroids.
Most of these pesticides are used as mixtures of isomers. Because they
degrade quickly, most related environmental problems are associated with
acute exposures.
A few points require mention before we leave the topic of pesticides.
The progression from OC to pyrethrin pesticides involves an increase in
degradability in the environment. However, the replacement of POP pesticides
with the more readily degradable pesticides comes at a cost of increased
mammalian toxicity. Figure 2.18 plots median lethal dose (LD50)* values for
pesticides fed orally to rats. There is a clear shift toward pesticides with higher
mammalian toxicity (lower LD50 values).
8. Herbicides
These biocides include products such as the bipyridiums paraquat and diquat, which have
nitrogen heterocyclic rings in their structures. Note that the rings are aromatic in that the
associated electrons stabilize the structures by being in resonance within the rings. The
herbicides also include triazines such as atrazine that can be problematic after entering
water bodies near agricultural fields. Phenoxy herbicides (e.g., 2,4-D) are also important in
the control of dicots and function by disrupting plant growth regulation.
9. Oxygen-Demanding Compounds
Putrescible materials possess high biochemical oxygen demands and in aquatic systems, can
reduce oxygen concentrations to stressful or lethal minima. Sewage-associated effluents are
one, but certainly not the only, important source of such materials. Poultry or meat
processing facilities and wood pulp operations can be locally damaging to waters into which
they discharge putrescible wastes.
hermaphrodite
10. Additional Emerging Organic Contaminants of Concern
Disinfection byproducts result from reactions of halogen-based disinfectants
with natural compounds of waters. Byproducts include trihalo-methane,
haloacetic acids, haloacetonitriles, and haloketones. Chlorination byproducts
have emerged as compounds of particular concern. Chlorine gas or
compounds used to disinfect drinking waters produce chlorinated organic
compounds with potentially toxic or carcinogenic effects. Chlorination
produces trihalomethanes such as the carcinogen chloroform (CHCl3). As one
sublethal effect example, Chlorination byproducts in drinking water are
suspected of modifying the menstrual cycle of women.
Perhaps among the most relevant classes of pharmaceuticals and veterinary
agents are hormone analogs and their metabolites. The hormone analogs and
their metabolites find their way into the environment via discharge from
sewage treatment plants, drainage from stock or poultry facilities, and runoff
from land to which biosolids have been applied.
C. Radiations
To this point, we have discussed contaminants based on their binding
chemistries and resulting compounds, that is, properties involving the behavior
of the outer orbital electrons of atoms and also structural properties of
compounds. Now, we will broaden the discussion to include changes involving
the state of inner orbital electrons and components of atomic nuclei.
The term radiation refers to the propagation of energy through space. This can
be in the form of photons, ejected atomic nuclei or their fragments, or
subatomic particles. Heat energy, visible and ultraviolet (UV) light, x-rays, and
gamma (y) rays are all examples of radiations that are photons of
electromagnetic energy. Radiations are also generated within the nuclei of
unstable elements: such nuclei are called radionuclidcs, and their emitted
radiations include alpha (a) particles (helium nuclei with a +2 charge), beta (p)
panicles (electrons or positrons with a -1 or +1 charge), and y photons.
Neutrons (which have no charge) and protons (which have a +1 charge) might
interact with other atomic nuclei but are not emitted from radioniidides
(although they are emitted if other atoms