Influence of the Properties of Chemicals on Their Environmental Fate - Organic Pollutants: An Ecotoxicological Perspective - page 69

(PCBs) and organochlorine insecticides into polar regions has been attributed to aer-
ial transport in the vapor state. In the second part of the topic, examples will be given
of pollutants that move readily into the air because of relatively high vapor pressure.
Much of the earth's surface is water, and an important aspect of the volatiliza-
tion of pollutants is their movement from water to air. The movement of solutes
between water and air is governed by Henry's law, which states that at equilibrium,
the concentration of a chemical in the vapor state bears a constant relationship to the
concentration in aqueous solution.
H′ = Conc.air/Conc. water = [n/V][1/S] = P/RTS
where H′ is a partition coefficient, n/V is the molar concentration of the vapor, S is
the saturation solubility of the solute in water, P is vapor pressure, R is the universal
gas constant, and T is the absolute temperature.
This equation can be simplified to give H = P/S, where H is now Henry's law
constant, which has dimensions of atm m 3 /mole. Values for H may be calculated or
measured (Mackay et al. 1979), and are now widely used in fugacity modeling (see
Section 3.2).
Another important determinant of the environmental fate of pollutants is their
chemical stability. Environmental chemicals that are highly resistant to chemical
degradation have been described as being “refractory,” or “recalcitrant” (see Crosby
1998). Many such chemicals are not very photochemically stable. When exposed
to solar radiation, they may be oxidized or may undergo molecular rearrangement.
Some chemicals, for example, organophosphates (OPs), carbamates, and pyrethroids,
are susceptible to hydrolysis, especially when exposed to water having high pH. By
contrast, other compounds only undergo very slow degradation and can therefore
be transported over large distances in air or water without substantial loss, unless
biodegradation is significant (see Section 3.3). Typical examples are certain highly
halogenated compounds such as p,p ′-DDE, dieldrin, higher chlorinated PCBs, and
TCDD (dioxin). Highly halogenated compounds tend to be resistant to oxidation and
other mechanisms of chemical degradation.
In most cases, chemical instability of pollutants limits risk to the environment
because it usually represents a reduction in toxicity. There are, however, exceptions
and the devil may be found in the detail. When dieldrin residues are exposed to solar
radiation, there is some conversion to the persistent and highly toxic photodieldrin.
When the OP malathion is stored under hot conditions over long periods, it is con-
verted to highly toxic isomalathion. When polycyclic aromatic hydrocarbons (PAHs)
are exposed to radiation, they are converted into products that are highly toxic to
fish. Such examples illustrate the dangers of hasty or superficial judgment in envi-
ronmental risk assessment, and the importance of rigorous case-by-case analysis.
Another factor that can influence the environmental distribution of a chemical
is the presence of charged groups. Some pollutants, such as the sodium or potas-
sium salts of phenoxyalkanoic herbicides, dinitrophenols, and tetra- or penta-chlo-
rophenol, exist as anions in solution. Others, such as the bipyridyl herbicides diquat
and paraquat, are present as cations. In either case, the ions may become bound to
organic macromolecules or minerals of soils or sediments that bear the opposite