Geology Reference
In-Depth Information
between the 1% and 0.1% of air are included, then
B
40% of the
chemicals become multi-media.
This approach of mapping chemicals into groups according to their
partitioning tendencies provides a rapid method of identifying relevant
environmental compartments where a chemical may reside. Key degra-
dation data relevant for that specific compartment can then be further
examined without the need for irrelevant data for other compartments.
6.5 CHEMICAL TRANSFORMATION AND DEGRADATION
Chemical degradation may proceed by a wide number of processes and
the reactivity of a chemical is governed by its stereochemistry (3-
dimensional structure), bond strengths and the presence of functional
groups. Both abiotic and biotic processes will transform a chemical
pollutant and the multitude of abiotic processes can be broadly grouped
under the following types of reaction: oxidation, reduction, hydrolysis
and photolysis. Indeed these generic reactions are not mutually exclu-
sive, as the generation of oxidants present in the atmosphere for exam-
ple, requires the action of sunlight on precursor molecules (e.g. the
formation of the OH radical from ozone). Several of the chlorinated
aromatic chemical groups illustrated in Figure 1 (and mentioned in
Section 6.4) are largely resistant to these transformation processes,
whereby their rates of reaction are very slow. However, even these
chemicals do degrade eventually, although their degradates may have a
similar longevity in the environment! Perhaps the fastest route of
degradation for these semi-volatile compounds is through oxidation
by the OH radical in the atmosphere, with half-lives typically on the
order of hours to days, compared to months or years in soils and
sediments. However, at any one time, the atmosphere may contain only
a very small fraction of the overall environmental 'inventory' so its
relevance as an important 'sink' for these chemicals is greatly reduced
compared to volatile organic compounds (VOCs). Nonetheless, oxida-
tion of PAHs present in the atmosphere can give rise to toxic analogues.
The reaction of higher volatility PAHs (generally the 2- to 4-ring PAHs
present predominantly in the vapour phase) with OH radicals during
daylight hours, or with nitrate (NO
3
) radicals during nighttime gives rise
to more polar hydroxy- and nitro-PAHs. The atmospheric formation of
nitro-PAHs may also occur via a daytime OH radical-initiated pathway
in the presence of NO
x
, although the formation yields of nitro-PAHs (or
nitroarenes) via this pathway are low compared to the yield of hydroxy-
PAHs.
33
Figure 5 illustrates a simplified reaction pathway of fluoran-
thene with the OH radical in the presence of NO
x,
resulting in the
