Geology Reference
In-Depth Information
1.2 ENVIRONMENTAL CHEMICAL PROCESSES
The chemical reactions affecting trace gases in the atmosphere generally
have quite significant activation energies and thus occur on a timescale of
minutes, days, weeks, or years. Consequently, the change to such chem-
icalsisdeterminedbytheratesoftheir reactions and atmospheric chem-
istry is intimately concerned with the study of reactions kinetics. On the
other hand, some processes in aquatic systems have very low activation
energies and reactions occur extremely rapidly. In such circumstances,
provided there is good mixing, the chemical state of matter may be
determined far more by the thermodynamic properties of the system than
by the rates of chemical processes and therefore chemical kinetics.
The environment contains many trace substances at a wide range of
concentrations and under different temperature and pressure conditions.
At very high temperatures such as can occur at depth in the solid earth,
thermodynamics may also prove important in determining, for example,
the release of trace gases from volcanic magma. Thus, the study of
environmental chemistry requires a basic knowledge of both chemical
thermodynamics and chemical kinetics and an appreciation of why one
or other is important under particular circumstances. As a broad
generalisation it may be seen that much of the chapter on atmospheric
chemistry is dependent on knowledge of reaction rates and underpinned
by chemical kinetics, whereas the chapters on freshwater and ocean
chemistry and the aqueous aspects of the soils are very much concerned
with equilibrium processes and hence chemical thermodynamics. It
should not however be assumed that these generalisations are univer-
sally true. For example, the breakdown of persistent organic pollutants
in the aquatic environment is determined largely by chemical kinetics,
although the partitioning of such substances between different environ-
mental media (air, water, soil) is determined primarily by their thermo-
dynamic properties and to a lesser degree by their rates of transfer.
1.3 ENVIRONMENTAL CHEMICALS
This topic is not concerned explicitly with chemicals as pollutants. This is
a topic covered by a companion volume on Pollution Science.This topic,
however, is nonetheless highly relevant to the understanding of chemical
pollution phenomena. The major areas of coverage are as follows:
(i) The chemistry of freshwaters. Freshwaters comprise three different
major components. The first is the water itself, which inevitably
contains dissolved substances, both inorganic and organic. Its
properties are to a very significant degree determined by the
