McKay, Donald. "Front matter" Multimedia Environmental Models ...

A formidable literature exists on the kinetics of gas phase organic substances,
notably hydrocarbons, with OH radicals. Quantitative structure activity relationships
have been developed in which each part of the molecule is assigned a rate constant
for abstraction of H by OH radicals, or for addition of OH radicals to unsaturated
bonds. Atkinson (2000) has reviewed these estimation methods and provides references
to compilations of rate constant data. Computer programs exist to estimate
these rate constants from molecular structure, for example from the Syracuse
Research Corporation website (www.syrres.com).
It is important to appreciate that the atmosphere is a very reactive medium in
which large quantities of chemical species are converted into oxidized products.
This is fortunate, because otherwise there would be more severe air pollution and
problems associated with the transport of these chemicals to remote regions.
6.6.5 Aqueous Oxidation and Reduction
Natural oxidizing agents include oxygen, hydrogen peroxide, ozone, and “engineered”
oxidants include chlorine, hypochlorite, chlorine dioxide, permanganate,
chromate, and ferrate. Natural reducing agents include sulphide, ferrous and manganous
ion, and organic matter, while “engineered” reductants include dithionite
and zero-valent (metal) iron. Oxidation usually involves the addition of oxygen but,
in more general terms, it is the removal of or abstraction of an electron. Reduction
involves electron addition. The potential or feasibility of such a reaction occurring
can be readily evaluated from the standard potential of the half reactions.
The kinetics are usually expressed using a second-order expression including
the concentration of the substance and the oxidant or reductant. In some cases, the
reactant is a solid (e.g., zero-valent iron), and an area-normalized value can be used.
Tratnyek and Macalady (2000) provide an excellent review of this literature and
give several examples of oxidation and reduction processes. Again, for our purposes,
a first-order rate constant can be estimated that includes the concentration of the
oxidising or reducing agent. This can be used to calculate the corresponding halflife
and D value.
6.6.6 Summary
It has been possible to provide only a brief account of the vast literature relating
to chemical reactivity in the environment. The air pollution literature is particularly
large and detailed. References have been provided to give the reader an entry to the
literature.
The susceptibility of a chemical in a specific medium to degrading reaction
depends both on the inherent properties of the molecule and on the nature of the
medium, especially temperature and the presence of candidate reacting molecules
or enzymes. In this respect, environmental chemicals are fundamentally different
from radioisotopes, which are totally unconcerned about external factors. Translation
and extrapolation of reaction rates from environment to environment and laboratory
to environment is therefore a challenging and fascinating task that will undoubtedly
keep environmental chemists busy for many more decades.
©2001 CRC Press LLC