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

sulfate to sulfide or by dechlorinating a molecule. The latter is very important as a
method of degrading organo-chlorine compounds, which are recalcitrant to direct
oxidation.
Howard (2000) has reviewed the principles surrounding biodegradation processes,
the laboratory and field test methods that are employed, and a variety of
methods by which biodegradation half-lives or classes can be estimated. One of the
most popular and accessible biodegradation estimation methods is the BIODEG
program, which is available from the Syracuse Research Corp. website
(www.syrres.com). It is well established that certain groupings of atoms impart
reactivity or recalcitrance to a molecule, thus a molecular structure can be examined
to identify how fast it is likely to degrade. Computer programs such as BIODEG
can do this automatically and assign a structure to a class such as “biodegrades fast”
with a half-life of days to weeks. The half-life may be reported for primary degradation,
i.e., loss of the parent compound, but also if interest is the time for complete
mineralization to CO 2 and water. The science of biodegradation is still a long way
from being able to estimate half-lives within an accuracy of a factor of three; indeed,
it may not be possible to estimate half-lives with greater accuracy.
In addition to the excellent review by Howard (2000), the reader will find
valuable material in the texts by Alexander (1994), Pitter and Choduba (1990), and
Schwarzenbach et al. (1993). Howard (2000) also lists databases, notably the
BIOLOG database of some 6000 chemicals.
6.6.2 Hydrolysis
In this process, the chemical species is subject to addition of water as a result
of reaction with water, hydrogen ion, or hydroxyl ion. All three mechanisms may
occur simultaneously at different rates; therefore, the overall rate can be very sensitive
to pH. Rates of environmental hydrolysis have been thoroughly reviewed by
Mabey and Mill (1978) and Wolfe and Jeffers (2000). For many organic compounds,
hydrolysis is not applicable.
A systematic method of testing for susceptibility to hydrolysis is to subject the
chemical to pH levels of 3, 7, and 11; observe the decay; and deduce rate constants
for acid, base, and neutral hydrolysis. These rate constants can be combined to give
an expression for the rate at any desired pH, namely,
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dC/dt = –k H[H + ]C – k OH[OH – ]C – k W[H 2O]C
Structure activity approaches can be used to correlate and predict these rate constants.
Often, the best approach is to seek data on a structurally similar substance.
Other useful references on hydrolysis include the Wolfe (1980), Pankow and
Morgan (1981), Zepp et al. (1975), Wolfe et al. (1977), and Jeffers et al. (1989).
6.6.3 Photolysis
The energy present in sunlight (photons) is often sufficient to cause chemical
reactions or the rupture of chemical bonds in molecules that are able to absorb this