An Introduction to Environmental Chemistry

not substantially changed either the compounds or the matrix. Bound residues
usually represent an extreme end-member of ageing. The size of each of these
fractions depends mainly on the length of contact time between the soil and
contaminant.
Although the attraction between mineral surfaces and organic contaminants
in soils can present a problem in cleaning up contaminated land, it can also be
put to use by environmental chemists as a way of cleaning up contaminated water
(Box 4.15).

4.10.2 Degradation of organic contaminants in soils

Degradation of organic contaminants in soils occurs typically by either chemical
or microbiological pathways. The effectiveness of degradation is largely deter-
mined by the contaminant availability (Section 4.10.1), although the degree of
persistence is influenced by the chemical structure of the contaminant. If the
chemical structure of the contaminant is similar to that of a natural substance it
is more likely to be degradable. In general, if the structure is complex the rate of
degradation is slower and is more likely to be incomplete. Resistance to biodegra-
dation is known as recalcitrance, which is caused by a number of factors:
1 Specific microbes or enzymes required for degradation may not be present in
the soil.
2 Unusual or complex substitutions in a molecule (e.g. chlorine (Cl), bromine
(Br) or fluorine (F)), or unusual bonds or bond sequences, may ‘confuse’ micro-
organisms that would otherwise recognize the molecule as a substrate (Box
4.16).
3 A high degree of aromaticity (see Section 2.7), i.e. strongly bonded structures
based on a number of fused benzene rings, results in molecules that are difficult
to break down.
4 Large, complex and heavy molecules tend to be less water soluble, and
therefore are physically unavailable to microorganisms that use intracellular
degradation processes.
Biodegradation of organic contaminants can be carried out by a single micro-
bial species in pure cultures, but in nature the efforts of a mixture of microbes
(a consortium) are usually required. The degradation process ranges from only
minor structural changes to the parent molecule, known as primary degradation,
to complete conversion to mineral constituents, for example CO 2 or H 2 O, and
termed mineralization:

eqn. 4.20

OH OH

OH

Phenol Catechol

primary degradation mineralization

CO 2 + H 2 O

The Chemistry of Continental Solids 125