An Introduction to Environmental Chemistry

The main styles of biodegradation and transformation are described in Box 4.16.

Even slight structural molecular transformations of the parent contaminant mol-

ecule brought about by biodegradation can alter significantly the original conta-

minant’s toxicity, mobility (Fig. 4.30) and its affinity for soil surfaces (Fig. 4.28).

4.10.3 Remediation of contaminated land

In many cases, contaminated land can be treated in order to rehabilitate it for

future use. The success of remediation techniques depends on the concentration,

type and availability of the contaminants present, and on-site factors, such as soil

texture, pH, availability of terminal electron acceptors (Section 4.6.5) and the age

The Chemistry of Continental Solids 129

Co-metabolism

eqn. 4

Although many aromatic compounds are subject to microbial degradation, the presence of
substituents (e.g. Cl, Br, CH 3 ) on the molecule can result in an increase in their resistance to
biodegradation. In these instances microbes that would normally recognize the molecule as a
substrate fail to do so because of the presence of the novel substituents, and hence do not
produce enzymes to destroy them. For example, biphenyl (unsubstituted) is readily degraded.
However, the polychlorinated biphenyls (PCBs), for example 2,3,2¢,4¢-tetrachlorinated biphenyl,
are recalcitrant. Interestingly where biphenyls and PCBs are present together, both are
degraded (eqn. 4). This is because the microbes recognize the biphenyl and produce enzymes
to degrade it. The same enzymes degrade the PCBs and thus both contaminants are removed.
This is co-metabolism, a process that accounts for the degradation of many xenobiotic (foreign
to life) compounds.

Accumulation on or within microbes

eqn. 5

Certain microorganisms, particularly those with high lipid (fat) contents, can absorb water-
insoluble chemicals, such as PCBs. Although this is not strictly biodegradation, it is a process
that removes pollutants from the environment.

Cl

Cl

Cl Cl

Cl Cl

Cl

PCB (in soil)

accumulation

Cl

Cl

Cl Cl

Cl Cl

Cl

PCB (in organism)

Cl

Cl Cl

Cl

co-metabolism

CO 2 + H 2 O

PCB Biphenyl

together