Geoscience Reference
In-Depth Information
SO
4
2-
or S
2
O
3
2-
, organic Fe compounds in Fe precipitates, and arsenite (mobile
and toxic) in arsenate (nontoxic). Aerobic respiration also has indirect effects on
the transformation of chemicals. The release of CO
2
, which affects redox potential
and pH, leads to the transformation of metal ions by chelation.
Anaerobic metabolism occurs under conditions in which the O
2
diffusion rate is
insufficient to meet the microbial demand, and alternative electron acceptors are
needed. The type of anaerobic microbial reaction controls the redox potential (Eh),
the denitrification process, reduction of Mn
4+
and SO
4
2-
, and the transformation of
selenium and arsenate. Keeney (
1983
) emphasized that denitrification is the most
significant anaerobic reaction occurring in the subsurface. Denitrification may be
defined as the process in which N-oxides serve as terminal electron acceptors for
respiratory electron transport (Firestone
1982
), because nitrification and NO
3
-
reduction to NH
4
+
produce gaseous N-oxides. In this case, a reduced electron-
donating substrate enhances the formation of more N-oxides through numerous
electrocarriers. Anaerobic conditions also lead to the transformation of organic
toxic compounds (e.g., DDT); in many cases, these transformations are more rapid
than under aerobic conditions.
Microbial methylation is a reaction that affects mainly properties of toxic,
inorganic trace elements, which involves the addition of a methyl group to the
contaminant molecule. It occurs under aerobic or anaerobic conditions. Mercury
methylation, for example, occurs under both conditions and leads to the release of
mercury into the atmosphere.
15.3 Biotransformation of Organic Contaminants
The microbial metabolic process is the major mechanism for the transformation of
toxic organic chemicals in the subsurface environment. The transformation process
may be the result of a primary metabolic reaction, when the organic molecule is
degraded by a direct microbial metabolism. Alternatively, the transformation
process may be an indirect, secondary effect of the microbial population on the
chemical and physical properties of the subsurface constituents. Bollag and Liu
(
1990
), considering behavior of pesticides, defined five basic processes involved in
microbially mediated transformation of toxic organic molecules in the soil upper
layer environment. These processes are described next.
Biodegradation is a process in which toxic organic molecules serve as substrate
for microbial growth. In this case, organic molecules are used by one or more
interacting microorganisms and metabolized into CO
2
and inorganic components.
In this way, microorganisms obtain their requirements for growth and toxic
organic molecules are completely decomposed, without producing metabolites
(which in some cases could be more toxic than the parent material). From an
environmental point of view, this process is highly effective and desirable; the
presence of a biodegradable compound in a subsurface site may enhance the
