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Fig. 13.8 Generalized
aerobic biodegradation
pathways for aromatic
hydrocarbons. Reprinted
from Abrajano et al. ( 2005 ).
Copyright 2005 with
permission of Elsevier
consumed by microorganisms or affected by additional processes causing natural
attenuation (Baedecker et al. 1993 ).
Persistence and degradation of chlorinated solvent contaminants in groundwater
systems have been the object of a large number of investigations over the last
20 years. In flowing groundwater systems, chlorinated solvents, such as trichlo-
roethylene (TCE) and perchloroethylene (PCE), act as persistent contaminants.
The degradation of TCE and PCE, and their transformation into more lightly
chlorinated ethenes such as cis-dichloro ethene (DCE) and vinyl chloride (VC), are
depicted in Fig. 13.9 . Chlorinated ethenes may be subject to microbial degradation
processes, including reductive dechlorination and aerobic and anaerobic oxidation.
In many natural environments, the initial reductive dechlorination drives the
transformation of PCE and TCE to DCE and VC, respectively. The combined
effects of methane-oxidizing cometabolism and anaerobic oxidation lead to the
transformation of DCE and VC into carbon dioxide, chloride, and water. Because
these biodegradation processes are all redox processes, the efficiency of biodeg-
radation is very sensitive to reduction-oxidation conditions (Chapelle 1996 ).
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