Environmental Engineering Reference
In-Depth Information
(1,1-DCE), and vinyl chloride (VC). These intermediate chlorinated ethenes are still
large threats to the environment whereas the end products of aerobic oxidation of TCE
such as chloride ions and carbon dioxide are less toxic or nontoxic. The complete
mineralization of TCE via aerobic degradation, therefore, makes it very attractive for
field applications.
However, it should be noted that a single bacterium has very limited capability of
mineralizing TCE and other chlorinated ethenes under aerobic conditions. To overcome
this limitation, some recent research efforts use a complex bacterial consortium to
degrade TCE (Meza et al., 2003). The mixed culture is composed of Pseudomonas
putida , Pseudomonas fluorescens , Mycobacterium sp. , Nocardia paraffinae , and
Nitrosomonas europeae . All these bacterial strains were found effective in degrading
chlorinated ethenes and their intermediates. When TCE was used as a sole substrate in a
batch study in the presence of the mixed bacterial culture, a degradation efficiency of
88% was achieved within 24 hours at the TCE concentration range of 228 to 900 mg L -1 .
This consortium was also evaluated to determine the efficiency for degradation of a
mixture of contaminants containing TCE, trichloroethane (TCA) and PCE. In the
presence of TCE and TCA, the TCA degradation rate was 90%, and the TCE
degradation rate was 80% after 24-h incubation.
Few research works have been done on the active enzymes involved in aerobic
degradation of TCE and other chlorinated aliphatic compounds. Shim (Shim 2000)
employed an Escherichia coli pure culture containing enzyme of toluene-o-xylene
monooxygenase (ToMO) or toluene o-monooxygenase (TOM) to degrade TCE and
mixtures of chlorinated aliphatic compounds. Toluene-o-xylene monooxygenase was
obtained from a strain of Pseudomonas stutzeri OX1 with Escherichia coli as the
expression host. Compared to the individual oxidation capability of ToMO and TOM on
TCE degradation, ToMO presented a higher degradation rate (82%) than TOM (62%).
For binary TCE mixtures with 1,1-DCE, cis-1,2-DCE, trans-1,2-DCE, and VC, ToMO
also achieved a substantially higher degradation than TOM. In the binary mixture of
TCE and VC, the presence of VC inhibited TCE degradation by TOM enzyme. The
significant capability of ToMO enzyme to degrade and mineralize TCE and related
compounds makes ToMO a potential candidate for aerobic degradation of mixtures of
chlorinated aliphatic compounds in the environment.
Under anaerobic conditions, the chlorinated aliphatic compounds such as PCE
and TCE are reductively dechlorinated by naturally existing microbes. To expedite
anaerobic degradation processes, various electron donors such as methanol, butyrate,
and lactate can be added for enhanced PCE and TCE reduction. The true electron donor
in the reductive dechlorination process is actually hydrogen gas (H 2 ) produced from the
fermentation of the biogenic electron donors. Although dechlorination microbial
populations have usually outcompeted H 2 -utilizing microorganisms such as
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