Anaerobic Degradation of Chlorinated Ethenes

At anaerobic sites, the solvents tetrachloroethene (PCE) and trichloroethene (TCE) can undergo reductive dechlorination to less-chlorinated ethenes, a process identified as halorespiration (Holliger and Schumacher, 1994). In this process H2 is transferred to the reactant which leads to loss of one chloride ion  

Similar reactions have been shown to reduce DCE to VC and VC to ethene. In general, reductive dechlorination to the ethenes occurs by sequential dechlorination from PCE to TCE to dichloroethene (DCE) to vinyl chloride (VC) and finally to ethene (Wiedemeier et al., 1999). 

The need for sufficient H2 concentrations to reduce chlorinated ethenes has been stressed by several authors (Wiedemeier et al., 1999 Loffler et al., 1999). From these requirements the link between redox conditions and the degradation rate becomes evident. First, the H2 concentration is regarded as a means to characterize the predominating metabolic pathway (Chapelle et al., 1996). Loffler et al. (1999) present threshold values of H2 concentrations to range from 5-95 nmol L for methanogenesis, 1-15 nmol L for sulfate reduction, 0.1-0.8 nmol L for Fe(III) reduction, and 0.3 nmol L for chlororespiration. Second, degradation rates can be expected to be proportional to substrate, that is H2 concentrations. 

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Heimann, A. C. Jakobsen, R. Experimental Evidence for a Lack of Thermodynamic Control on Hydrogen Concentrations during Anaerobic Degradation of Chlorinated Ethenes. Environ. Sci. Technol. 2006, 40, 3501-3507. 

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