Redox conditions and the efficiency of chlorinated ethene biodegradation: Laboratory studies

The potential for biodegradation of highly reduced groundwater contaminants is greatest under aerobic conditions and least under CO2-reducing (methanogenic) conditions. Laboratory studies conducted using [1,2-14C] vinyl chloride (VC) indicate the same pattern applies to the anaerobic oxidation of relatively reduced chloroethylenes. Recent studies, showing that CH4 can be a significant product of microbial degradation of VC under methanogenic conditions, clarified mechanisms underlying anaerobic VC mineralization and emphasized the redox dependence of this process. A microcosm study conducted with stream bed sediments demonstrated rapid degradation of [1,2-14C] VC and simultaneous production of 14CO2 and 14CH4. The results of acetate mineralization studies indicated that these sediments contained active acetotrophic methanogens. VC degradation involved an initial transformation to acetate via oxidative acetogenesis followed by acetotrophic methanogenesis to yield CO2 and CH4 as final products. Based on these recent results, a conceptual model for anaerobic microbial degradation of VC to non-chlorinated products can be proposed.