The absence and application of stable carbon isotopic fractionation during the reductive dechlorination of polychlorinated biphenyls

A bacterial enrichment culture (specific to doubly flanked chlorine removal) reductively dechlorinated 2,3,4,5-tetrachlorobiphenyl (2,3,4,5-CB) to 2,3,5-trichlorobiphenyl (2,3,5-CB) in aqueous media. Approximately 90% conversion to 2,3,5-CB occurred after 90 days, with no other products formed. The delta C-13 values of 2,3,4,5-CB and 2,3,5-CB were relatively constant over the course of the reaction, indicating a very small or no isotope effect. In addition, compound-specific delta C-13 analysis performed for every congener in three different lots of Aroclor 1268 showed an intrinsic isotopic trend of decreasing C-13 abundance with increasing chlorine content, similar to observations in other commercial mixtures of polychlorinated biphenyls (PCBs). The results of this laboratory study suggest that microbial reductive dechlorination of PCBs in contaminated sediments will create congeners with more depleted delta C-13 values than native PCBs of similar chlorination. Such information may provide additional evidence for the occurrence of this process and aid in further understanding the biogeochemistry of these compounds.