Transformation of Hexachloroethane in a Sulfidic Natural Water

Abstract

To investigate the roles that hydrogen sulfide and polysulfides could play as reductants in natural waters, reactions of hexachloroethane and pentachloroethane were examined in samples obtained from the anoxic hypolimnion of Lower Mystic Lake, MA. Experiments were conducted by monitoring the disappearance of pentachloroethane or hexachloroethane and appearance of reaction products; measured pseudo-first-order reaction rate constants for hexachloroethane were compared to predictions based on measured polysulfide and bisulfide concentrations and second-order rate constants. For pentachloroethane, 80% of the reaction could be attributed to abiotic dehydrohalogenation. An additional 9% resulted from abiotic reductive elimination to trichloroethylene. Reaction of hexachloroethane was predominately abiotic. At least 61% occurs via reductive elimination to tetrachloroethylene; a further 7−10% involves hydrogenolysis to pentachloroethane (indicative of a free-radical pathway). More than 20% of the observed hexachloroethane disappearance in filter-sterilized samples can be attributed to reaction with H2S and Sn2- species, with the latter playing a predominant role. These (and related reductants) could promote reductive dehalogenation through dissociative electron-transfer processes involving a free radical intermediate; alternatively, they may serve as “nucleophilic reductants”. Results of this study demonstrate that aqueous polysulfide species, in particular, should not be overlooked as potential “environmental reductants” in sulfate-reducing environ ments.