Web Release Date: March 24,
Transformation of Hexachloroethane in a Sulfidic Natural Water
Received for review August 5, 1997 Revised manuscript received
January 20, 1998 Accepted January 23, 1998 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.
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