tetrachloroethane by FeS produced among other products
trans-DCE, which subsequently decreased in concentration
no more than approximately 10% over the course of 7 days.
This finding suggests that trans-DCE is not a significant
intermediate in the transformation of TCE or PCE to
acetylene, since it is transformed by FeS slowly enough that
if it were an intermediate, it would be present above detection
limits, and no trans-DCE was detected in these experiments
at any time.
genolysis products (e.g., ref 45). While the detection or
nondetection of acetylene is potentially confounded by the
numerous pathways for transformation of this compound in
natural systems, including hydrolysis and hydrogenation,
analysis for acetylene in addition to other potential TCE and
PCE reaction products in samples from contaminated sites
may provide insight into the pathway(s) of TCE and PCE
degradation in the environment.
The results described here also illustrate the utility of
calculating and reporting TCE and PCE branching ratios to
allow quantitative evaluation of the relative rates of TCE and
PCE hydrogenolysis and reductive elimination reactions and
expected product distributions. Such information is very
relevant for understanding the natural transformation of TCE
and PCE and for evaluating the suitability of a particular
reductant for a treatment application.
Several pathways for hydrogenolysis and reductive elimi-
nation of TCE and PCE have previously been proposed (6-8,
16, 28, 33, 37, 38). These pathways are illustrated for TCE in
Figure 4, but analogous reaction pathways for PCE can also
be considered. The detection of products of both hydro-
genolysis (cis-DCE) and reductive elimination (acetylene) in
the transformation of TCE by FeS suggests a pathway
involving a common intermediate or intermediates such as
the cis-dichlorovinyl radical (Figure 4(i)) or the cis-dichlo-
rovinyl anion (Figure 4(ii)). Through isotope studies, Glod et
al. (28) found evidence for the intermediacy of species similar
to both (i) and (ii) in the transformation of TCE by cobalamin/
titanium(III) citrate. Sivavec and Horney (6) detected hy-
drocarbon products hypothesized to arise from radical
coupling reactions in the degradation of TCE by FeS (troilite).
Detection of both hydrogenolysis and reductive elimina-
tion products in the degradation of TCE (and PCE) by FeS
suggests a common rate-limiting elementary reaction step
with a common metastable intermediate such as (i) or (ii)
(Figure 4). However, it is not possible, based on the evidence
reported here, to disprove the hypothesis that the products
of hydrogenolysis and reductive elimination are formed by
independent pathways that do not share a common reaction
intermediate. It is possible, for example, that dichloroelimi-
nation of TCE to chloroacetylene takes place in a single
elementary reaction step that does not share a common
intermediate with pathways for cis-DCE formation. Evidence
was recently reported that the initial step in the reductive
elimination of chloroethylenes by zero-valent metals is an
essentially concerted two-electron reduction (7). Nucleo-
philes such as sulfides and polysulfides have been shown to
promote related dihaloelimination reactions (39-43 and
references cited therein) by pathways that may involve a
single two-electron reduction reaction (44), and it is certainly
possible that the sulfide functional groups in FeS promote
such a reaction. Such a pathway is illustrated for TCE in
Figure 4. However, the following observations are not
consistent with the idea of a concerted two-electron reduction
of TCE or PCE by FeS. Addition of 1 mM cysteine to FeS
slurries produced diminished rates of disappearance of both
TCE and PCE. This is consistent with previous results (15)
in which the transformation of hexachloroethane by FeS was
significantly slowed by cysteine, speculated to be due to
adsorption of cysteine to surface iron atoms. Despite the
expected decrease in the values of kobs and k1-k4 for TCE
and PCE in the presence of cysteine (calculated as described
previously and shown in Tables 1 and 3), cysteine did not
significantly affect the TCE and PCE branching ratios (Table
3). This suggests a common rate-limiting elementary reaction
step in the formation of acetylene and cis-DCE from TCE
and in the formation of acetylene and TCE from PCE, since
it seems unlikely that addition of cysteine would affect the
rates of two independent rate-limiting steps to the same
extent. Additional studies are needed to gain further evidence
for this hypothesis.
Acknowledgments
We thank Tom Yavaraski for invaluable technical assistance
in the laboratory and three anonymous reviewers for
comments that improved the manuscript. Funding for this
research was provided by the Great Lakes and Mid-Atlantic
Center for Hazardous Substance Research under Grant
R-819605 from the Office of Research and Development, U.S.
Environmental Protection Agency (U.S. EPA). Partial funding
of the research activities of the center was provided by the
State of Michigan Department of Natural Resources. Funding
for this research was also provided by the U.S. EPA, U.S.
Department of Energy, National Science Foundation, and
Office of Naval Research Joint Program on Bioremediation
(EPA-G-R-825958). The content of this publication does not
necessarily reflect the views of these agencies.
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