mercaptojuglone would be too high to conclude that it
functions as a nucleophile in SN reactions at carbon (eqs
ppm; L. Roberts, personal communication) with that in
distilled water containing 5 mM hydrogen sulfide. Rates
of reduction in the lake water were a factor of 12 higher
than measured rates in the clean system. Traces of
pentachloroethane were detected in the lake water samples,
indicating that pentachloroethyl radicals were formed. The
reaction mechanism was postulated to be a two-electron
reductive elimination (eq 18, Figure 7). The elevated rates
in the lake water were explained by invoking reaction with
polysulfides. This work demonstrates that quinone func-
tionalgroups in naturalorganic matter mayserve as electron
transfer mediators in solutions containing hydrogen sulfide
and that, at the pH of the lakewater (6.8), equilibrium
concentrations of polysulfides would not be high enough
to account for a 12-fold increase in the rate. It is possible
that electron transfer reactions produced pentachloroethyl
radicals in that system. However, the second electron
transfer and elimination reactions to form tetrachloroethene
may have been faster than hydrogen-atom abstraction such
that little pentachloroethane was observed, in analogy to
the observations made in the present work.
These results demonstrate the need to studythe interplay
between the biogeochemistry of natural systems and the
transformation ofxenobiotic compounds. This work shows
that naturallyoccurring organosulfur compounds transform
halogenated alkanes. The extent of formation of organo-
sulfur compounds has been shown to vary systematically
with environmental parameters such as lake trophic state
and oxygen availability (40). Hence, systematic variations
in the rates of reduction of xenobiotics in sediments also
may occur.
1
6 and 19 in Figure 7) in these solutions. However, the
three electron-donating hydroxygroups in mercaptojuglone
hydroquinone would make it a somewhat better nucleophile
-
than phenyl-S , and thus the computations above cannot
be regarded as conclusive. Furthermore, nucleophilic
attack at chlorine (eq 18 in Figure 7) would be expected to
be quite different than such attack at carbon.
Correlation of the disappearance rate of a series of
polyhalogenated alkanes with the change in free energy
upon reaction with mercaptojuglone according to Marcus
theory indicates that transfer of the first electron to these
compounds is rate-limiting (16). This supports the reaction
of hexachloroethane according to eq 16 in Figure 7 in the
initial, rate-limiting step. However, the one-electron
oxidation potential for mercaptojuglone used in the Marcus
plot could be expected to be a surrogate measure of the
nucleophilicity of the mercaptojuglone, as discussed above.
Thus, no conclusions can be drawn from the present results
as to whether the reaction mechanism is a one- or a two-
electron transfer.
Environm ental Im plications. The results presented
here indicate that completelyreduced mercaptojuglone was
the reactive species in the reduction of the halogenated
alkanes in solutions containing juglone and hydrogen
sulfide. Evidence for this hypothesis include the kinetics
of these solutions compared to the kinetics in solutions
containing species such as hydrogen sulfide, polysulfides,
or juglone hydroquinone alone and compared to reported
rates of reaction of these species with polyhalogenated
alkanes. The possible reaction of nitro aromatic com-
pounds and halogenated alkanes with mercaptoquinones
formed in addition reactions in model systems (37) and in
solutions containing natural organic matter and hydrogen
sulfide (13, 38) needs to be reconsidered.
Acknowledgments
The authors wish to thank R. Stierli, who performed a
number of the kinetic measurements. N. Urban, J. Bus-
chmann, and L. Roberts kindly reviewed the manuscript.
The results presented here are especially applicable to
sulfate-reducing environments. Organosulfur compounds
and polysulfides will be formed by the mechanisms
discussed here in such environments when humic sub-
stances come into contact with hydrogen sulfide. The
addition of inorganic sulfur species onto organic matter
appears to occur in marine and freshwater sediments and
peat (14). Quinone functional groups in humic substances,
which alone may not be efficient electron transfer agents,
may become more efficient electron transfer agents through
the addition of hydrogen sulfide. Polysulfides are formed
in the environment through, among other pathways, the
oxidation of hydrogen sulfide in the reduction of humic
substances. Concentrations of 0.2 mM S0 have been
reported in marine porewater and salt marsh water (39).
Although the results presented here demonstrate that
polysulfides are not more highly reactive with respect to
hexachloroethane reduction than hydrogen sulfide at
neutral pH, at higher pH their equilibrium concentrations
increase and reaction rates are greatly increased. Halo-
genated alkanes entering sulfate-reducing environments
could be expected to be transformed at faster rates due to
the reactivity of hydrogen sulfide, organosulfur, and
polysulfide species.
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