Reductive Outer-Sphere Single Electron Transfer Is an Exception Rather than the Rule in Natural and Engineered Chlorinated Ethene Dehalogenation

摘要

Chlorinated ethenes (CEs) such as perchloroethylene, trichloro-ethylene and dichloroethylene are notorious groundwater contaminants. Although reductive dehalogenation is key to their environmental and engineered degradation, underlying reaction mechanisms remain elusive. Outer-sphere reductive single electron transfer (OS-SET) has been proposed for such different processes as Vitamin B_(12)-dependent biodegradation and zerovalent metal-mediated dehalogenation. Compound-specific isotope effect (~(13)C/~(12)C, ~(37)Cl/~(35)Cl) analysis offers a new opportunity to test these hypotheses. Denned OS-SET model reactants (CO_2 radical anions, S~(2-)-doped graphene oxide in water) caused strong carbon (ε_C = -7.9‰ to -11.9‰), but negligible chlorine isotope effects (ε_(Cl) = -0.12‰ to 0.04‰) in CEs. Greater chlorine isotope effects were observed in CHCl_3 (ε_C = -7.7‰, ε_(Cl) --2.6‰), and in CEs when the exergonicity of C-CI bond cleavage was reduced in an organic solvent (reaction with arene radical anions in glyme). Together, this points to dissociative OS-SET (SET to a σ~* orbital concerted with C-Cl breakage) in alkanes compared to stepwise OS-SET (SET to a π~* orbital followed by C-Cl cleavage) in ethenes. The nonexistent chlorine isotope effects of chlorinated ethenes in all aqueous OS-SET experiments contrast strongly with pronounced CI isotope fractionation in all natural and engineered reductive dehalogenations reported to date suggesting that OS-SET is an exception rather than the rule in environmental transformations of chlorinated ethenes.