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

Chlorinated ethenes (CEs) such as perchloroethylene, trichloroethylene 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 (C-13/C-12, Cl-37/Cl-35) analysis offers a new opportunity to test these hypotheses. Defined OS-SET model reactants (CO2 radical anions, S2--doped graphene oxide in water) caused strong carbon (epsilon(C) = 7.9 parts per thousand to -11.9 parts per thousand, but negligible chlorine isotope effects (epsilon(Cl) = 0.12 parts per thousand to 0.04 parts per thousand) in CEs. Greater chlorine isotope effects were observed in CHCl3 (epsilon(C) = -7.7 parts per thousand, epsilon(Cl) = -2.6 parts per thousand), and in CEs when the exergonicity of C-Cl 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 sigma* orbital concerted with C-Cl breakage) in alkanes compared to stepwise OS-SET (SET to a pi* 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,Cl 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.