Reductive Transformation of Tetrachloroethene Catalyzed by Sulfide-Cobalamin in Nano-Mackinawite Suspension

Abstract

In this study, we demonstrated the reductive substitution of aquo-cobalamin(III) (Cbl(III)-H2O) by sulfide-cobalamin(III) (Cbl(III)-S2-) in the presence of S2-. Cbl(III)-H2O was transformed to Cbl(III)-S2- via ligand substitution of H2O by S2- at its axial position, due to the weakness of the bond strength of cobalt with H2O. Cbl(III)-S2- was reduced to sulfide-cobalamin(II) and partially further to sulfide-cobalamin(I) as the concentration of S2- increased from 0.1 to 5.0 mM. In the presence of nanomackinawite (nFeS), Cbl(III)-S2- species were fully adsorbed onto the nFeS surface, and its reactivity was enhanced by the reactive surface chemical species. The dechlorination kinetic rate constant of tetrachloroethene by Cbl(III)-S2- in nFeS suspension (k(nFeS-Cbl(III)-S)(2-) = 0.008 +/- 0.001 h(-1)) was 4 times greater than that by Cbl(III)-H2O, showing a potential role for S2--Cbl(II) as a more effective electron transfer mediator than S-n(2-)-Cbl(II) and OH-Cbl(II). The experimental results provide fundamental knowledge on potential biogeochemical reactions relevant to interactions between S2- and bacterial coenzyme in nFeS suspension, which can significantly enhance reductive dechlorination of chlorinated compounds in contaminated sites.