Enhancement of reductive dechlorination of tetrachloroethene by iron reductants with cobalamin (III)

Abstract

Cobalamin (III) is a coenzyme produced by anaerobic microorganisms (e.g., methanogens, acetogens, and sulfate-reducing bacteria) and it is abundantly presence in anaerobic reducing environments. In this thesis, the ability of cobalamin (III) as an electron transfer mediator in reductive dechlorination of tetrachloroethene (PCE) by a variety of iron reductants was investigated. Reduction of cobalamin (III) to cobalamin (II) and cobalamin (I) by different iron reductants (e.g., nano-sized zero valent iron (nZVI) and nano-mackinawite (nFeS)) play a vital role to shuttle electrons from iron reductants to the target contaminants. A remarkable enhancement of reductive dechlorination of PCE was observed in iron reductants suspension with cobalamins species, while no significant reductive dechlorination of PCE was observed in the iron reductants suspension at natural condition (pH 8.3). Complexation of aquocobalamin (III) with sulfide (S2-) also enhanced reductive dechlorination of PCE in nFeS suspension. Cobalamin (II), sulfidecobalamin(II), and sulfidecobalamin(I) were identified as predominant cobalamin species during the reductive dechlorination of PCE. Reductive dechlorination of PCE by iron reductants with cobalamin (III) also significantly affected by the environmental conditions (e.g., suspension pH and concentrations of cobalamin (III), nZVI, and PCE). The formation of sulfidecobalamin(II) on the nFeS surface also affected the byproduct distribution of PCE (PCE →acetylene →1,3-butadiene) via reductive β-elimination and isomerization reactions. Experimental results from this study can provide basic knowledge to identify potential reaction mechanism for enhanced reductive dechlorination of chlorinated organics by biogeochemical reactions possibly observed in natural reducing environments.