Reductive Dechlorination of PCE and TCE by Vitamin B12 and ZVMs

Abstract

The reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) catalyzed by vitamin B12 was examined when zero valent metals (ZVMs) were used as bulk electron donors in batch reactors. UV-visible spectra showed that zinc reduces vitamin B12 Co(III) to vitamin B12 Co(I) through B12 Co(II) and iron reduces vitamin B12 Co(III) to vitamin B12 Co(II). Thus iron, the most popular ZVM reductant, does not have enough reduction potential to reduce vitamin B12 to the super-reduced vitamin B12(I), which has been shown to be an active species in reductive dechlorination. Dechlorination of PCE and TCE by iron and zinc in the presence of vitamin B12 showed that the zinc and vitamin B12 combination greatly enhances the reaction rates for both PCE and TCE, but iron and vitamin B12 result in an increase in reactivity only for PCE degradation, not for TCE degradation in comparon with metals only. This result indicates vitamin B12(I) is active towards both PCE and TCE degradation while vitamin B12(II) is active only towards PCE. Calculated activation energies for the dechlorination of PCE in the presence of Vitamin B12 showed that vitamin B12 lowered the activation energy about 40-60 kJ mol−1 for both metals.