REDUCTIVE DECHLORINATION OF PCE AND TCE BY VITAMIN B_(12) AND ZVMs

摘要

The reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) catalyzed by vitamin B_(12) was examined when zero valent metals (ZVMs) were used as bulk electron donors in batch reactors. UV-visible spectra showed that zinc reduces vitamin B_(12) Co(Ⅲ) to vitamin B_(12) Co(Ⅰ) through B_(12) Co(Ⅱ) and iron reduces vitamin B_(12) Co(Ⅲ) to vitamin B_(12) Co(Ⅱ). Thus iron, the most popular ZVM reductant, does not have enough reduction potential to reduce vitamin B_(12) to the super-reduced vitamin B_(12)(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 B_(12) showed that the zinc and vitamin B_(12) combination greatly enhances the reaction rates for both PCE and TCE, but iron and vitamin B_(12) result in an increase in reactivity only for PCE degradation, not for TCE degradation in comparon with metals only. This result indicates vitamin B_(12)(Ⅰ) is active towards both PCE and TCE degradation while vitamin B_(12)(Ⅱ) is active only towards PCE. Calculated activation energies for the dechlorination of PCE in the presence of Vitamin B_(12) showed that vitamin B_(12) lowered the activation energy about 40-60 kJ mol~(-1) for both metals.