Reductive dechlorination of tetrachloroethylene and trichloroethylene catalyzed by vitamin B-12 in homogeneous and heterogeneous systems

The reduction of tetrachloroethylene (PCE) and trichloroethylene (TCE) catalyzed by vitamin B-12 was examined in homogeneous and heterogeneous (B-12 bound to agarose) batch systems using titanium(III) citrate as the bulk reductant. The solution and surface-mediated reaction rates at similar B-12 loadings were comparable, indicating that binding vitamin B-12 to a surface did not lower catalytic activity. No loss in PCE reducing activity was observed with repeated usage of surface-bound vitamin B-12 Carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction, relative to controls. In addition to sequential hydrogenolysis, a second competing reaction mechanism for the reduction of PCE and TCE by B-12, reductive beta-elimination, is proposed to account for the observation of acetylene as a significant reaction intermediate. Reductive beta-elimination should be considered as a potential pathway in other reactive systems involving the reduction of vicinal polyhaloethenes. Surface-bound catalysts such as vitamin B-12 may have utility in the engineered degradation of aqueous phase chlorinated ethenes.