COBALAMIN-MEDIATED REDUCTION OF CIS-DICHLOROETHENE AND TRANS-DICHLOROETHENE, 1,1-DICHLOROETHENE, AND VINYL-CHLORIDE IN HOMOGENEOUS AQUEOUS-SOLUTION - REACTION-KINETICS AND MECHANISTIC CONSIDERATIONS

Since cobalamin is involved in the enzymatic reduction of halogenated ethenes by a variety of anaerobic bacteria and since cobalamin has bee n suggested as electron transfer mediator for the treatment of halogen ated solvents, its reactions with such compounds are presently of grea t interest In this paper, it is shown that, in homogeneous aqueous sol ution containing titanium(III) citrate as the bulk electron donor, sup erreduced cobalamin reductively dechlorinated cis-and trans-dichloroet hene (cis-DCE and trans-DCE), 1,1-dichloroethene (1,1-DCE), and vinyl chloride (VC) in pH-dependent reactions to ethene and ethane. Evidence is given that the initial step was the addition of cob(I)alamin to th e chlorinated ethenes (CEs) with simultaneous protonation. Only for 1, 1-DCE at high pH, a dissociative electron transfer mechanism as sugges ted for tetrachloroethene (PCE) and trichloroethene (TCE) in earlier w ork was important. 1,1-DCE reacted about 30 times faster than VC, 600 times faster than trans-DCE, and 3000 times faster than cis-DCE. Acety lene and ethene were found to react at similar rates as 1,1-DCE and VC , respectively. However, at more positive redox potentials, the reduct ive cleavage of the addition products, particularly of the adducts of acetylene, ethene, and VC with cob(I)alamin, may become very slow, thu s preventing the regeneration of cob(I)alamin. The results of this stu dy demonstrate that, at more negative potentials and at low pH, cobala min is a potent electron transfer mediator for the complete dehalogena tion of PCE and TCE without significant accumulation of VC.