Cobalamin-Mediated Reduction of cis- 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 been suggested as electron transfer mediator for the treatment of halogenated solvents, its reactions with such compounds are presently of great interest. In this paper, it is shown that, in homogeneous aqueous solution containing titanium(III) citrate as the bulk electron donor, superreduced cobalamin reductively dechlorinated cis - and trans -dichloroethene ( 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 the chlorinated ethenes (CEs) with simultaneous protonation. Only for 1,1-DCE at high pH, a dissociative electron transfer mechanism as suggested for tetrachloroethene (PCE) and trichloroethene (TCE) in earlier work 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. Acetylene and ethene were found to react at similar rates as 1,1-DCE and VC, respectively. However, at more positive redox potentials, the reductive cleavage of the addition products, particularly of the adducts of acetylene, ethene, and VC with cob(I)alamin, may become very slow, thus preventing the regeneration of cob(I)alamin. The results of this study demonstrate that, at more negative potentials and at low pH, cobalamin is a potent electron transfer mediator for the complete dehalogenation of PCE and TCE without significant accumulation of VC.