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Investigation of mechanisms for the reductive dechlorination of chlorinated ethylenes using electroanalytical techniques.

Chlorinated ethenes in the environment can dechlorinate by accepting electrons from electron donors fortuitously found in nature under a wide range of conditions in the sub-surface by both abiotic and biotic pathways. A source of electron donors is a range of dehalorespiratory bacteria, in which Co(I) in vitamin B12, in the presence of strong reductants such as Ti(III) citrate, aids dehalogenation. In this work we investigated a range of reducing conditions using electron donors of varying strengths and proposed mechanisms for dechlorination. Initially, the reducing abilities of one of the strongest electron donors known, tetrakis(dimethylamino)ethylene (TDAE), was studied. Subsequently, the reducing ability of a cobalt salt under reducing conditions was examined. Molasses was used as a source of hydrogen. The reactions were followed using electrochemistry, UV-Vis spectroscopy and ion chromatography. TDAE was found to form adducts with tetrachloroethylene and trans-dichloroethylene and to reduce trichloroethylene and cis-dichloroethylene very rapidly. On application of a negative potential, cobalt, in the presence of molasses, was also found to dechlorinate trichloroethylene rapidly. Dechlorination of PCE was considerably slower.