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

Chlorinated ethenes in the environment can dechlorinate by accepting electr ons from electron donors fortuitously found in nature under a wide range of conditions in the sub-surface by both abiotic and biotic pathways. A sourc e of electron donors is a range of dehalorespiratory bacteria, in which Co( I) in vitamin B-12, in the presence of strong reductants such as Ti(III) ci trate, aids dehalogenation. In this work we investigated a range of reducin g conditions using electron donors of varying strengths and proposed mechan isms for dechlorination. Initially, the reducing abilities of one of the st rongest electron donors known, tetrakis( dimethylamino) ethylene (TDAE), wa s studied. Subsequently, the reducing ability of a cobalt salt under reduci ng 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 an d trans-dichloroethylene and to reduce trichloroethylene and cis-dichloroet hylene very rapidly. On application of a negative potential, cobalt, in the presence of molasses, was also found to dechlorinate trichloroethylene rap idly. Dechlorination of PCE was considerably slower.