Carbon and Chlorine Isotope Fractionation during Aerobic Oxidation and Reductive Dechlorination of Vinyl Chloride and cis-1,2-Dichloroethene

The study investigated carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride (VC) and cis-1,2-dichloroethene (cDCE). The experimental data followed a Rayleigh trend. For aerobic oxidation, the average carbon isotope enrichment factors were -7.2 parts per thousand and -8.5 parts per thousand for VC and cDCE, respectively, while average chlorine isotope enrichment factors were only -0.3 parts per thousand for both compounds. These values are consistent with an initial transformation by epoxidation for which a significant primary carbon isotope effect and only a small secondary chlorine isotope effect is expected. For reductive dechlorination, larger carbon isotope enrichment factors of -25.2 parts per thousand for VC and -18.5 parts per thousand for cDCE were observed consistent with previous studies. Although the average chlorine isotope enrichment factors were larger than those of aerobic oxidation (-1.8 parts per thousand for VC, -1.5 parts per thousand for cDCE), they were not as large as typically expected for a primary chlorine isotope effect suggesting that no cleavage of C-Cl bonds takes place during the initial rate-limiting step. The ratio of isotope enrichment factors for chlorine and carbon were substantially different for the two reaction mechanisms suggesting that the reaction mechanisms can be differentiated at the field scale using a dual isotope approach.