Distinguishing abiotic and biotic transformation of tetrachloroethylene and trichloroethylene by stable carbon isotope fractionation

Significant carbon isotope fractionation was observed during FeS-mediated reductive dechlorination of tetrachloroethylene (PCE) and trichloroethylene (TCE). Bulk enrichment factors (epsilon(bulk)) for PCE were -30.2 +/- 4.3 parts per thousand (pH 7), -29.54 +/- 0.83 parts per thousand (pH 8), and -24.6 +/- 1.1 parts per thousand (pH 9). For TCE, epsilon(bulk) values were -33.4 +/- 1.5 parts per thousand (pH 8) and -27. 9 +/- 1.3 parts per thousand (pH 9). A smaller magnitude of carbon isotope fractionation resulted from microbial reductive dechlorination by two isolated pure cultures (Desulfuromonas michiganensis strain BB1 (BB1) and Sulfurospirillum multivorans (Sm)) and a bacterial consortium (BioDechlor INOCULUM (BDI)). The epsilon(bulk) values for biological PCE microbial dechlorination were -1.39 +/- 0.21 parts per thousand (BB1), -1.33 +/- 0.13 parts per thousand (Sm), and -7.12 +/- 0.72 parts per thousand (BDI), while those for TCE were -4.07 +/- 0.48 parts per thousand (BB1), -12.8 +/- 1.6 parts per thousand (Sm), and -15.27 +/- 0.79 parts per thousand (BDI). Reactions were investigated by calculation of the apparent kinetic isotope effect for carbon (AKIE(C)), and the results suggest that differences in isotope fractionation for abiotic and microbial dechlorination resulted from the differences in rate-limiting steps during the dechlorination reaction. Measurement of more negative epsilon(bulk) values at sites contaminated with PCE and TCE may suggest the occurrence of abiotic reductive dechlorination by FeS.