| Abstract
| - We investigated the extent and variability of C and Clisotope fractionation during the reduction of polychlorinatedethanes to evaluate the potential use of Cl isotopeanalysis for the assessment of contaminant transformationin subsurface environments. Kinetic isotope effects (KIE)for C and Cl for the reductive β-elimination of 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA), pentachloroethane (PCA),and hexachloroethane by Cr(II) used as model reductantin homogeneous solution were compared to KIEs measuredfor dehydrochlorination of 1,1,2,2-TeCA and PCA. Sinceisotopic reactions of polychlorinated compounds arecomplicated by the simultaneous presence of several Clisotopologues and intramolecular isotopic competition, wepresent a procedure for the determination of KIEs for Clfrom the initial reactant and final product Cl isotope ratios.Despite different reaction mechanisms, that is reductionvia dissociative inner-sphere electron transfer by Cr(H2O)62+and base-catalyzed, concerted elimination, respectively,apparent KIEs for C of both pathways fall within a similarrange (1.021−1.031). In contrast, KIEs for Cl are significantlyhigher for reductive β-elimination (1.013−1.021) thanfor dehydrochlorination (1.000−1.006). These results suggestthat reductive transformations of polychlorinated contaminants might be identified on the basis of combined Cand Cl isotope analysis.
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