Potential for identifying abiotic chloroalkane degradation mechanisms using carbon isotopic fractionation

Nancy Vanstone, Martin Elsner, Georges Lacrampe-Couloume, Scott Mabury, Barbara Sherwood Lollar

Research output: Contribution to journalArticlepeer-review

45 Scopus citations

Abstract

Degradation of 1,1- and 1,2-dichloroethane (1,1-DCA, 1,2-DCA) and carbon tetrachloride (CCl4) on Zn0 was investigated using compound specific isotope analysis (CSIA) to measure isotopic fractionation factors for chloroalkane degradation by hydrogenolysis, by α-elimination, and by β-elimination. Significant differences in enrichment factors (ε) and associated apparent kinetic isotope effects (AKIE) were measured for these different reaction pathways, suggesting that carbon isotope fractionation by β-elimination is substantially larger than fractionation by hydrogenolysis or by α-elimination. Specifically, for 1,1-DCA, the isotopic composition of the reductive α-elimination product (ethane) and the hydrogenolysis product (chloroethane) were the same, indicating that cleavage of a single C-Cl bond was the rate-limiting step in both cases. In contrast, for 1,2-DCA, ε = εreactive position = -29.7 ± 1.5‰, and the calculated AKIE (1.03) indicated that β-elimination was likely concerted, possibly involving two C-Cl bonds simultaneously. Compared to 1,1-DCA hydrogenolysis, the AKIE of 1.01 for hydrogenolysis of CCl4 was much lower, indicating that, for this highly reactive organohalide, mass transfer to the surface was likely partially rate-limiting. These findings are a first step toward delineating the relative contribution of these competing pathways in other abiotic systems such as the degradation of chlorinated ethenes on zerovalentiron (ZVI), iron sulfide, pyrite, or magnetite, and, potentially, toward distinguishing between degradation of chlorinated ethenes by abiotic versus biotic processes.

Original languageEnglish
Pages (from-to)126-132
Number of pages7
JournalEnvironmental Science and Technology
Volume42
Issue number1
DOIs
StatePublished - 1 Jan 2008
Externally publishedYes

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