TY - JOUR
T1 - Potential for identifying abiotic chloroalkane degradation mechanisms using carbon isotopic fractionation
AU - Vanstone, Nancy
AU - Elsner, Martin
AU - Lacrampe-Couloume, Georges
AU - Mabury, Scott
AU - Lollar, Barbara Sherwood
PY - 2008/1/1
Y1 - 2008/1/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=37549035038&partnerID=8YFLogxK
U2 - 10.1021/es0711819
DO - 10.1021/es0711819
M3 - Article
C2 - 18350886
AN - SCOPUS:37549035038
SN - 0013-936X
VL - 42
SP - 126
EP - 132
JO - Environmental Science and Technology
JF - Environmental Science and Technology
IS - 1
ER -