Insight into Methyl tert-Butyl Ether (MTBE) Stable Isotope Fractionation from Abiotic Reference Experiments.

Methyl group oxidation, SN2-type hydrolysis, and SN1-type hydrolysis are suggested as natural transformation mechanisms of MTBE. This study reports for the first time MTBE isotopic fractionation during acid hydrolysis and for oxidation by permanganate. In acid hydrolysis, MTBE isotopic enrichment factors were C = -4.9” ± 0.6” for carbon and H = -55” ± 7” for hydrogen. Position-specific values were C,reactive position = -24.3” ± 2.3” and H,reactive position = -73” ± 9”, giving kinetic isotope effects KIEC = 1.025 ± 0.003 and KIEH = 1.08 ± 0.01 consistent with an SN1-type hydrolysis involving the tert-butyl group. The characteristic slope of 2Hbulk/13Cbulk bulk,H/ bulk,C = 11.1 ± 1.3 suggests it may identify SN1-type hydrolysis also in settings where the pathway is not well constrained. Oxidation by permanganate was found to involve specifically the methyl group of MTBE, similar to aerobic biodegradation. Large hydrogen enrichment factors of H = -109” ± 9” and H,reactive position = -342” ± 16” indicate both large primary and large secondary hydrogen isotope effects. Significantly smaller values reported previously for aerobic biodegradation suggest that intrinsic fractionation is often masked by additional non-fractionating steps. For conservative estimates of biodegradation at field sites, the largest values reported should, therefore, be used

Identifying Abiotic Chlorinated Ethene Degradation: Characteristic Isotope Patterns in Reaction Products with Nanoscale Zero-Valent Iron.

Carbon isotope fractionation is of great interest in assessing chlorinated ethene transformation by nanoscale zero-valent iron at contaminated sites, particularly in distinguishing the effectiveness of an implemented abiotic degradation remediation scheme from intrinsic biotic degradation. Transformation of trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and vinyl chloride (VC) with two types of nanoscale iron materials showed different reactivity trends, but relatively consistent carbon isotope enrichment factors (?) of -19.4‰ 1.8‰ (VC), -21.7‰ 1.8‰ (cis-DCE), and -23.5‰ 2.8‰ (TCE) with one type of iron (FeBH), and from -20.9‰ 1.1‰ to -26.5‰ 1.5‰ (TCE) with the other (FeH2). Products of the dichloroelimination pathway (ethene, ethane, and acetylene) were consistently 10‰ more isotopically depleted than those of the hydrogenolysis pathway (cis-DCE from TCE, VC from cis-DCE), displaying a characteristic pattern that may serve as an indicator of abiotic dehalogenation reactions and as a diagnostic parameter for differentiating the effects of abiotic versus biotic degradation. In contrast, the product-related enrichment factors of each respective pathway varied significantly in different experiments. Because such variation would not be expected for independent pathways with constant kinetic isotope effects, our data give preliminary evidence that the two pathways may share an irreversible first reaction step with subsequent isotopically sensitive branching

Potential for identifying abiotic chloroalkane degradation mechanisms using carbon isotopic fractionation.

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 ?-elimination, and by ?-elimination. Significant differences in enrichment factors (?) and associated apparent kinetic isotope effects (AKIE) were measured for these different reaction pathways, suggesting that carbon isotope fractionation by ?-elimination is substantially larger than fractionation by hydrogenolysis or by ?-elimination. Specifically, for 1,1-DCA, the isotopic composition of the reductive ?-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, ? = ?reactive position = -29.7 ± 1.5‰, and the calculated AKIE (1.03) indicated that ?-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 zerovalent iron (ZVI), iron sulfide, pyrite, or magnetite, and, potentially, toward distinguishing between degradation of chlorinated ethenes by abiotic versus biotic processes

Hydrogen isotopic enrichment: An indication of biodegradation at a petroleum hydrocarbon contaminated field site.

Compound-specific carbon and hydrogen isotope analysis was used to investigate biodegradation of benzene and ethylbenzene in contaminated groundwater at Dow Benelux BV industrial site.