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

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posted on 15.08.2007, 00:00 authored by Martin Elsner, Jennifer McKelvie, Georges Lacrampe Couloume, Barbara Sherwood Lollar
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 εC = −4.9‰ ± 0.6‰ for carbon and εH = −55‰ ± 7‰ for hydrogen. Position-specific values were εC,reactive position = −24.3‰ ± 2.3‰ and εH,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 Δδ2Hbulk/Δδ13Cbulk ≈ εbulk,H/ εbulk,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 εH = −109‰ ± 9‰ and εH,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 ε values reported should, therefore, be used.