Insight into methyl tert-butyl ether (MTBE) stable isotope Fractionation from abiotic reference experiments

Methyl group oxidation, SN2-type hydrolysis, and SNI-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 cc = -4.9% +/- 0.6% for carbon and EH = epsilon(H) -55% +/- 7%o for hydrogen. Positionspecific values were epsilon(C),(reactive position) = -24.3% +/- 2.3%+/- and CH,reactive position = -73%o 9%o, giving kinetic isotope effects KlEc = 1.025 +/- 0.003 and KIEH = 1.08 +/- 0.01 consistent with an SNI-type hydrolysis involving the tertbutyl group. The characteristic slope of Delta delta H-2(bulk)/Delta delta C-13(bulk) epsilon(bulk/H) epsilon(bulk), 61C = 11.1 +/- 1.3 suggests it may identify SN1type hydrolysis also in settings where the pathway is not well constrained. Oxidation by permanganate was found to involve specifically the methyl group of IVITBE, similarto aerobic biodegradation. Large hydrogen enrichment factors Of epsilon(H) = -109%o 9%o and EH,reactive position = -342% +/- 16% indicate both large primary and large secondary hydrogen isotope effects. Significantly smaller values reported previouslyfor aerobic biodegradation suggest that intrinsic fractionation is often masked by additional nonfractionating steps. For conservative estimates of biodegradation at field sites, the largest e values reported should, therefore, be used.