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

Methyl group oxidation, S_N2-type hydrolysis, and S_N1-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 ε_C = −4.9‰ ± 0.6‰ for carbon and ε_H = −55‰ ± 7‰ for hydrogen. Position-specific values were ε_{C,reactive position} = −24.3‰ ± 2.3‰ and ε_{H,reactive position} = −73‰ ± 9‰, giving kinetic isotope effects KIE_C = 1.025 ± 0.003 and KIE_H = 1.08 ± 0.01 consistent with an S_N1-type hydrolysis involving the tert-butyl group. The characteristic slope of Δδ²H_bulk/Δδ¹³C_bulk ≈ ε_bulk,H/ ε_bulk,C = 11.1 ± 1.3 suggests it may identify S_N1-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 ε_H = −109‰ ± 9‰ and ε_{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 ε values reported should, therefore, be used.