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 epsilon(C) = -4.9 per thousand +/- 0.6 per thousand for carbon and epsilon(H) = -55 per thousand +/- 7 per thousand for hydrogen. Position-specific values were epsilon(C), reactive position = -24.3 per thousand +/- 2.3 oer thousand and epsilon(H,reactive position) = -73 per thousand +/- 9 per thousand, giving kinetic isotope effects KIE(C) = 1.025 +/- 0.003 and KIE(H) = 1.08 +/- 0.01 consistent with an SN1-type hydrolysis involving the tert-butyl group. The characteristic slope of deltadelta2H(bulk)/deltadelta13C(bulk) approximately epsilon(bulk,H)/ epsilon(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 epsilon(H) = -109 per thousand +/- 9 per thousand and epsilon(H,reactive position) = -342 per thousand +/- 16 per thousand 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 epsilon values reported should, therefore, be used.
Elsner
Martin
M
Stable Isotope Laboratory, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada. martin.elsner@gsf.de
McKelvie
Jennifer
J
Couloume
Georges Lacrampe
GL
Lollar
Barbara Sherwood
BS
D000332:Aerobiosis; D001673:Biodegradation, Environmental; D002247:Carbon Isotopes; D005591:Chemical Fractionation / Q000379:methods*; D004602:Elements; D006859:Hydrogen; D006868:Hydrolysis; D007700:Kinetics; D017895:Manganese Compounds / Q000737:chemistry; D008738:Methyl Ethers / Q000737:chemistry*; D010084:Oxidation-Reduction; D010087:Oxides / Q000737:chemistry
D002247:Carbon Isotopes; D004602:Elements; D017895:Manganese Compounds; D008738:Methyl Ethers; D010087:Oxides; C048856:permanganic acid; C043243:methyl tert-butyl ether; D006859:Hydrogen