es070531o_si_003.tif (12.4 MB)
Download fileInsight into Methyl tert-Butyl Ether (MTBE) Stable Isotope Fractionation from Abiotic Reference Experiments
figure
posted on 15.08.2007, 00:00 authored by Martin Elsner, Jennifer McKelvie, Georges Lacrampe Couloume, Barbara Sherwood LollarMethyl 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.