Carbon isotope fractionation during aerobic biodegradation of n-alkanes and aromatic compounds in unsaturated sand

Abstract

Microcosm experiments were conducted to quantify carbon isotope fractionation during aerobic biodegradation of n-alkanes (from C 3 to C 10 ) and monoaromatic hydrocarbons in unsaturated alluvial sand. In single compound experiments with n-alkanes, the largest enrichment factor was obtained for propane (À10.8 ± 0.7‰). The magnitude of the enrichment factor decreased with increasing number of carbon atoms from propane to n-decane (À0.2 ± 0.1‰). This trend can partly be explained by the decreasing probability that a 13 C is located at the reacting site in the molecule with increasing chain length. After correcting for the presence of non-reacting positions, a chain length dependence of the calculated apparent isotope effect persisted. This observation suggests that transport and binding steps before the actual reaction step become increasingly rate limiting with increasing chain length. For aromatic compounds tested individually, the enrichment factor was the largest (À1.4 ± 0.1‰) for benzene (B), followed by toluene (T) (À0.8 ± 0.1‰) and m-xylene (X) (À0.6 ± 0.1‰). Enrichment factors for BTX were systematically smaller than for n-alkanes with equivalent number of carbons, which is likely related to different biodegradation mechanisms. The study demonstrates that significant carbon isotope fractionation occurs during aerobic biodegradation of n-alkanes and aromatic compounds under unsaturated conditions and that the magnitude of isotope enrichment is linked to molecule size and molecule structure.

Analytical modelling of stable isotope fractionation of volatile organic compounds in the unsaturated zone

“…ω = 0. . b From (Bouchard et al, 2008c), MCP and MCH from (Bouchard et al, 2005). c n-hexane, n-octane and toluene from .…”

“…Where α is the isotope fractionation factor for the compound of interest taken from (Bouchard et al, 2008c) and (Bouchard et al, 2005). For the column study, the biodegradation rate constant from the previous study was used as an initial value that requires adjustment as the biomass composition might have changed.…”

Carbon isotopic fractionation during vaporization of low molecular weight hydrocarbons (C6–C12)

“…Table 4 lists the carbon isotope enrichment factors of the LMWHs considered here and in previous studies, along with values calculated using Eq. (5) (Craig 1953;Cerling et al 1991;Bouchard et al 2008c;Jeannottat and Hunkeler 2012). All the experimental values of e (e e ) for the C 6 -C 9 LMWHs are negative, indicating the enrichment of 13 C in the residual liquids and ''normal isotope fractionation'' during vaporization.…”

“…The theoretical fractionation factor between the diffusion coefficients is given by the following equation (Craig 1953;Cerling et al 1991;Bouchard et al 2008c;Jeannottat and Hunkeler 2012):…”

Origin and fate of deep-sea seeping methane bubbles at Kuroshima Knoll, Ryukyu forearc region, Japan

“…In addition, Strapoc et al (2007) found kinetic isotope fractionation in propane presumably produced through anaerobic microbial consumption. The kinetic isotope fractionation during aerobic microbial consumption of C 2 -C 4 hydrocarbons has been quantified in laboratory incubation experiments as well (Kinnaman et al, 2007;Bouchard et al, 2008). Thus, microbial consumption of C 3 (significantly), C 4 (moderately), and C 2 (partly) in thermogenic hydrocarbons could explain the observed molecular and isotopic fractionations of hydrocarbons.…”