Microcosm experiments were conducted to quantify carbon isotope fractionation during aerobic biodegradation of -alkanes (from C to C) and monoaromatic hydrocarbons in unsaturated alluvial sand. In single compound experiments with -alkanes, the largest enrichment factor was obtained for propane (−10.8 ± 0.7‰). The magnitude of the enrichment factor decreased with increasing number of carbon atoms from propane to -decane (−0.2 ± 0.1‰). This trend can partly be explained by the decreasing probability that a 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 (−1.4 ± 0.1‰) for benzene (B), followed by toluene (T) (−0.8 ± 0.1‰) and -xylene (X) (−0.6 ± 0.1‰). Enrichment factors for BTX were systematically smaller than for -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 -alkanes and aromatic compounds under unsaturated conditions and that the magnitude of isotope enrichment is linked to molecule size and molecule structure.
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