C and Cl isotope fractionation of 1,2-dichloroethane displays unique δ13C/δ37Cl patterns for pathway identification and reveals surprising C-Cl bond involvement in microbial oxidation.
This study investigates dual element isotope fractionation during aerobic biodegradation of 1,2-dichloroethane (1,2-DCA) via oxidative cleavage of a C–H bond (Pseudomonas sp. strain DCA1) versus C–Cl bond cleavage by SN2 reaction (Xanthobacter autotrophicus GJ10 and Ancylobacter aquaticus AD20). Compound-specific chlorine isotope analysis of 1,2-DCA was performed for the first time, and isotope fractionation (εbulkCl) was determined by measurements of the same samples in three different laboratories using two gas chromatography–isotope ratio mass spectrometry systems and one gas chromatography–quadrupole mass spectrometry system. Strongly pathway-dependent slopes (Δδ13C/Δδ37Cl), 0.78 ± 0.03 (oxidation) and 7.7 ± 0.2 (SN2), delineate the potential of the dual isotope approach to identify 1,2-DCA degradation pathways in the field. In contrast to different εbulkC values [−3.5 ± 0.1‰ (oxidation) and −31.9 ± 0.7 and −32.0 ± 0.9‰ (SN2)], the obtained εbulkCl values were surprisingly similar for the two pathways: −3.8 ± 0.2‰ (oxidation) and −4.2 ± 0.1 and −4.4 ± 0.2‰ (SN2). Apparent kinetic isotope effects (AKIEs) of 1.0070 ± 0.0002 (13C-AKIE, oxidation), 1.068 ± 0.001 (13C-AKIE, SN2), and 1.0087 ± 0.0002 (37Cl-AKIE, SN2) fell within expected ranges. In contrast, an unexpectedly large secondary 37Cl-AKIE of 1.0038 ± 0.0002 reveals a hitherto unrecognized involvement of C–Cl bonds in microbial C–H bond oxidation. Our two-dimensional isotope fractionation patterns allow for the first time reliable 1,2-DCA degradation pathway identification in the field, which unlocks the full potential of isotope applications for this important groundwater contaminant.