Hydrogen isotope fractionation during biodegradation of 1,2-dichloroethane: potential for pathway identification using a multi-element (C, Cl and H) isotope approach

Abstract

Even though multi-element isotope fractionation patterns provide crucial information to identify contaminant degradation pathways in the field, those involving hydrogen are still lacking for many halogenated groundwater contaminants and degradation pathways. This study investigates for the first time hydrogen isotope fractionation during both aerobic and anaerobic biodegradation of 1,2-dichloroethane (1,2-DCA) using five microbial cultures. Transformation-associated isotope fractionation values (ε_bulk^H) were: -115 ± 18¿ (aerobic C-H bond oxidation), -34 ± 4¿ and -38 ± 4¿ (aerobic C-Cl bond cleavage via hydrolytic dehalogenation), -57 ± 3¿ and -77 ± 9¿ (anaerobic C-Cl bond cleavage via reductive dihaloelimination). The dual element C-H isotope approach (ΛC-H = Δδ2H/Δδ13C ≈ ε_bulk^H/ε_bulk^C, where Δδ2H and Δδ13C are changes in isotope ratios during degradation) resulted in clearly different ΛC-H values: 28 ± 4 (oxidation), 0.7 ± 0.1 and 0.9 ± 0.1 (hydrolytic dehalogenation), 1.76 ± 0.05 and 3.5 ± 0.1 (dihaloelimination). This result highlights the potential of this approach to identify 1,2-DCA degradation pathways in the field. In addition, distinct trends were also observed in a multi (i.e., Δδ2H vs Δδ37Cl vs Δδ13C) isotope plot, which opens further possibilities for pathway identification in future field studies. This is crucial information to understand the mechanisms controlling natural attenuation of 1,2-DCA and to design appropriate strategies to enhance biodegradation.