Hydrogen Isotope Fractionation during the 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 with which 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) were -115 ± 18‰ (aerobic C-H bond oxidation), -34 ± 4‰ and -38 ± 4‰ (aerobic C-Cl bond cleavage via hydrolytic dehalogenation), and -57 ± 3‰ and -77 ± 9‰ (anaerobic C-Cl bond cleavage via reductive dihaloelimination). The dual-element C-H isotope approach (Λ = ΔδH/ΔδC ≈ ε/ε, where ΔδH and ΔδC are changes in isotope ratios during degradation) resulted in clearly different Λ values: 28 ± 4 (oxidation), 0.7 ± 0.1 and 0.9 ± 0.1 (hydrolytic dehalogenation), and 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., ΔδH versus ΔδCl versus ΔδC) 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.

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of Abiotic Degradation of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies

“…27 Halogen isotope effects for aerobic microbial degradation of CH 3 Cl (or CH 3 Br) have not been published yet but it is conceivable that they show a similarly large isotope fractionation as reported for other halogenated alkanes. Aerobic microbial degradation of 1,2dichloroethane, for instance, caused an ε Cl of at least −3.8 mUr 64 which was similar to the ε Cl reported for anaerobic microbial degradation (−4.2 mUr). 26 In contrast, CH 3 X in the troposphere mainly degrades via OH and Cl radical reactions 1 which cause a C−H bond dissociation.…”

Compound-specific chlorine isotope fractionation in biodegradation of atrazine

“…Therefore, information from a third element, chlorine, would be highly valuable. Also on the mechanistic end, information gained from a change in the chlorine isotope value could lead to a more reliable differentiation of transformation pathways and contribute to a better mechanistic understanding of the underlying chemical reaction 31 . Along these lines, triple element (3D) isotope analysis was already accomplished for chlorinated alkanes 31,32 and alkenes 33,34 .…”

Ideas and perspectives: Same Carbon Different Elements – An Insight into Position-Specific Isotope Patterns Within a Single Compound

“…A compound often consists of different elements, for instance, H and O in H2O (Dansgaard, 1964;Craig, 1961), N and O in NO3 - (Casciotti and McIlvin, 2007;Wankel et al, 2009), S and O in SO4 2- (Antler et al, 2013), or C and H in organic compounds (Elsner, 2010;Palau et al, 2017). The isotope fractionation relationship between these different elements, i.e.…”