Hydrogen Isotope Fractionation during the Biodegradation of 1,2-

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 (εbulk
H ) 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 (ΛC−H = Δδ2H/Δδ13C ≈ εbulk
H /εbulk
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), 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.,
Δδ2H versus Δδ37Cl versus 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.