Identifying abiotic chlorinated ethene degradation: Characteristic isotope patterns in reaction products with nanoscale zero-valent iron

Carbon isotope fractionation is of great interest in assessing chlorinated ethene transformation by nanoscale zero-valent iron at contaminated sites, particularly in distinguishing the effectiveness of an implemented abiotic degradation remediation scheme from intrinsic biotic degradation. Transformation of trichloroethylene (TCE), cis-dichloroethylene (cis-DCE), and vinyl chloride (VC) with two types of nanoscale iron materials showed different reactivity trends but relatively consistent, carbon isotope enrichment factors (epsilon) of -19.4 parts per thousand +/- 1.8 parts per thousand (VC), -21.7 parts per thousand +/- 1.8 parts per thousand (cis-DCE), and -23.5 parts per thousand +/- 2.8 parts per thousand (TCE) with one type of iron (Fe-BH), and from -20.9 parts per thousand +/- 1.1 parts per thousand to -26.5 parts per thousand +/- 1.5 parts per thousand (TCE) with the other (Fe-H2). Products of the dichloroelimination pathway (ethene, ethane, and acetylene) were consistently 10 parts per thousand more isotopically depleted than those of the hydrogenolysis pathway (cis-DCE from TCE, VC from cis-DCE), displaying a characteristic pattern that may serve as an indicator of abiotic dehalogenation reactions and as a diagnostic parameter for differentiating the effects of abiotic versus biotic degradation. In contrast the product-related, enrichment factors of each respective pathway varied significantly in different experiments. Because such variation would not be expected for independent pathways with constant kinetic isotope effects, our data give preliminary evidence that the two pathways may share an irreversible first reaction step with subsequent isotopically sensitive branching.