Mechanistic Investigation of Chlorinated Ethylene Degradation using Chlorine and Carbon Isotope Fractionation

Abstract

Chloroethenes are large-scale industrial products, and detected as toxic contaminants in the environment. Their reductive dechlorination is a clear remediation approach, which has been the focus of several studies, using biotic and abiotic model systems. Despite the progress toward understanding the underlying process the formation of toxic and harmless products remains incompletely understood, and some of the proposed mechanistic hypotheses have led to inconsistencies. A recently developed analytical method of continuous flow compound specific chlorine isotope analysis was used in this study to further uncover the underlying mechanisms of reductive dechlorination. In the first instance, the newly created chlorine isotope data was analyzed towards the basic question how isotope effects of chlorine are manifested in the respective products during biodegradation. The developed mathematical framework gave first insights into position specific chlorine isotope effects during biodegradation of chloroethenes. From their interpretation, the structural selectivity in the biotic reductive dechlorination of TCE could be allocated to two chlorine substituents. This information allowed a systematic discussion with respect to the hypothesized mechanisms. Further degradation experiments with two different microbial strains were investigated by combined analysis of carbon and chlorine isotope effects in dual isotope plots in comparison to model reactions that are commonly used to mimic microbial dechlorination. Similar mechanisms were indicated for biodegradation and reactions with cobalamin (Vitamin B12), the enzymatic cofactor of dehalogenase enzymes. In contrast a different mechanism was indicated for reactions with cobaloxime, a commonly used mimicking reagent for cobalamin. The results demonstrate the strength of dual isotope plots as an indicator of the authenticity of a model reaction for the actual system with respect to the underlying mechanisms. The method of two dimensional isotope analysis was further investigated for its application towards the environmental clean-up technology of permeable reactive barriers (PRB) with zero-valent iron (ZVI). Dual isotope plots and product related carbon isotope fractionation were explored here as two discrete approaches to distinguish the effectiveness of transformation by ZVI as opposed to natural biodegradation. The results of this work exemplify the potential of chlorine and carbon isotope analysis to assess the sustainable removal of contaminants and their degradation pathways directly in real-world transformations. Moreover, it opens the perspective for future work to pinpoint mechanisms of the important environmental dehalogenation reactions by applying the approach on further model reactions with distinct mechanisms.