Pinpointing Chlorinated Ethylene Dehalogenation Mechanisms with Carbon, Chlorine and Hydrogen Isotope Effect Studies

• Applicant: Professor Dr. Martin Elsner
• Subject Area: Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
• Term: from 2009 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 152515687

Chlorinated ethylenes are prevalent groundwater contaminants, which may undergo reductive dehalogenation. Currently, mechanistic insight is incomplete, product formation little understood, and available insight is difficult to transfer from lab to field. To bridge this gap with isotope effects of multiple elements, in the past period we have (a) firmly established compound-specific 37Cl/35Cl besides 13C/12C isotope analysis and explored the first dual element isotope fractionation patterns of chlorinated ethylenes in (b) microbial degradation, (c) dehalogenation by Fe(0) and (d) reactions with chemical model reactants (Vitamin B12 - the cofactor of all dehalogenases). Based on characteristically different patterns in dual element isotope fractionation (Cl vs. C), our results give a first hot lead that two or more mechanisms are at work in microbial reductive dehalogenation. In the second period I therefore aim to pinpoint the mechanisms behind these differences with (a) targeted experiments involving carefully chosen model reactants and (b) measuring besides carbon and chlorine also hydrogen isotope effects. The approach pillars on the hypotheses (i) that two pathways which were previously considered distinct - nucleophilic substitution and nucleophilic addition of Vitamin B12 - share the same initial step; (ii) that they may be deconvolved in experiments at different pH; (iii) that single electron transfer to chlorinated ethylenes can be simulated both in water and in organic solvents; (iv) that information from hydrogen isotope fractionation as third observable allows deconvolving the three mechanistic endmembers. Such detailed mechanistic understanding will make it possible to better understand the formation of toxic vs. non-problematic product in natural and engineered degradation reactions and may help developing better strategies for remediation of contaminated sites.

• DFG Programme: Research Grants
• International Connection: Switzerland
• Cooperation Partner: Professor Dr. Kristopher McNeill