Modeling 3D-CSIA data: Carbon, chlorine, and hydrogen isotope fractionation during reductive dechlorination of TCE to ethene

Boris M Van Breukelen,Héloïse A.A Thouement,Philip E Stack,Mindy Vanderford,Paul Philp,Tomasz Kuder

doi:10.1016/j.jconhyd.2017.07.003

Abstract

Reactive transport modeling of multi-element, compound-specific isotope analysis (CSIA) data has great potential to quantify sequential microbial reductive dechlorination (SRD) and alternative pathways such as oxidation, in support of remediation of chlorinated solvents in groundwater. As a key step towards this goal, a model was developed that simulates simultaneous carbon, chlorine, and hydrogen isotope fractionation during SRD of trichloroethene, via cis-1,2-dichloroethene (and trans-DCE as minor pathway), and vinyl chloride to ethene, following Monod kinetics. A simple correction term for individual isotope/isotopologue rates avoided multi-element isotopologue modeling. The model was successfully validated with data from a mixed culture Dehalococcoides microcosm. Simulation of Cl-CSIA required incorporation of secondary kinetic isotope effects (SKIEs). Assuming a limited degree of intramolecular heterogeneity of δ37Cl in TCE decreased the magnitudes of SKIEs required at the non-reacting Cl positions, without compromising the goodness of model fit, whereas a good fit of a model involving intramolecular CCl bond competition required an unlikely degree of intramolecular heterogeneity. Simulation of H-CSIA required SKIEs in H atoms originally present in the reacting compounds, especially for TCE, together with imprints of strongly depleted δ2H during protonation in the products. Scenario modeling illustrates the potential of H-CSIA for source apportionment.

Highlights

At many contaminated sites, monitored natural attenuation (MNA) of chlorinated ethenes is the preferred and cost-effective remediation approach (Meckenstock et al, 2015)
In order to explore the potential use of H-compound-specific isotope analysis (CSIA) in source apportionment of TCE versus primary contaminants tetrachloroethene (PCE) source zones, we extended the H-CSIA model with the PCE to TCE step to assess the δ2H values of TCE and daughter products in scenarios of pure and mixed PCE and TCE sources
The developed numerical model serves as a template model to interpret C, H, and Cl CSIA data in sequential reductive dechlorination (SRD) of halogenated hydrocarbons in general, with the aim of investigating (S)KIEs, intramolecular halogen isotope ratio heterogeneity, and protonation effects

Summary

Introduction

At many contaminated sites, monitored natural attenuation (MNA) of chlorinated ethenes is the preferred and cost-effective remediation approach (Meckenstock et al, 2015). Microbial sequential reductive dechlorination (SRD) of chlorinated ethenes is usually the main transformation process in MNA. Degradation may, occur via alternative transformation pathways such as (an)aerobic oxidation (Bradley, 2011; Bradley and Chapelle, 2011; Chu et al, 2004; Pooley et al, 2009) and chemical reduction (Damgaard et al, 2013; Darlington et al, 2013; Ferrey et al, 2004; Lee and Batchelor, 2002) of lower and higher chlorinated ethenes, respectively. Assessment of the alternative pathways of chlorinated ethene destruction is more difficult, since the degradation products (Cl−, CO2) blend with the natural background levels. Less sustainable remedies, such as pump and treat, may be instituted or continued unnecessarily

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