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

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 δCl 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 δH during protonation in the products. Scenario modeling illustrates the potential of H-CSIA for source apportionment.

A modified method to calculate dual-isotope slopes for the natural attenuation of organic pollutants in the environment

“…A third factor, it can be completed in one-step regression, thus reducing the transmission of errors. Last but not least, because ΛOLR is approximately equal to Λα (Elsner et al 2007, Vavilin &Rytov 2016 (derivation process in literature (Elsner et al 2007) and revised below), distinct mechanisms usually lead to different ΛOLR (Braeckevelt et al 2012) Combined with the above advantages, ΛOLR has been widely used involving twodimensional isotopes (Badin et al 2014, Solano et al 2018) (or three (Kuder et al 2013, Van Breukelen et al 2017).…”

Modelling of C/Cl isotopic behaviour during chloroethene biotic reductive dechlorination: Capabilities and limitations of simplified and comprehensive models

“…Several studies have been carried out to address this challenge, some of which additionally successfully applied models to evaluate the fate of chloroethenes plumes [ 9 – 13 ]. However, for simplification purposes, most models considered one element only [ 9 , 11 , 14 , 15 ], and some models simulated heavy and light isotopes of one element of each compound as separate species [ 14 , 15 ] while others considered isotopologues with regards to one [ 11 , 16 ] or two elements [ 13 ]. Such a variety of simplification types reflects the need for a trade-off between including all isotopologues or isotopocules (i.e., isotopomers of all isotopologues of one compound; isotopomers being isomers of isotopologues) and avoiding an excessive complexity leading to computationally demanding models.…”

“…However, secondary isotope effects, which affect elements located in non-reacting bonds, were neglected in this work whichalso ignored PCE. Recent studies have shown that secondary Cl isotopic effects are measurable during chloroethene reductive dechlorination and should therefore not be discounted [ 16 – 18 ]. However, few models so far have considered secondary isotopic effects since they were formerly generally assumed to be negligible [ 13 , 19 , 20 ].…”

“…However, few models so far have considered secondary isotopic effects since they were formerly generally assumed to be negligible [ 13 , 19 , 20 ]. To address this need, Höhener [ 18 ] and van Breukelen et al [ 16 ] recently incorporarted secondary isotopic effects in isotope fractionation models though both studies considered isotopes of all elements independantly. Van Breukelen et al [ 16 ] applied a correction term to the reaction rate of each isotope/isotopologue so that the independent isotope networks correspond to the overall reaction progress.…”

“…To address this need, Höhener [ 18 ] and van Breukelen et al [ 16 ] recently incorporarted secondary isotopic effects in isotope fractionation models though both studies considered isotopes of all elements independantly. Van Breukelen et al [ 16 ] applied a correction term to the reaction rate of each isotope/isotopologue so that the independent isotope networks correspond to the overall reaction progress. On the other hand, Höhener et al applied a “Cretnik correction” which involved the introduction of an offset.…”