Modeling Chlorine Isotope Trends during Sequential Transformation of Chlorinated Ethenes

Abstract

There is increasing interest in combined carbon−chlorine compound-specific isotope analysis (CSIA) to differentiate between contaminant sources and to assess transformation processes. However, the significant abundance of polychlorinated molecules with several heavy chlorine isotopes complicates the evaluation of chlorine isotope trends. Therefore, the goal of this study was to develop a conceptual and mathematical framework that describes the expected chlorine isotope fractionation patterns during multistep transformation of chlorinated compounds. Reductive dechlorination of chlorinated ethenes served as an example. The study demonstrates that chlorine isotope trends can be simulated by reproducing the average behavior of light and heavy isotopes or by explicitly simulating molecules with different numbers of heavy isotopes (isotopologues). The calculations reveal that initial chlorine isotope ratios of products equal the isotope ratios of their parent compounds in the absence of secondary isotope effects, while steadily increasing during transformation. The slopes in dual isotope plots are linear for reactant and product during a one-step reaction. They become nonlinear for products that are degraded further but converge to characteristic slopes. Consideration of different scenarios reveals that combined carbon−chlorine isotope analysis bears high potential to differentiate between contaminant sources, to elucidate reaction mechanisms in laboratory studies, and to identify transformation processes in the field.

Chlorine Isotope Effects from Isotope Ratio Mass Spectrometry Suggest Intramolecular C-Cl Bond Competition in Trichloroethene (TCE) Reductive Dehalogenation

“…In addition, our calculations can explain the different numerical values in the TCE degradation experiment when  13 C/ 37 Cl of TCE is compared to C 13 C/ 37 Cl of cis-DCE. While the slope for TCE is equal to ε TCE,carbon /ε TCE,chlorine [31], the slope for cis-DCE corresponds to ε TCE,carbon /ε TCE->cis-DCE , chlorine . The observation that ε TCE->cis-DCE,chlorine < ε TCE,chlorine (see above) therefore explains the steeper slope for the cis-DCE data.…”

“…Information on the magnitude of either position-specific isotope effect may therefore be retrieved from analysis of isotope ratios in the products (Cl − and the less chlorinated ethene). This approach was discussed in previous work [31] and was pursued in a recent experimental study by Kuder et al [30]. There, it was a priori excluded that more than one C-Cl bond is reactive in TCE-in contradiction to the scenarios of Scheme 1.…”

“…The average secondary isotope effect is further given by (31) so that the primary isotope effect in PCE can be calculated:…”

Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools

“…Hunkeler et al, 2009 predicted that the vertical spacing between dual isotope slopes associated with chlorinated ethenes during reductive dechlorination reflected the C enrichment factors when the slopes run parallel to each other. The reason is that the kinetic Cl isotope effect acts on the cleaved chloride so that the chlorine isotope ratio of TCE is expected to match that of PCE (as experimentally confirmed by Cretnik et al, 2014), whereas C isotope values differ by the C enrichment factor ε C .…”

Unravelling long-term source removal effects and chlorinated methanes natural attenuation processes by C and Cl stable isotopic patterns at a complex field site

“…Moreover, dual isotopic studies (2D-CSIA) with carbon and chlorine isotope analyses have allowed more precise identification of pollutants' origin and fate as dual isotope slopes (Λ) reflect ongoing degradation mechanisms and can be compared with characteristic slopes from laboratory studied reactions (Hunkeler et al, 2009). However, an aged source of contamination would have undergone fractionation processes in the unsaturated zone distinctly or not to those occurred in the saturated zone Hunkeler, 2012, 2013) and, thus, it would hamper reliable degradation processes discrimination.…”