Carbon and Chlorine Isotope Analysis to Identify Abiotic Degradation Pathways of 1,1,1-Trichloroethane

This study investigates dual C−Cl isotope fractionation during 1,1,1-TCA transformation by heat-activated persulfate (PS), hydrolysis/dehydrohalogenation (HY/DH) and Fe(0). Compound-specific chlorine isotope analysis of 1,1,1-TCA was performed for the first time, and transformation-associated isotope fractionation εCbulk and ε CIbulk values were −4.0 ± 0.2‰ and no chlorine isotope fractionation with PS, −1.6 ± 0.2‰ and −4.7 ± 0.1‰ for HY/DH, −7.8 ± 0.4‰ and −5.2 ± 0.2‰ with Fe(0). Distinctly different dual isotope slopes (Δ δ13C/Δδ37Cl): ∞ with PS, 0.33 ± 0.04 for HY/DH and 1.5 ± 0.1 with Fe(0) highlight the potential of this approach to identify abiotic degradation pathways of 1,1,1-TCA in the field. The trend observed with PS agreed with a C−H bond oxidation mechanism in the first reaction step. For HY/DH and Fe(0) pathways, different slopes were obtained although both pathways involve cleavage of a C−Cl bond in their initial reaction step. In contrast to the expected larger primary carbon isotope effects relative to chlorine for C−Cl bond cleavage, εCbulk Clbulk was observed for HY/DH and in a similar range for reduction by Fe(0), suggesting the contribution of secondary chlorine isotope effects. Therefore, different magnitude of secondary chlorine isotope effects could at least be partly responsible for the distinct slopes between HY/DH and Fe(0) pathways. Following this dual isotope approach, abiotic transformation processes can unambiguously be identified and quantified

Carbon and chlorine isotope analysis to identify abiotic degradation pathways of 1,1,1-trichloroethane

This study investigates dual C-Cl isotope fractionation during 1,1,1-TCA transformation by heat-activated persulfate (PS), hydrolysis/dehydrohalogenation (HY/DH) and Fe(0). Compound-specific chlorine isotope analysis of 1,1,1-TCA was performed for the first time, and transformation-associated isotope fractionation ε_bulk^C and ε_bulk^Cl were: -4.0±0.2¿ and no chlorine isotope fractionation with PS, -1.6±0.2¿ and -4.7±0.1¿ for HY/DH, -7.8±0.4¿ and -5.2±0.2¿ with Fe(0). Distinctly different dual isotope slopes (Δδ13C/Δδ37Cl): ∞ with PS, 0.33±0.04 for HY/DH and 1.5±0.1 with Fe(0) highlight the potential of this approach to identify abiotic degradation pathways of 1,1,1-TCA in the field. The trend observed with PS agreed with a C-H bond oxidation mechanism in the first reaction step. For HY/DH and Fe(0) pathways, different slopes were obtained although both pathways involve cleavage of a C-Cl bond in their initial reaction step. In contrast to the expected larger primary carbon isotope effects relative to chlorine for C-Cl bond cleavage, ε_bulk^C<ε_bulk^Cl was observed for HY/DH and in a similar range for reduction by Fe(0), suggesting the contribution of secondary chlorine isotope effects. Therefore, different magnitude of secondary chlorine isotope effects could at least be partly responsible for the distinct slopes between HY/DH and Fe(0) pathways. Following this dual isotope approach abiotic transformation processes can unambiguously be identified and quantified

C, Cl and H compound-specific isotope analysis to assess natural versus Fe(0) barrier-induced degradation of chlorinated ethenes at a contaminated site

Compound-specific isotopic analysis of multiple elements (C, Cl, H) was tested to better assess the effect of a zero-valent iron-permeable reactive barrier (ZVI-PRB) installation at a site contaminated with tetrachloroethene (PCE) and trichloroethene (TCE). The focus was on (1) using 13C to evaluate natural chlorinated ethene biodegradation and the ZVI-PRB efficiency; (2) using dual element 13C-37Cl isotopic analysis to distinguish biotic from abiotic degradation of cis-dichloroethene (cis-DCE); and (3) using 13C-37Cl-2H isotopic analysis of cis-DCE and TCE to elucidate different contaminant sources. Both biodegradation and degradation by ZVI-PRB were indicated by the metabolites that were detected and the 13C data, with a quantitative estimate of the ZVI-PRB efficiency of less than 10% for PCE. Dual ele- ment 13C-37Cl isotopic plots confirmed that biodegradation was the main process at the site including the ZVI-PRB area. Based on the carbon isotope data, approximately 45% and 71% of PCE and TCE, respec- tively, were estimated to be removed by biodegradation. 2H combined with 13C and 37Cl seems to have identified two discrete sources contributing to the contaminant plume, indicating the potential of 2 H to discriminate whether a compound is of industrial origin, or whether a compound is formed as a daughter product during degradation

Hydrogen isotope fractionation during biodegradation of 1,2-dichloroethane: potential for pathway identification using a multi-element (C, Cl and H) isotope approach

Even though multi-element isotope fractionation patterns provide crucial information to identify contaminant degradation pathways in the field, those involving hydrogen are still lacking for many halogenated groundwater contaminants and degradation pathways. This study investigates for the first time hydrogen isotope fractionation during both aerobic and anaerobic biodegradation of 1,2-dichloroethane (1,2-DCA) using five microbial cultures. Transformation-associated isotope fractionation values (ε_bulk^H) were: -115 ± 18¿ (aerobic C-H bond oxidation), -34 ± 4¿ and -38 ± 4¿ (aerobic C-Cl bond cleavage via hydrolytic dehalogenation), -57 ± 3¿ and -77 ± 9¿ (anaerobic C-Cl bond cleavage via reductive dihaloelimination). The dual element C-H isotope approach (ΛC-H = Δδ2H/Δδ13C ≈ ε_bulk^H/ε_bulk^C, where Δδ2H and Δδ13C are changes in isotope ratios during degradation) resulted in clearly different ΛC-H values: 28 ± 4 (oxidation), 0.7 ± 0.1 and 0.9 ± 0.1 (hydrolytic dehalogenation), 1.76 ± 0.05 and 3.5 ± 0.1 (dihaloelimination). This result highlights the potential of this approach to identify 1,2-DCA degradation pathways in the field. In addition, distinct trends were also observed in a multi (i.e., Δδ2H vs Δδ37Cl vs Δδ13C) isotope plot, which opens further possibilities for pathway identification in future field studies. This is crucial information to understand the mechanisms controlling natural attenuation of 1,2-DCA and to design appropriate strategies to enhance biodegradation

Distinct dual C-Cl isotope fractionation patterns during anaerobic biodegradation of 1,2-dichloroethane: potential to characterize microbial degradation in the field

This study investigates, for the first time, dual C-Cl isotope fractionation during anaerobic biodegradation of 1,2-dichloroethane (1,2-DCA) via dihaloelimination by Dehalococcoides and Dehalogenimonas-containing enrichment cultures. Isotopic fractionation of 1,2-DCA (εbulkC and εbulkCl) for Dehalococcoides (−33.0 ± 0.4¿ and −5.1 ± 0.1¿) and Dehalogenimonas-containing microcosms (−23 ± 2¿ and −12.0 ± 0.8¿) resulted in distinctly different dual element C-Cl isotope correlations (Λ = Δδ13C/Δδ37Cl ≈ εbulkC/εbulkCl), 6.8 ± 0.2 and 1.89 ± 0.02, respectively. Determined isotope effects and detected products suggest that the difference on the obtained Λ values for biodihaloelimination could be associated with a different mode of concerted bond cleavage rather than two different reaction pathways (i.e., stepwise vs concerted). Λ values of 1,2-DCA were, for the first time, determined in two field sites under reducing conditions (2.1 ± 0.1 and 2.2 ± 2.9). They were similar to the one obtained for the Dehalogenimonas-containing microcosms (1.89 ± 0.02) and very different from those reported for aerobic degradation pathways in a previous laboratory study (7.6 ± 0.1 and 0.78 ± 0.03). Thus, this study illustrates the potential of a dual isotope analysis to differentiate between aerobic and anaerobic biodegradation pathways of 1,2-DCA in the field and suggests that this approach might also be used to characterize dihaloelimination of 1,2-DCA by different bacteria, which needs to be confirmed in future studies