Carbon and Chlorine Isotope Fractionation Patterns Associated with Different Engineered Chloroform Transformation Reactions

Abstract

To use compound-specific isotope analysis for confidently assessing organic contaminant attenuation in the environment, isotope fractionation patterns associated with different transformation mechanisms must first be explored in laboratory experiments. To deliver this information for the common groundwater contaminant chloroform (CF), this study investigated for the first time both carbon and chlorine isotope fractionation for three different engineered reactions: oxidative C-H bond cleavage using heat-activated persulfate, transformation under alkaline conditions (pH ∼ 12) and reductive C-Cl bond cleavage by cast zerovalent iron, Fe(0). Carbon and chlorine isotope fractionation values were -8 ± 1‰ and -0.44 ± 0.06‰ for oxidation, -57 ± 5‰ and -4.4 ± 0.4‰ for alkaline hydrolysis (pH 11.84 ± 0.03), and -33 ± 11‰ and -3 ± 1‰ for dechlorination, respectively. Carbon and chlorine apparent kinetic isotope effects (AKIEs) were in general agreement with expected mechanisms (C-H bond cleavage in oxidation by persulfate, C-Cl bond cleavage in Fe(0)-mediated reductive dechlorination and E1 elimination mechanism during alkaline hydrolysis) where a secondary AKIE (1.00045 ± 0.00004) was observed for oxidation. The different dual carbon-chlorine (ΔδC vs ΔδCl) isotope patterns for oxidation by thermally activated persulfate and alkaline hydrolysis (17 ± 2 and 13.0 ± 0.8, respectively) vs reductive dechlorination by Fe(0) (8 ± 2) establish a base to identify and quantify these CF degradation mechanisms in the field.

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

“…4, Rodríguez-Fernández et al, 2018a, 2018b than to CF oxidation or alkaline hydrolysis (OX and AH, Fig. 4, Torrentó et al, 2017), in accordance with the redox conditions ( Fig. 1) and with the presence of DCM indicative of hydrogenolysis (Fig.…”

“…4) or with CF alkaline hydrolysis (AH, Fig. 4) (ANCOVA, p N 0.05) (Torrentó et al, 2017). CF oxidation in the pit source's influence area might be supported by an Eh evolution from Fe-controlled conditions in 2006 and 2007 towards more oxidising conditions in the latter campaigns (Fig.…”

“…Abiotic reactions like oxidation (e.g. by persulfate) and CF alkaline hydrolysis have been proved as efficient CF remediation strategies (Torrentó et al, , 2017. Under anoxic conditions, direct or indirect "hydrolytic reduction" of CF and cometabolic hydrogenolysis are described, the latter being associated to methanogens, fermenting bacteria and sulfate reducers (Cappelletti et al, 2012).…”

“…These studies are mainly based on carbon isotope measurements to confirm CT degradation in the non-saturated zone (Kirtland et al, 2003), to characterize the very depleted δ 13 C of CT and CF industrially produced from methane (Nijenhuis et al, 2013), to confirm CF degradation, along with concentration data, in a multi-contaminant polluted aquifer (Hunkeler et al, 2005) and/or to evaluate CF remediation treatment by alkaline hydrolysis in Òdena site (NE Spain) . New methods for chlorine isotope analyses of CMs and reference Λ values have only became available recently (Heckel et al, 2017a(Heckel et al, , 2017bTorrentó et al, 2017;Rodríguez-Fernández et al, 2018a, 2018b. Thus, there are not previous field site studies testing 2D-CSIA approach potential to identify CMs degradation pathways in complex fractured systems with multi-contaminant spills like that of Òdena.…”

Metagenomic- and cultivation-based exploration of anaerobic chloroform biotransformation in hypersaline sediments as natural source of chloromethanes

“…The enzymes responsible for reductive dehalogenation in OHRB are mainly corrinoid cofactor dependent reductive dehalogenases (RDases) such as a CF RDase (CfrA) identified from Dehalobacter -containing microbial consortia (39). CF can also be abiotically dechlorinated under anoxic conditions via hydrogenolysis to DCM, or via reductive elimination to CH 4 (40–42).…”