Dual C-Cl Isotope Analysis for Characterizing the Reductive Dechlorination of α- and γ-Hexachlorocyclohexane by Two Strains and an Enrichment Culture.

Hexachlorocyclohexanes (HCHs) are persistent organic contaminants that threaten human health. Microbial reductive dehalogenation is one of the most important attenuation processes in contaminated environments. This study investigated carbon and chlorine isotope fractionation of α- and γ-HCH during the reductive dehalogenation by three anaerobic cultures. The presence of tetrachlorocyclohexene (TeCCH) indicated that reductive dichloroelimination was the first step of bond cleavage. Isotope enrichment factors (ε and ε) were derived from the transformation of γ-HCH (ε, from -4.0 ± 0.5 to -4.4 ± 0.6 ‰; ε, from -2.9 ± 0.4 to -3.3 ± 0.4 ‰) and α-HCH (ε, from -2.4 ± 0.2 to -3.0 ± 0.4 ‰; ε, from -1.4 ± 0.3 to -1.8 ± 0.2 ‰). During α-HCH transformation, no enantioselectivity was observed, and similar ε values were obtained for both enantiomers. The correlation of C and Cl fractionation (Λ = ΔδC/ΔδCl ≈ ε/ε) of γ-HCH (from 1.1 ± 0.3 to 1.2 ± 0.1) indicates similar bond cleavage during the reductive dichloroelimination by the three cultures, similar to α-HCH (1.7 ± 0.2 to 2.0 ± 0.3). The different isotope fractionation patterns during reductive dichloroelimination and dehydrochlorination indicates that dual-element stable isotope analysis can potentially be used to evaluate HCH transformation pathways at contaminated field sites.