Multi-element (H, C, Cl) isotope analysis to characterize reductive transformation of α-, β-, γ-, and δ-HCH isomers by cobalamin and Fe nanoparticles.

Hexachlorocyclohexane (HCH), a typical persistent organic pollutant, poses a serious threat to both human health and the environment. The degradability of HCH isomers (α, β, γ, and δ) varies significantly under anoxic aqueous conditions and the corresponding reductive transformation mechanisms remain elusive. This work employed multi-element (H, C, Cl) stable isotope analysis to characterize the reductive dehalogenation mechanisms of HCH isomers using cobalamin (vitamin B) reduced with Ti and Fe nanoparticles. The isotopic fractionation of HCH isomers varied from -2.8 ± 0.5 to -7.0 ± 0.7 ‰ for carbon (ε), from not significant to -62.4 ± 5.2 ‰ for hydrogen (ε), and from -1.4 ± 0.2 to -4.7 ± 0.5 ‰ for chlorine (ε), respectively. Dual C-Cl isotopic fractionation values (Λ) for α-, β-, γ- and δ-HCH during the transformation by B were determined to be 2.0 ± 0.2, 1.5 ± 0.2, 1.1 ± 0.1, and 1.9 ± 0.3, respectively. The Λ values of β- and δ-HCH in the reaction with Fe nanoparticles were found to be similar (1.9 ± 0.3 and 1.9 ± 0.2). However, the apparent kinetic isotope effect AKIE/AKIE values suggested that the bond cleavage mechanism of δ-HCH may differ from that of other isomers. The comparison of the angles θ by multi-element isotope plot showed a distinct differentiation between the pathways of anaerobic transformation of HCH isomers and aerobic pathways reported in the literature. Therefore, multi-element isotope analysis could offer a new perspective for characterizing the fate of HCH isomers.