Multi-element (2H, 13C, 37Cl) isotope analysis to characterize reductive transformation of α-, β-, γ-, and δ-HCH isomers by cobalamin and Fe0 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 (²H, ¹³C, ³⁷Cl) stable isotope analysis to characterize the reductive dehalogenation mechanisms of HCH isomers using cobalamin (vitamin B_₁₂) reduced with Ti³⁺ and Fe⁰ nanoparticles. The isotopic fractionation of HCH isomers varied from -2.8 ± 0.5 to -7.0 ± 0.7 ‰ for carbon (ε_C), from not significant to -62.4 ± 5.2 ‰ for hydrogen (ε_H), and from -1.4 ± 0.2 to -4.7 ± 0.5 ‰ for chlorine (ε_Cl), respectively. Dual C-Cl isotopic fractionation values (Λ_C/Cl) for α-, β-, γ- and δ-HCH during the transformation by B₁₂ were determined to be 2.0 ± 0.2, 1.5 ± 0.2, 1.1 ± 0.1, and 1.9 ± 0.3, respectively. The Λ_C/Cl values of β- and δ-HCH in the reaction with Fe⁰ nanoparticles were found to be similar (1.9 ± 0.3 and 1.9 ± 0.2). However, the apparent kinetic isotope effect AKIE_C/AKIE_Cl 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.