Compound specific stable isotope analysis (CSIA) to characterize transformation mechanisms of α-hexachlorocyclohexane

A systematic investigation of environmentally relevant transformation processes of alpha-hexachlorocyclohexane (α-HCH) was performed in order to explore the potential of compound specific stable isotope analysis (CSIA) to characterize reaction mechanisms. The carbon isotope enrichment factors (ɛC) for the chemical transformations of α-HCH via direct photolysis, indirect photolysis (UV/H2O2), hydrolysis, electro-reduction or reduction by Fe0 were quantified and compared to those previously published for biodegradation. Hydrogen abstraction by hydroxyl radicals generated by UV/H2O2 led to ɛC of −1.9 ± 0.2‰ with an apparent kinetic carbon isotope effect (AKIEC) of 1.012 ± 0.001. Dehydrochlorination by alkaline hydrolysis yielded ɛC of −7.6 ± 0.4‰ with AKIEC of 1.048 ± 0.003. Dechlorination either by homolytic bond cleavage in direct photolysis (ɛC = −2.8 ± 0.2‰) or single-electron transfer in electro-reduction (ɛC = −3.8 ± 0.4‰) corresponded to AKIEC of 1.017 ± 0.001 and 1.023 ± 0.003, respectively. Dichloroelimination catalyzed by Fe0 via two-electron transfers resulted in ɛC of −4.9 ± 0.1‰. AKIEC values assuming either a concerted or a stepwise mechanism were 1.030 ± 0.0006 and 1.015 ± 0.0003, respectively. Contrary to biodegradation, no enantioselectivity of α-HCH was observed in chemical reactions, which might be used to discriminate chemical and biological in situ transformations.