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

A systematic investigation of environmentally relevant transformation processes of alpha-hexachlorocyclohexane (alpha-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 (epsilon(C)) for the chemical transformations of alpha-HCH via direct photolysis, indirect photolysis (UV/H2O2), hydrolysis, electro-reduction or reduction by Fe-0 were quantified and compared to those previously published for biodegradation. Hydrogen abstraction by hydroxyl radicals generated by UV/H2O2 led to epsilon(C) of -1.9 +/- 0.2 parts per thousand with an apparent kinetic carbon isotope effect (AKIE(C)) of 1.012 +/- 0.001. Dehydrochlorination by alkaline hydrolysis yielded epsilon(C) of -7.6 +/- 0.4 parts per thousand with AKIE(C) of 1.048 +/- 0.003. Dechlorination either by homolytic bond cleavage in direct photolysis (epsilon(C) = 2.8 +/- 0.2 parts per thousand) or single-electron transfer in electro-reduction (epsilon(C) = -3.8 +/- 0.4%,) corresponded to AKIE(C) of 1.017 +/- 0.001 and 1.023 +/- 0.003, respectively. Dichloroelimination catalyzed by Fe-0 via two-electron transfers resulted in epsilon(C) of -4.9 +/- 0.1 parts per thousand. AKIE(C) 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 alpha-HCH was observed in chemical reactions, which might be used to discriminate chemical and biological in situ transformations. (C) 2014 Elsevier B.V. All rights reserved.