Dual C-Cl isotope analysis for characterizing the anaerobic transformation of α, β, γ, and δ-hexachlorocyclohexane in contaminated aquifers

摘要

Hexachlorocyclohexanes (HCHs) are widespread and persistent environmental pollutants, which cause heavy contamination in soil, sediment and groundwater. An anaerobic consortium, which was enriched on beta-HCH using a soil sample from a contaminated area of a former pesticide factory, was capable to transform alpha, beta, gamma, and delta-HCH via tetrachlorocyclohexene isomers stoichiometrically to benzene and chlorobenzene. The carbon and chlorine isotope enrichment factors (epsilon(C) and epsilon(Cl)) of the dehalogenation of the four isomers ranged from -1.9 0.3 to-6.4 +/- 0.7 parts per thousand and from-1.6 +/- 0.2 to-3.2 +/- 0.6 parts per thousand, respectively, and the correlation of delta Cl-37 and delta C-13 (L values) of the four isomers ranged from 1.1 +/- 0.1 to 2.4 +/- 0.2. The evaluation of L and the apparent kinetic isotope effects (AKIE) for carbon and chlorine may lead to the hypothesis that the two eliminated chlorine atoms of alpha- and gamma-HCH were in axial positions, the same as for the beta-HCH conformer which has six chlorine atoms in axial positions after ring flip. The dichloroelimination of delta-HCH resulted in distinct AKIE and L values as one chlorine atom is in axial whereas the other chlorine atoms are in the equatorial positions. Significant chlorine and carbon isotope fractionations of HCH isomers were observed in the samples from a contaminated aquifer (Bitterfeld, Germany). The Cl-37/Cl-35 and C-13/C-12 isotope fractionation patterns of HCH isomers from laboratory experiments were used diagnostically in a model to characterize microbial dichloroelimination in the field study. The comparison of isotope fractionation patterns indicates that the transformation of HCH isomers at the field was mainly governed by microbial dichloroelimination transformation. (c) 2020 Elsevier Ltd. All rights reserved.