Review and evaluation of a chiral enrichment model for chlordane enantiomers in the environment

Abstract

Chlordane is an organochlorine pesticide that contains a mixture of components, some of which are chiral. Analysis of enantiomers can provide insights into the mechanisms of their sources, pathway, and fate. The objectives of this work were: (i) to review the published literature that reported chlordane enantiomer concentrations in various environmental compartments; (ii) to evaluate the applicability of a model developed by Hegeman and Laane (2002) for predicting the relative distribution of various chiral pesticides in the environment. This model predicts that enantiomer deviation increases in the following order: air, water, soil, molluscs, fish/birds, marine mammals, specific tissues, and enzymes; (iii) to determine the relative ranking of plants within the model. All data were converted into an enantiomer fraction and aggregated by environmental compartment using bootstrap averaging to compare to model predictions. Enantiomer distributions of cis-chlordane, trans-chlordane, oxychlordane, and heptachlor exo-epoxide were reported in soil, air, water/sediments, plants, and animals. Based on aggregated average estimates for enantiomer fraction, the deviation from racemic increased in the following order for cis-chlordane: water/sediments = lower trophic aquatic organisms < air < fish < soil < plants (all combined) < seal < polar bear < bird < whale; for trans-chlordane: fish < water/sediments < lower trophic aquatic organisms < air < soil < plants (all combined) < seal < whales < bird < polar bear; and for heptachlor exo-epoxide: plants (all combined) < fish < soil = whale < water/sediments < bird < air < seal < polar bear. Depending on the species used to represent marine mammals, the Spearman’s rank-order correlation coefficients of observed data against model predictions were + 0.75 or + 0.84 for cis-chlordane, + 0.41 or + 0.49 for trans-chlordane, and −0.59 or + 0.12 for heptachlor exo-epoxide. This suggests that the model is applicable for cis- and trans-chlordane, but not for heptachlor exo-epoxide. The fundamental mechanisms behind the observed enantiomeric enrichment are of eminent importance in understanding the enantioselective deviations.