Air-soil and air-water exchange of chiral pesticides

• The enantiomers of chiral pesticides are often metabolized at different rates in soil and water, leading to non-racemic residues. This paper reviews enantioselective metabolism of organochlorine pesticides (OCPs) in soil and water, and the use of enantiomers to follow transport and fate processes. Residues of chiral OCPs and their metabolites are frequently non-racemic in soil, although exceptions occur in which the OCPs are racemic. In soils where enantioselective degradation and/or metabolite formation has taken place, some OCPs usually show the same degradation preference; e.g. depletion of (+)trans-chlordane (TC) and (-)cis-chlordane (TC), and enrichment of the metabolite (+)heptachlor exo-epoxide (HEPX). The selectivity is ambivalent for other chemicals; preferential loss of either (+) or (-)o,p'-DDT and enrichment of either (+) or (-)oxychlordane (OXY) occurs in different soils.Non-racemic OCPs are found in air samples collected above soil which contains non-racemic residues. The enantiomer profiles of chlordanes in ambient air suggests that most chlordane in northern Alabama air comes from racemic sources (e.g. termiticide emissions), whereas a mixture of racemic and non-racemic (volatilization from soil) sources supplies chlordane to air in the Great Lakes region. Chlordanes and heptachlor exo-epoxide (HEPX) are also non-racemic in arctic air, probably the result of soil emissions from lower latitudes.The (+) enantiomer of alpha-hexachlorocyclohexane (alpha-HCH) is preferentially metabolized in the Arctic Ocean, arctic lakes and watersheds, the North American Great Lakes and the Baltic Sea. In some marine regions (Bering and Chukchi seas, parts of the North Sea) the preference is reversed and (-)alpha-HCH is depleted. Volatilization from seas and large lakes can be traced by the appearance of non-racemic alpha-HCH in the air boundary layer above the water. Estimates of microbial degradation rates for alpha-HCH in the eastern Arctic Ocean and an arctic lake have been made from the enantiomer fractions (EFs) and mass balance in the water column. Apparent pseudo first-order rate constants in the eastern Arctic Ocean are 0.12 y(-1) for (+)alpha-HCH 0.030 y(-1) for (-)alpha-HCH, and 0.037 y(-1) for achiral gamma-HCH. These rate constants are 3-10 times greater than those for basic hydrolysis in seawater. Microbial breakdown may compete with advective outflow for long-term removal of HCHs from the Arctic Ocean. Rate constants estimated for the arctic lake are about 3-8 times greater than those in the ocean.

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