Enantioselective Degradation in Sediment and Aquatic Toxicity to Daphnia magna of the Herbicide Lactofen Enantiomers

Many pesticides in use are chiral compounds containing stereoisomers. However, the environmental behavior and fate of such compounds with respect to enantioselectivity so far has received little attention. In this study, the degradation of lactofen and its main metabolites (acifluorfen, an achiral compound; desethyl lactofen, a chiral compound) in sediment were investigated under laboratory conditions using enantioselective HPLC, and the enantioselectivities of individual enantiomers of lactofen and desethyl lactofen in acute toxicity to Daphnia magna were studied. The calculated LC50 values of S(+)-, rac-, and R-(-)-lactofen were 17.689, 4.308, and 0.378 mu g/mL, respectively, and the calculated LC50 values of S-(+)-, rac-, and R-(-)-desethyl lactofen were 21.327, 13.684, and 2.568 mu g/mL, respectively. Therefore, the acute toxicities of lactofen and desethyl lactofen enantiomers were enantioselective. In sediments, S-(+)-lactofen or S-(+)-desethyl lactofen was preferentially degraded, resulting in relative enrichment of the R-(-)-form. Lactofen and desethyl lactofen were both configurationally stable in sediment, showing no interconversion of S- to R-enantiomers or vice versa. Furthermore, the conversion of lactofen to desethyl lactofen proceeded with retention of configuration. These results for major differences in acute toxicity and degradation of the enantiomers may have some implications for better environmental and ecological risk assessment for chiral pesticides.