Enantioselective sorption of the chiral fungicide metalaxyl on soil from non-racemic aqueous solutions: Environmental implications

Abstract

Mechanisms governing the enantioselectivity of the processes that determine the behavior of chiral pol-lutants in the environment need to be better understood. Understanding these mechanisms should helpimprove predictions of the hazards and risks chiral compounds can pose to people and the environment.We report the results of batch sorption experiments indicating that the sorption of the chiral fungicidemetalaxyl on soil from non-racemic initial solutions was enantioselective. While from a racemic initialsolution the two enantiomers of metalaxyl were sorbed on the soil to the same extent, increasing the frac-tion of R-enantiomer in the initial solution led to enhanced sorption of this enantiomer and to reducedsorption of the S-enantiomer. Considering the shape of the sorption isotherms (S-type) and the sorp-tion behavior of model sorbents, we attributed this effect to molecular interactions between metalaxylenantiomer species at the sorbed state, where R-R metalaxyl interactions appeared to be more favor-able than R–S metalaxyl interactions. We discuss important environmental implications of the proposedmechanism, such as those related to the fact that the biological degradation of metalaxyl is known to bean enantioselective process that can yield non-racemic residues in soils shortly after application of thefungicide as a racemic mixture.