Sorption experiments with malathion and metalaxyl and selected minerals were conducted and analyzed by achiral and chiral HPLC.  Chiral analysis of sorption experiments with racemic malathion indicated nonenantioselective sorption, resulting in racemic enantiomeric fractions (EFs) for all sorbents.  However, sorption of the individual enantiomers of malathion, R-(+)- and S-(-)-malathion, resulted in enantiomerization to racemic EFs after contact with the sorbents.  Construction of isotherms revealed that the malathion racemate and enantiomers have different isotherm shapes, possibly indicating different sorption mechanisms.  Sorption of racemic metalaxyl was also found to be nonenantioselective with racemic EFs for all sorbents.  However sorption of metalaxyl-M (consisting of 97% R-(-)-metalaxyl and 3% S-(+)-metalaxyl) resulted in an increase of EF with bentonite and montmorillonite indicating the possibility of enantioselective sorption.  Although enantiomerization of chiral pesticides has been observed in aprotic solvents, this research presents the first evidence of enantiomerization during sorption for two current-use chiral pesticides.

Sorption and enantiomerization of malathion and metalaxyl by minerals

248th National Meeting (2014)

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