ENVIRONMENTAL BEHAVIOR OF ACETAMIDE PESTICIDE STEREOISOMERS .2. STEREOSELECTIVE AND ENANTIOSELECTIVE DEGRADATION IN SEWAGE-SLUDGE AND SOIL

The degradation of five acetamide pesticides (alachlor, acetochlor, metalaxyl, metolachlor and dimethenamid) with different types of stereoisomerism (axial- and/or C-chirality) was studied in soil and sewage sludge using chiral high-resolution gas chromatography/mass spectrometry (HRGC/MS). Overall degradation rates in both media were in the order of alachlor > acetochlor > dimethenamid > metolachlor > metalaxyl. The results indicated the degradation of metalaxyl, metolachlor, and dimethenamid to be stereo- and/or enantioselective, particularily with respect to C-chirality. For acetochlor (axial-chiral), the results were inconclusive due to interference in the detection of this compound. Metalaxyl showed different enantioselectivity in the two media with the 1' S-(+)-enantiomer faster degraded in sewage sludge and the fungicidal more active 1' R(-)-enantiomer faster degraded in soil. alpha-Hexachlorocyclohexane (alpha-HCH) and its enantioselective degradation were used as an internal control to confirm biotic action in the sewage sludge. Surface water (lakes, drainage canal) and rain contained detectable quantities (2-120 ng/L) of some of these pesticides. The peak area ratios in all but one sample (drainage canal) were close to those of the technical products, indicating little if any biological degradation.