NMR Investigation of the Behavior of an Organothiophosphate Pesticide, Methyl Parathion, Sorbed on Clays

The decomposition of methyl parathion (an organothiophosphate pesticide) sorbed on partially hydrated kaolin and montmorillonite clays (in Ca²⁺, Cu²⁺, Zn²⁺, and Al³⁺ forms) at high concentration (typically 1−10 wt %) has been examined by nuclear magnetic resonance (NMR), using solid-state ³¹P NMR (based on magic-angle spinning and cross polarization or direct polarization) and liquid-state ³¹P NMR of DMSO and acetone extracts. The results indicate that methyl parathion is initially physisorbed, appearing by solid-state ³¹P NMR to exhibit substantial molecular-level motion. The signals due to unreacted methyl parathion diminish and are replaced by new ³¹P NMR peaks resulting from hydrolysis, isomerization, and oxidation reactions over periods ranging from hours to years. ³¹P NMR characteristics indicate that these decomposition products are much more tightly bound to the clay than is methyl parathion. Methyl parathion decomposition is most effectively catalyzed by partially hydrated Cu(II)- and Al-montmorillonites (but with different product distributions); Ca-montmorillonite and kaolin were least effective.