NMR Investigation of the Behavior of an Organothiophosphate Pesticide, Chlorpyrifos, Sorbed on Montmorillonite Clays

Abstract

Phosphorus-31 nuclear magnetic resonance (31P NMR) was used to explore the decomposition of chlorpyrifos (an organothiophosphate pesticide) sorbed at high concentration (typically 2-10 wt %) on partially hydrated montmorillonite clays in four different cation-exchanged forms (Ca2+, Cu2+, Zn2+, and Al3+). Solid-state 31P NMR (using magic-angle spinning and cross polarization or direct polarization) and liquid-solution 31P NMR of DMSO and acetone extracts indicate that chlorpyrifos is initially physisorbed, appearing by solid-state 31P NMR to exhibit significant motion on the molecular level, which results in almost liquidlike solid-state spectra. Over periods ranging from hours to years, the signals due to unreacted chlorpyrifos diminish and are replaced by new 31P NMR peaks resulting from hydrolysis, isomerization, mineralization, and oxidation reactions. The 31P NMR signal characteristics indicate that these decomposition products are much more tightly bound to the clay than is chlorpyrifos. Partially hydrated Cu(II)- and Al-montmorillonites most effectively catalyzed chlorpyrifos decomposition (but with different product distributions); Ca-montmorillonites (and, as previously shown, kaolin) were least effective. Solid-state 13C and 27Al NMR spectra were less useful for following the decomposition of chlorpyrifos than those obtained by 31P NMR. Pesticide loading levels (1-10% w/w) that are very much higher than those typically found in the environment were used to facilitate 31P NMR detection of less-than-dominant decomposition species.