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Metal (Hydr)oxide Surface-Catalyzed Hydrolysis of Chlorpyrifos-Methyl, Chlorpyrifos-Methyl Oxon, and Paraoxon

U.S. Environmental Protection Agency, 960 College Station Road, Athens, GA 30605

Alan T. Stone^*

Dep. of Geography and Environmental Engineering, G.W.C. Whiting School of Engineering, Johns Hopkins Univ., Baltimore, MD 21218

*Corresponding author (dog_zats{at}jhuvms.hcf.jhu.edu).

ABSTRACT

This work is concerned with the susceptibility of pesticides and their transformation products to mineral-surface-catalyzed hydrolysis in soils. Experiments were performed in an aqueous medium containing pH buffer [2.0–5.0 mM acetate or 3-(N-morpholino) propane sulphonic acid (MOPS)], 10 mM NaCl, and 0 to 25% methanol. Addition of 10 g L^–1 TiO₂, -FeOOH, and Al₂O₃ catalyzes the hydrolysis of the thionate (P=S) and oxonate (P=O) forms of chlorpyrifosmethyl [O,O-dimethyl O-(3,5,6-trichloro-2-pyridinyl) phosphorothioate and O,O-dimethyl-O,3,5,6-trichloro-2-pyridyl phosphate]. Paraoxon (O,O-diethyl O-p-nitrophenyl phosphate) is also subject to surface-catalyzed hydrolysis, while zinophos (O,O-diethyl O-2-pyrazinyl phosphorothioate) is not. The effects of pH and the identity of the metal (hydr)oxide surface are discussed in light of three possible mechanisms of catalysis: (i) metal ion coordination of the parent ester through the thionate-S or oxonate-O to enhance the electrophilicity of the P site; (ii) metal ion coordination and induced deprotonation of water to create a reactive nucleophile; and (iii) metal ion coordination of the leaving group to facilitate its exit. Because different methanol concentrations were employed to ensure solubility, ester-to-ester comparisons must be made with caution. It is apparent, however, that surface-catalyzed hydrolysis reactions provide a unique opportunity to explore the reactivity properties of soil mineral surfaces.

This work was supported by the USEPA National Center for Environmental Research and Quality Assurance (Grant R81-8894).

Received for publication August 27, 1996.