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

Abstract

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 m M acetate or 3‐( N ‐morpholino) propane sulphonic acid (MOPS)], 10 m M NaCl, and 0 to 25% methanol. Addition of 10 g L −1 TiO 2 , α‐FeOOH, and Al 2 O 3 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.