Oxide Surface-Catalyzed Hydrolysis of Carboxylate Esters and Phosphorothioate Esters

Pathways and rates of organic pollutant transformation may be altered through interaction with the surfaces of oxides and other soil minerals. We examined oxide surface-catalyzed hydrolysis of representative carboxylate esters and phosphorothioate esters. Ester and hydrolysis product concentrations were monitored in acetate-buffered suspensions containing 10 g L oxide. Carboxylate esters found susceptible to catalysis in this work possess an auxiliary ligand donor group that, along with the carbonyl O group, form a chelate with surface-bound metal atoms. Esters possessing an O-donor auxiliary group (e.g., phenyl salicylate) are susceptible to catalysis by AlO, TiO, and FeOOH surfaces. Esters possessing a N-donor auxiliary group (e.g., phenyl picolinate) are no longer susceptible to catalysis by AlO, reflecting the poor affinity of N-donor groups for Al complexation. Methyl chlorpyrifos is the most susceptible of the three phosphorothioate esters to oxide-catalyzed hydrolysis; it possesses an auxiliary ligand donor group analogous to the carboxylate esters examined. Methyl parathion and ronnel, which do not possess a suitable auxiliary ligand donor group, are also susceptible to oxide-catalyzed hydrolysis, indicating that chelate formation is not required in order for surface catalysis to occur. The presence of methanol and acetonitrile cosolvents diminishes the catalytic effect. The widespread use of these cosolvents in fate studies may have caused rates of oxide and mineral surface-catalyzed hydrolysis reactions to be underestimated.