Abiotic hydrolysis of pesticides in the aquatic environment.
Hydrolysis of a pesticide is basically a reaction with a water molecule involving specific catalysis by proton or hydroxide, and sometimes inorganic ions such as phosphate ion, present in the aquatic environment that play a role in general acid-base catalysis. In this review, the basic profiles of hydrolysis such as pH and temperature dependencies are clarified for each class of pesticides together with typical reaction mechanisms. Although these hydrolytic profiles depend on the chemical structure and functional group(s) of a pesticide molecule, they are not always consistent within a chemical class of pesticides. For example, organophosphorus pesticides are primarily susceptible to alkaline hydrolysis with less acidic catalysis, but some of phosphorodithioates are found to be acid labile. In the case of carbamates, the pKa value of a leaving group is known to control their hydrolysis mechanism, whether BAC2 or E1cB. As one of the predictive approaches, the linear free-energy relationship has been successfully applied to hydrolysis of a series of organophosphorus and carbamate pesticides under conditions in which the reaction mechanism does not change. However, it still seems advantageous to estimate a priori hydrolytic profiles of pesticides, either because there is insufficient precision in the methodology or the chemical class is limited, or because of pH and temperature dependencies of hydrolysis. Therefore, it would still be practical, for the present, when investigating abiotic hydrolysis of a new pesticide that a laboratory study be effectively designed on the basis of accumulated knowledge of hydrolytic profiles for the essential chemical structure and functional groups and conducted to obtain pH- and temperature-rate profiles. Various instrumental techniques have been applied to chemical identification of degradates, leading to clarification of the reaction mechanisms involved, but greater use of computational methods such as ab initio and semiempirical molecular orbital calculations would be highly recommended for better understanding at a molecular level by considering the solvent effect (hydration). The chemical identification of degradates is usually cumbersome and challenging, especially when these are unstable species. The recent progress of LC-MS allows unstable or polar degradates to be identified more efficiently, and such knowledge will help researchers to hydrolytic processes more easily understand. Although testing guidelines being harmonized throughout the world afford valuable information on the basic profiles of hydrolysis, recent investigations on interactions of pesticides with dissolved organic matter and catalytic or inhibitive effects caused by metal ions, metal oxides, and clay seem to raise the question as to what degree laboratory and field data differ and how laboratory data can be more precisely extrapolated to field data. Moreover, pesticides are usually applied as a suitable formulation, and thus the effects of surfactants and other formulation reagents on hydrolysis should be examined in more detail. To assess the fate and impact of pesticides and their degradates in real aquatic environments, these concerns should be further examined using the various analytical techniques together with simulation models.
Sumitomo Chemical Co., Ltd., Environmental Health Science Laboratory, 2-1 Takatsukasa 4-chome, Takarazuka, Hyogo 665-8555, Japan.
Toshiyuki
T
Katagi
D004784:Environmental Monitoring / Q000379:methods; D006801:Humans; D006868:Hydrolysis; D007700:Kinetics; D010575:Pesticides / Q000032:analysis* / Q000737:chemistry; D014874:Water Pollutants, Chemical / Q000032:analysis*
D010575:Pesticides; D014874:Water Pollutants, Chemical