Photodegradation of pesticides on plant and soil surfaces.

Photodegradation is an abiotic process in the dissipation of pesticides where molecular excitation by absorption of light energy results in various organic reactions, or reactive oxygen species such as OH*, O3, and 1O2 specifically or nonspecifically oxidize the functional groups in a pesticide molecule. In the case of soil photolysis, the heterogeneity of soil together with soil properties varying with meteorological conditions makes photolytic processes difficult to understand. In contrast to solution photolysis, where light is attenuated by solid particles, both absorption and emission profiles of a pesticide are modified through interaction with soil components such as adsorption to clay minerals or solubilization to humic substances. Diffusion of a pesticide molecule results in heterogeneous concentration in soil, and either steric constraint or photoinduced generation of reactive species under the limited mobility sometimes modifies degradation mechanisms. Extensive investigations of meteorological effects on soil moisture and temperature as well as development of an elaborate testing chamber controlling these factors seems to provide better conditions for researchers to examine the photodegradation of pesticides on soil under conditions similar to the real environment. However, the mechanistic analysis of photodegradation has just begun, and there still remain many issues to be clarified. For example, how photoprocesses affect the electronic states of pesticide molecules on soil or how the reactive oxygen species are generated on soil via interaction with clay minerals and humic substances should be investigated in greater detail. From this standpoint, the application of diffuse reflectance spectroscopy and usage or development of various probes to trap intermediate species is highly desired. Furthermore, only limited information is yet available on the reactions of pesticides on soil with atmospheric chemical species. For photodegradation on plants, the importance of an emission spectrum of the light source near its surface was clarified. Most photochemical information comes from photolysis in organic solvents or on glass surfaces and/or plant metabolism studies. Epicuticular waxes may be approximated by long-chain hydrocarbons as a very viscous liquid or solid, but the existing form of pesticide molecules in waxes is still obscure. Either coexistence of formulation agents or steric constraint in the rigid medium would cause a change of molecular excitation, deactivation, and photodegradation mechanisms, which should be further investigated to understand the dissipation profiles of a pesticide in or on crops in the field. A thin-layer system with a coat of epicuticular waxes extracted from leaves or isolated cuticles has been utilized as a model, but its application has been very limited. There appear to be gaps in our knowledge about the surface chemistry and photochemistry of pesticides in both rigid media and plant metabolism. Photodegradation studies, for example, by using these models to eliminate contribution from metabolic conversion as much as possible, should be extensively conducted in conjunction with wax chemistry, with the controlling factors being clarified. As with soil surfaces, the effects of atmospheric oxidants should also be investigated. Based on this knowledge, new methods of kinetic analysis or a device simulating the fate of pesticides on these surfaces could be more rationally developed. Concerning soil photolysis, detailed mechanistic analysis of the mobility and fate of pesticides together with volatilization from soil surfaces has been initiated and its spatial distribution with time has been simulated with reasonable precision on a laboratory scale. Although mechanistic analyses have been conducted on penetration of pesticides through cuticular waxes, its combination with photodegradation to simulate the real environment is awaiting further investigation.