Sorption of organophosphorus insecticides

Sorption of organophosphorus insecticides to soil particles depends primarily on compound hydrophobicity and the fraction of natural organic matter (NOM) in the soil (19, 20). Mineral phases appear to exert more influence on sorption processes for organophosphorus insecticides than for more extensively investigated hydrophobic organic compounds (e.g., DDT). The sorption of most hydrophobic organic compounds is dominated by NOM when the fraction of soil organic matter, exceeds 0.002 (21). For... 

Felsot, A. and Dahm, P.A. Sorption of organophosphorus and carbamate insecticides by soil, J. Agric. Food Chem., 27 (3) 557-563, 1979. 

Formulation and initial placement influence the susceptibility of organophosphorus insecticides to transport in surface runoff, as well as their degradation by abiotic and microbial processes. Formulation affects the kinetics of insecticide release into soil water and overland flow, as well as sorption to soil solids and plant surfaces. Spray adjuvants affect initial placement by influencing the amount of insecticide depositing on foliar and soil surfaces. Initial placement determines the relative importance of such processes as volatilization, photolysis, biodegradation, and leaching out of the zone of interaction with overland flow. 

Sorption to soil solids and plant cuticular material represents an important process influencing the chemodynamic behavior of insecticides, including their transport in surface runoff Sorption phenomena affect the volatilization, hydrolysis, photolysis and microbial transformation of organophosphorus insecticides. Furthermore, species sorbed to soil particles are transported by erosion processes rather than as solutes in the water phase. Sorption to foliar surfaces reduces the amount of pesticide mobilized by washoff. 

Several other aspects of organophosphorus insecticide sorption require further study. Although sorption-desorption hysteresis (20), aging and bound residue formation (30) have been noted, the molecular mechanisms responsible for these phenomena have not been elucidated. The kinetics of organophosphorus insecticide release from soil particles including the effects of biosurfactants (31) have not been extensively investigated, nor have the effects of formulation ingredients on sorption processes (32). 

The effect of formulation and spray adjuvants on insecticide efficacy has received considerable attention from the pesticide industry. However, few detailed mechanistic studies on the role these additives play in environmental fate processes have appeared in the open literature. Application of laboratory-derived process information to field scenarios is hindered by the fact that most laboratory investigations have used technically pure (unformulated) organophosphorus insecticides. Including the effects of formulation ingredients on such processes as volatilization and sorption to soil solids would allow laboratory studies to better predict the environmental behavior of these compounds. 

During transport with surface runoff, organophosphorus insecticides redistribute themselves between the dissolved, colloidal and suspended particle phases. Such phase redistribution during overland transport has not been investigated and the common assumption of phase equilibrium at the field outlet has not been tested. The validation of physically based numerical models of pesticide transport in surface runoff will require careful laboratory and field experimentation that includes the effects of infiltration and sorption (77). 

While the occurrence of most processes affecting organophosphorus insecticide concentrations in surface runoff are well documented, many are relatively poorly understood on a mechanistic level. Key processes requiring further study include sorption to plant and soil surfaces, and transfer from soil water into overland flow. Sorption to foliar surfaces determines the amount of applied insecticide available for washoff onto the soil surface or into surface runoff The kinetics of desorption from plant cuticular materials and the effect of adjuvants on foliar sorption processes need additional research. 

---