Some models for the adsorption kinetics of pesticides in soil.

Three models describing adsorption-desorption kinetics of pesticides in soil, that could be incorporated into computer programs on pesticide movement in soil, were discussed, The first model involved single first-order rate equations for adsorption and desorption. Results from an analytical and a numerical solution for local equilibration were compared. Concentration-time relationships for the solution and absorbed phases were calculated for different rate constants, initial conditions, and partition ratios at equilibrium. The second model described simultaneous adsorption-desorption equilibration with two mechanisms, both with their own rate constants. After a comparatively fast equilibration with the first mechanism, there was a gradual increase in extent of overall-adsorption, accompanied with a shift to greater amounts adsorbed by the second mechanism. With the thrid model, adsorption equilibration occurred by diffusion into a stagnant region. With diffusion distances ranging from 0.1 to 4.0 cm, the time needed for approach to adsorption equilibrium varied from about 0.25 days to about one year. Some of the possibilities of these models were discussed considering published experimental results.