Adsorption equilibriums of principal herbicides on paddy soils in Japan

Herbicides used in paddy fields during the flooding season can easily cause pollution by run-off into rivers or by other routes. It is very important to know the adsorption characteristics that influence their fate in the soil. The adsorption equilibriums have often been expressed by Henry equations, and the values of equilibrium constant, Kd, are estimated from the adsorption constants, K(OC), based on organic carbon contents of soils. There is little information concerning the equilibrium values expressed by the Freundlich equations, and insufficient information on the actual concentration levels in the paddy field. Therefore, adsorption equilibriums of the five principal herbicides: esprocarb, mefenacet, pretilachlor, simetryn and thiobencarb, on five kinds of paddy soil in Japan were investigated. It was found that their equilibrium values were better expressed by the Freundlich equation for concentration levels for the paddy fields, and that the values for the adsorption coefficient, n, varied from 1.0 to 1.6. Values for the coefficient, k, were in the range of 29-420 mg(1 - 1/n) l(1/n)/kg-dry, and the values were poorly related to solubilities in water or to the octanol-water partition coefficients of the herbicides. For each herbicide, except for simetryn, the values of k among the soils differed by 2-3 times, and no correlation could be found with the organic carbon contents, specific surface areas, pH, cation exchange capacity or major minerals of the soils. The adsorption equilibriums calculated from the values of adsorption constant Kd by the values of K(OC) in the literature were found to be very different from the experimental equilibriums. From the experimental values of coefficient k and n of the Freundlich equation, the maximum runoff concentrations of the herbicides were preliminarily estimated by a simple equilibrium model.