Adsorption and Desorption Isotherms of the Nonpolar Fungicide Chlorothalonil in a Range of Temperate Maritime Agricultural Soils

Abstract

A batch equilibrium experiment was conducted to determine the adsorption and desorption isotherms of chlorothalonil for a range of agricultural soils in Ireland. The sorption isotherms in tillage soils were described by the Freundlich model in a nonlinear form while in the grassland soil, the adsorption was almost linear. The experimental sorption data fit the Freundlich (R2 > 0.99) and the linear (R2 > 0.99) model very well. Chlorothalonil exhibited fast initial adsorption within the first hour until steady state, after which the sorption potential decreased and varied by about 3 % up to 10 h. Desorption equilibrium took twice the time needed for adsorption. The adsorption of chlorothalonil onto the soils studied was strong and the experimental Freundlich adsorption coefficients (Kf) ranged from 17.74 to 78.19 (mg1 − 1/n kg−1) (L)1/n, and these were correlated with cation exchange capacity and organic carbon content. All tillage soils exhibited L-type isotherm, whereas Elton grassland soil showed near C-type (linear) isotherm, probably due to the highest organic carbon content among other soil. Desorption process revealed hysteresis with the Freundlich desorption coefficients being greater than for adsorption, meaning that not all chlorothalonil adsorbed could be easily desorbed. Only 3 to 8 % was desorbed in the single desorption step during the batch equilibrium experiment. Calculated Koc values showed that chlorothalonil has slight to low mobility in the soils studied associated with high adsorption, and hence may constitute a greater risk to surface waters by runoff than to ground waters by leaching.