Effect of soil pH and organic matter on desorption hysteresis of chlorimuron-ethyl in two typical Chinese soils

Purpose Chlorimuron-ethyl is a sulfonylurea herbicide widely used to control many annual broadleaf weeds, and the residue for a long period posed a heavy hazard to rotational crops or vegetables. Knowledge about effect of soil pH and organic matter on desorption processes of chlorimuron-ethyl remains obscure. In this study, desorption behavior of chlorimuron-ethyl as a function of soil pH and organic matter was investigated. Materials and methods Two typical soils in northeastern China, brown earth (luvisols) and black soil (phaeozem), were used in this study. Soils were treated with peroxide to remove organic matter. Desorption kinetics of chlorimuron-ethyl in the original and H(2)O(2)-treated soils were simulated. The adsorption and desorption experiments were conducted at three different pH values with the original soils using batch equilibration methods. For the soils after having removed organic matter, the lower concentrations of adsorption were not used for desorption study because the concentrations expected were lower than the limit of detection of the method. Results and discussion The pseudo-second-order equation could well describe the desorption kinetics and the desorption velocity increased significantly from the soils after the removal of organic matter. The desorption data were well fitted with the Freundlich isotherm with high regression coefficients (R(2)> 0.97). Besides, the apparent hysteresis was observed for the desorption of chlorimuron-ethyl from the two soils, and the desorption from black soil was weaker than that from brown earth due to the higher content of organic matter and clay, and lower proportion of sand. The desorption of chlorimuron-ethyl from both soils became more difficult with an increase in the initial concentrations as a result of the gradient effect of concentrations. The degree of hysteresis increased as soil pH increased due to the strengthened binding energy of adsorption interactions. Conclusions The desorption of chlorimuron-ethyl from brown earth and black soil was significantly hysteretic, and the degree of hysteresis was greater for black soil than for brown earth. The increase of soil pH would enhance the retention of chlorimuron-ethyl and thus effectively retard chlorimuron-ethyl from entering the aqueous phase. The method of removing organic matter by H(2)O(2) treatment was not fit for researching effect of organic matter on the desorption of chlorimuron-ethyl from original soils.