Study of Sorption and Desorption of Oxadiargyl and Thiobencarb in Calcareous Paddy Soils of Mazandaran Province

Herbicide persistence is somewhat favorable to control unwanted plants in the soil. Herbicide residues, however, may be a major concern for human health and the environment. Adsorption on soil particles and degredation, among different procesess, have important role on bioactivity and persistence of herbicides in the soil and other natural systems. It is, therefore, important to investigate herbicide adsorption from controlling herbicides and movement in the soil points of view. Most of the paddy soils are calcareous in the Mazandaran province and there is no complete information available about herbicide interaction with soils. Study of herbicides' behavior is necessary to obtain data needed for risk assessment modeling, which has been rarely conducted in Iran. To meet this purpose, adsorption and desorption isotherms of thiobencarb and oxadiargyl were determined for Dashtnaz and Gharakhil soils. Adsorption experiments of the herbicides were done based on batch method. Concentrations of the herbicides in the extract were analyzed using GC-MS. Adsorbed herbicide was calculated by subtracting the initial concentration and the equilibrium concentration. Desorption isotherms of the herbicides were measured in four successive steps, immediately after adsorption experiment. Amount of herbicide retained by soil was calculated after each desorption step. Adsorption isotherms confirmed that both herbicides were strongly adsorbed on soil particles. Furthermore, desorption data indicated that after four successive desorption steps, less than 9% thiobencarb and 1% oxadiargyl were released. This denotes that electrolyte ions in solution cannot adequately compete with and replace adsorbed thiobencarb and oxadiargyl molecules. This would lead to a considerable hysteresis between adsorption and desorption isotherms as was observed experimentally. Overall, it was concluded that both herbicides are among non-persistent and immobile herbicides in the paddy soils. Key words: Adsorption, Desorption, Thiobencarb, Oxadiargyl, Water and Soil Contamination, Rice.