Adsorption-Desorption Behavior of Thiram onto Humic Acid

摘要

The adsorption/desorption behavior of pure thiram (Thi-P) and formulated thiram (Thi-F) onto commercial humic acids (HA) was studied using a batch equilibration procedure. Results of adsorption kinetic experiments sfiowed that thiram adsorption is a fast process since 85% of the equilibrium concentration is reached within two hours. Experimental K_D values between 0.110 to 0.210 L g~(-1) were obtained for the adsorption of both Thi-P and Thi-F onto HA, suggesting that thiram is strongly sorted by humic acids. In general, for both Thi-P and Thi-F, the lower the initial thiram concentration, the stronger is its adsorption (higher K_D and percentage adsorption values). The adsorption isotherms were found to match the BET model. The results show that thiram adsorption onto condensed humic acids cannot be explained only in terms of specific interactions, such as those identified in studies of adsorption of thiram with humic acids in solution. The comparison of sorption and desorption results allowed the observation of hysteresis phenomena. Desorption K_D values were consistently higher than those for adsorption at the same equilibrium concentration. Hysteresis was lower for the formulated thiram suggesting that adsorption is more reversible in the presence of the formulation components turning the pesticide more susceptible to be leached.