Modeling adsorption-desorption processes of Cu on montmorillonite and the effect of competitive adsorption with a cationic pesticide

Abstract

The effect of the ionic strength on adsorption of Cu on Ca-montmorillonite (SAz-1) was studied at concentrations ranging from 31 to 516 µM. An adsorption model was employed in the analysis of the data. When the background electrolyte was NaClO4 the ionic interchange was suppressed at 0.5 M, and Cu adsorption was taking place on edge sites, reaching a plateau at about 24 mmol/kg. A further increase in ionic strength did not make any effect on Cu adsorption, suggesting that the heavy metal was being adsorbed by inner sphere complexes on the edge sites. When the electrolyte used was NaCl the amounts of Cu adsorbed were reduced. The model predicted well the adsorption data by considering adsorption of CuCl⁺ species. Adsorption-desorption processes of Cu on Ca-montmorillonite in media of 0.01 and 0.1 M NaCl showed hysteresis. Model calculations predict the desorbed amounts fairly well. According to the model the hysteresis is mainly attributed to the heterogeneity of sites for the adsorption of Cu. The hysteresis arising from the planar sites is largely due to reduced competition for adsorption and enhancement in the magnitude of the surface potential. The presence of the cationic pesticide chlordimeform reduces strongly the sorption of the metal on the planar positions unlike the edge sites. However, Cu sorption increases on the clay treated previously with chlordimeform which was due to the opening of the clay platelets after some molecules of the pesticide are adsorbed, facilitating the subsequent penetration of the metal and its adsorption on planar positions. This cooperative adsorption was due to the fact that the loading of the pesticide on the clay was a very small amount of the CEC.