Adsorption of Cd and Zn on montmorillonite in the presence of a cationic pesticide

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Abstract
The adsorption of Cd and Zn on a standard montmorillonite (SAZ-1) in the presence of the cationic pesticide chlordimeform, both in solution (simultaneous adsorption) and when the pesticide is previously adsorbed on the clay (successive adsorption) has been studied. The adsorption of Zn decreases when the chlordimeform concentration increases in both simultaneous and successive adsorptions, following the sequence: chlordimeform-free solutions > successive > simultaneous. In the adsorption of Cd the sequence is different: successive > chlordimeform-free solutions > simultaneous, indicating that a small amount of pesticide adsorbed favours Cd adsorption. In all cases, the apparent affinity of the metal for adsorption on montmorillonite, on the basis of distribution coefficients, KD, is higher for lower metal surface coverage, and decreases largely with the amount of the metal adsorbed. This indicates the existence of high affinity sites on the clay, probably those of variable charge edge regions. The contribution of edge and interlamellar positions to adsorption of Zn and Cd on SAZ-1 has been studied.