Copper Sorption on Kaolinite

Abstract

<p>In order to improve our ability to predict sorption reactions, the rates of copper sorption on kaolinite were measured to determine the number and type of mechanisms responsible for trace metal sorption on clay minerals. From a theoretical consideration of Cu⁺² chemistry and kaolinite surface properties, it is probable that copper sorption is controlled by several mechanisms, with different sorption rates. These mechanisms include nonspecific coulombic attraction, coordination to one or two surface sites, induced surface precipitation, structural penetration, and sorption induced by kaolinite agglomeration. Cu⁺² sorption on kaolinite, in a 0.05 M KNO₃ medium at pH 6.2, was followed with a specific ion electrode for up to 2.5 weeks. The kinetic data revealed an instantaneous sorption, occurring within the first 1 to 5 seconds, followed by four concurrent first-order reactions. The mean rate constants of the first-order reactions are 0.7 ± 0.7, (8 ± 4) * 10⁻³, (14 ± 6) * 10⁻⁴, and (9 ± 6) * 10⁻⁵ (min.⁻¹).</p> <p>In order to characterize and identify the apparent mechanisms resolved by kinetics, experiments were designed to test the properties of Cu sorbed on kaolinite. These tests included (1)varying the salt concentration (2)changing the pH, (3)measuring the H⁺/Cu⁺² exchange ratio, and (4)desorbing Cu⁺² with water, Sr⁺², ⁶⁴Cu⁺², nitric acid and cyanide. The results of these tests indicate that the instantaneous sorption is caused by complexation to surface sites, with a small amount of nonspecific sorption. The first-order reactions are attributed to surface induced precipitation, coordination to two surface sites, and Cu fixation in the kaolinite structure. Copper sorption induced by centrifuging suggests that when clays are agglomerated, as in a clay floc, metal sorption may be enhanced. Sorption mechanisms observed on kaolinite may be applied to other clays, if differences in surface properties are taken into consideration.</p>