Binary adsorption of nickel and zinc from aqueous solutions onto the Serbian natural clinoptilolite

Sorption of the Zn2+ and Ni2+ ions by the Na+-enriched natural clinoptilolite (Na-CLI) from aqueous solutions containing both Zn2+ and Ni2+ ions at a range of concentrations has been studied at 298, 308, and 318K. It was found that the sorption of a particular ion by Na-CLI increases with the increase in the initial concentration of the solution of that ion. At a given initial concentration, the sorption degree increases with temperature. Also, the sorption of a particular ion is greater when the initial solution contains a second ion than it is when the particular ion is present alone. The sorption kinetics is in accord with the pseudo-second-order reaction rate model. The values of the k(2) rate constant do not show a regular change with the initial Zn2+/Ni2+ concentrations probably because the presence of the second ion has a significant effect on the reaction rate. Thermodynamic study has shown that the sorption is exothermic, occurring spontaneously in the 298-318K range and with a positive entropy change.