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The sorption of Zn2+, Ni2+, and Co2+ on ZnO was studied as a function of pH, temperature, and concentration of the metal cations. The extent of sorption was found to increase with the increase in pH and concentration and decrease with the increase in temperature. The potentiometric titrations data showed that the sorption mechanism of the transition metal cations changed from adsorption to precipitation with the increase in pH. The sorption and precipitation of the metal cations were explained with the help of a new equation derived from the law of mass action. FTIR studies revealed the formation of a double hydroxide of Zn and metal ions concerned on the surface of the solid. The dissolution studies of the ZnO were also employed to evaluate the sorption mechanism of the metal cations on the ZnO. The decrease in the extent of adsorption of the metal cations with temperature was due to an increase in dissolution of the ZnO.

, indicating that the mixed oxide was more selective as an exchanger towards Cd 2+ ions relative to its components, SiO 2 and Fe(OH) 3 . The sorption data fitted the linear forms of the Kurbatov and Langmuir adsorption equations. The sorption of metal cations was accompanied by the release of H + ions to the bulk phase. On average 1 mol H + was released for every cation sorbed. The values of the binding constants were used to estimate the apparent thermodynamic parameters, DH and DS. The phenomenon of enthalpy/entropy compensation showed that the adsorption of metal ions by the mixed oxide occurred typically through a cation-exchange mechanism.