Adsorption of Pb 2+, Cd 2+ and Sr 2+ ions onto natural and acid-activated sepiolites

Abstract

The adsorption mechanisms of Pb 2+, Cd 2+ and Sr 2+ ions on natural sepiolite and the influence of acid treatment on the adsorption capacity of sepiolite were investigated. The point of zero charge, pH pzc, is 7.4 ± 0.1 for natural sepiolite and 6.9 ± 0.1 for partially acid-activated sepiolite. The shift of the pH pzc of sepiolites toward lower pH values in solutions of Pb 2+ and Cd 2+ ions indicates that specific adsorption of these cations on natural and acid-activated sepiolites occurred, which was more pronounced for Pb 2+ ions than for Cd 2+ ions. There was no shift of the pH pzc in solution containing Sr 2+ ions, suggesting that specific adsorption of this cation did not occur. The affinity for ion exchange with the Mg 2+ ions from the sepiolite structure was the highest for Pb 2+ ions, then Cd 2+, whereas the affinity for Sr 2+ ions was negligible. The adsorption isotherms suggest that the sequence of the efficiency of the sepiolites adsorption is Pb 2+ > Cd 2+ > Sr 2+. The retention of Pb 2+ and Cd 2+ ions by sepiolite occurs dominantly by specific adsorption and exchange of Mg 2+ ions from the sepiolite structure. The concentration in the external outer sphere of the Stern layer by electrostatic forces is the dominant mechanism for the retention of Sr 2+ ions on the surface of sepiolites. Despite increases in the surface areas upon acid activation, improvements in the adsorption were not observed, as a result of the decreasing number of Mg–OH groups, as main centers for specific adsorption, and the number of Mg 2+ ions available for ion exchange with Pb 2+, Cd 2+ and Sr 2+ ions. The removal of Mg–OH groups from the sepiolite also resulted in a decrease in the point of zero charge and an increase of the relative surface acidity, from 2.6 ± 0.1 for the natural to 6.4 ± 0.1 for the acid-activated sepiolite.