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

摘要

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.