Adsorption of Pb2+, Cd2+ and Sr2+ ions onto natural and acid-activated sepiolites
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The adsorption mechanisms of Pb2+, Cd2+ and Sr2+ ions on natural sepiolite and the influence of acid treatment on the adsorption capacity of sepiolite were investigated. The point of zero charge, pHpzc, is 7.4 ± 0.1 for natural sepiolite and 6.9 ± 0.1 for partially acid-activated sepiolite. The shift of the pHpzc of sepiolites toward lower pH values in solutions of Pb2+ and Cd2+ ions indicates that specific adsorption of these cations on natural and acid-activated sepiolites occurred, which was more pronounced for Pb2+ ions than for Cd2+ ions. There was no shift of the pHpzc in solution containing Sr2+ ions, suggesting that specific adsorption of this cation did not occur. The affinity for ion exchange with the Mg2+ ions from the sepiolite structure was the highest for Pb2+ ions, then Cd2+, whereas the affinity for Sr2+ ions was negligible. The adsorption isotherms suggest that the sequence of the efficiency of the sepiolites adsorption is Pb2+ > Cd2+ > Sr2+. The retention of Pb2+ and ...Cd2+ ions by sepiolite occurs dominantly by specific adsorption and exchange of Mg2+ 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 Sr2+ 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 Mg2+ ions available for ion exchange with Pb2+, Cd2+ and Sr2+ 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.
Keywords:Sepiolite / Acid treatment / Adsorption / Heavy metals / Sr2+ ion
Source:Applied Clay Science, 2007, 37, 1-2, 47-57