Adsorption of Pb 2+ , Cd 2+ and Sr 2+ ions onto natural and acid-activated sepiolites Slavica Lazarević a , Ivona Janković-Častvan a , Dušan Jovanović b , Slobodan Milonjić c , Djordje Janaćković a , Rada Petrović a, ⁎ a Faculty of Technology and Metallurgy, University of Belgrade, Karnegijeva 4, 11000 Belgrade, Serbia b Institute of Chemistry, Technology and Metallurgy, Njegoševa 12, Belgrade, Serbia c The Vinča Institute of Nuclear Sciences, POB 522, 11001 Belgrade, Serbia Received 14 July 2006; received in revised form 9 November 2006; accepted 20 November 2006 Available online 3 January 2007 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+ N Cd 2+ N 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. © 2006 Elsevier B.V. All rights reserved. Keywords: Sepiolite; Acid treatment; Adsorption; Heavy metals; Sr 2+ ion 1. Introduction Sepiolite (Si 12 O 30 Mg 8 (OH) 4 (H 2 O) 4 ·8H 2 O) is a nat- ural, fibrous clay mineral with fine microporous chan- nels of dimensions 0.37 × 1.06 nm running parallel to the length of the fibers. The size of the fibers is variable, but an average of 800 × 25 × 4 nm is typical, thus resulting in Applied Clay Science 37 (2007) 47 – 57 www.elsevier.com/locate/clay ⁎ Corresponding author. Tel.: +381 11 3370489; fax: +381 11 3370387. E-mail address: radaab@tmf.bg.ac.yu (R. Petrović). 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2006.11.008