Journal of Colloid and Interface Science 331 (2009) 263–274 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis Electrolyte ion effects on Cd 2+ binding at Al 2 O 3 surface: Specific synergism versus bulk effects Chariklia Kosma a , Georgia Balomenou a , George Salahas b , Yiannis Deligiannakis a,∗ a Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece b Inst. Educ. Technol., Dept. Green House Crops Floriculture, Mesolongi 30200, Greece article info abstract Article history: Received 2 August 2008 Accepted 6 November 2008 Available online 20 December 2008 Keywords: Electrolyte Cd 2+ Ternary species Synergistic Ionic strength FITEQL γ -Al 2 O 3 Cd 2+ binding on γ -Al 2 O 3 was studied in the presence of the common electrolyte ions Mg 2+ , SO 2− 4 , and NO − 3 at high and low concentrations. Direct measurements were performed for Cd 2+ as well as for electrolyte ion adsorption as a function of pH. The experimental data reveal that Cd 2+ binding on γ -Al 2 O 3 is modulated by the electrolyte ions in a complex manner. At high electrolyte concentration, Cd 2+ uptake by γ -Al 2 O 3 is inhibited. Theoretical analysis by a surface complexation model shows that this effect can be attributed partially to bulk, ionic strength, and effect of the electrolyte, but the most significant inhibition is due to direct competition between Mg 2+ and Cd 2+ ions for the ≡SO − surface sites of γ -Al 2 O 3 . At low concentration of electrolyte ions, Cd 2+ uptake by γ -Al 2 O 3 can be enhanced due to synergistic co-adsorption of Cd 2+ and electrolyte anions, particularly SO 2− 4 and to a lesser extent NO − 3 . The theoretical analysis shows that this co-adsorption is due to formation of ternary surface species (≡SOH 2 SO 4 Cd) and (≡SOH 2 NO 3 Cd) which enhance Cd-uptake at pH values well below the point of zero charge of the γ -Al 2 O 3 .  2008 Elsevier Inc. All rights reserved. 1. Introduction Interfacial chemistry of metal oxides and hydroxides is impor- tant because of their role in the environment and use in indus- trial processes [1]. Metal hydroxides are characterized by a pH- dependent surface charge as a result of the adsorption/desorption of protons [1,2]. Aluminum oxide is a common adsorbent in aque- ous electrolyte solutions [3]. One of its valuable properties is the presence of both acid and base active sites on the surface, which exert an inherent influence on sorption properties. Acid–base prop- erties of aluminum oxides have been extensively studied in the past (see [3,4] and references therein). Anthropogenic activities re- lease heavy metals into the environment, leading to contamination of the food chain and exposure of plants, animals, and humans. Among the released heavy metals, cadmium (Cd 2+ ) is of primary environmental concern [3,5]. The binding of simple inorganic electrolyte anions, NO − 3 or SO 2− 4 , onto colloidal substrates under different pH conditions has been extensively studied and reviewed in the literature. Typical experimental scenarios involve the addition of low to moderate concentrations of inorganic electrolytes to aqueous suspensions of oxides, with the amount of ion binding being determined usu- * Corresponding author. E-mail address: ideligia@cc.uoi.gr (Y. Deligiannakis). ally by either charge titration or electrokinetic methodologies [1,2, 6–8]. Although electrolyte ions are usually approximated as non- specifically-bound ions, they can be bound via specific, usually weak interactions with surface groups [9]. Thus, nonzero elec- trolyte ion affinities can be estimated from the analysis of the charging behavior of oxides with a surface using an appropriate surface complexation model [9]. More specifically, recent works on goethite showed that electrolyte ions, i.e., NO − 3 or SO 2− 4 , may inter- act with surfaces through the formation of inner sphere complexes as a function of loading, pH, and salt level [8]. Rahnemaie et al. have analyzed the role of Mg 2+ , as well as the binding of SO 2− 4 on goethite, based on H-binding data [10]. The H-binding data were analyzed by the so-called multisite complexation (MUSIC) charge distribution (CD) [10,11] approach to surface complexation. This analysis suggested that only inner sphere complexation for electrolyte anions was needed to fit the H-binding data and that allowing for outer sphere complexation did not improve the fit [9]. Wijnja and Schulthess [12] and Peak et al. [13] have studied SO 2− 4 interaction with goethite using in situ ATR-FTIR spectroscopy. Both studies [12,13] showed that sulfate ions adsorb as a combination of inner- and outer-sphere complexes. The inner-sphere complex was suggested to be a monodentate complex [12,13]. Peak et al. [13] have shown that inner- as well as outer-sphere complexes are present at pH < 6, while outer-sphere complexation determine SO 2− 4 adsorption at pH > 6 [13]. Rietra et al. [14] have studied in 0021-9797/$ – see front matter  2008 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2008.11.023