title: Electrolyte ion effects on Cd2+ binding at Al2O3 surface: specific synergism versus bulk effects.
abstract: Cd(2+) binding on gamma-Al(2)O(3) was studied in the presence of the common electrolyte ions Mg(2+), SO(4)(2-), 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 gamma-Al(2)O(3) is modulated by the electrolyte ions in a complex manner. At high electrolyte concentration, Cd(2+) uptake by gamma-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 [triple bond]SO(-) surface sites of gamma-Al(2)O(3). At low concentration of electrolyte ions, Cd(2+) uptake by gamma-Al(2)O(3) can be enhanced due to synergistic co-adsorption of Cd(2+) and electrolyte anions, particularly SO(4)(2-) and to a lesser extent NO(3)(-). The theoretical analysis shows that this co-adsorption is due to formation of ternary surface species ([triple bond]SOH(2)SO(4)Cd) and ([triple bond]SOH(2)NO(3)Cd) which enhance Cd-uptake at pH values well below the point of zero charge of the gamma-Al(2)O(3).
authors.affiliation: Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, 30100 Agrinio, Greece.
authors.forename: Chariklia
authors.initials: C
authors.lastname: Kosma
authors.forename: Georgia
authors.initials: G
authors.lastname: Balomenou
authors.forename: George
authors.initials: G
authors.lastname: Salahas
authors.forename: Yiannis
authors.initials: Y
authors.lastname: Deligiannakis