Electrolyte ion effects on Cd 2+ binding at Al 2O 3 surface: Specific synergism versus bulk effects

Abstract

Cd 2+ binding on γ-Al 2O 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 2O 3 is modulated by the electrolyte ions in a complex manner. At high electrolyte concentration, Cd 2+ uptake by γ-Al 2O 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 2O 3. At low concentration of electrolyte ions, Cd 2+ uptake by γ-Al 2O 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 2SO 4Cd) and ( SOH 2NO 3Cd) which enhance Cd-uptake at pH values well below the point of zero charge of the γ-Al 2O 3.