Electrolyte ion effects on Cd2+ binding at Al2O3 surface: Specific synergism versus bulk effects.

Cd2+ binding on γ-Al2O3 was studied in the presence of the common electrolyte ions Mg2+, SO2−4, and NO−3 at high and low concentrations. Direct measurements were performed for Cd2+ as well as for electrolyte ion adsorption as a function of pH. The experimental data reveal that Cd2+ binding on γ-Al2O3 is modulated by the electrolyte ions in a complex manner. At high electrolyte concentration, Cd2+ uptake by γ-Al2O3 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 Mg2+ and Cd2+ ions for the ≡SO− surface sites of γ-Al2O3. At low concentration of electrolyte ions, Cd2+ uptake by γ-Al2O3 can be enhanced due to synergistic co-adsorption of Cd2+ and electrolyte anions, particularly SO2−4 and to a lesser extent NO−3. The theoretical analysis shows that this co-adsorption is due to formation of ternary surface species (≡SOH2SO4Cd) and (≡SOH2NO3Cd) which enhance Cd-uptake at pH values well below the point of zero charge of the γ-Al2O3. [Copyright &y& Elsevier]