Ternary [Al 2O 3–electrolyte–Cu 2+] species: EPR spectroscopy and surface complexation modeling

Abstract

Cu 2+ binding on γ-Al 2O 3 is modulated by common electrolyte ions such as Mg 2+, SO 4 2 - , and PO 4 3 - in a complex manner: (a) At high concentrations of electrolyte ions, Cu 2+ uptake by γ-Al 2O 3 is inhibited. This is partially due to bulk ionic strength effects and, mostly, due to direct competition between Mg 2+ and Cu 2+ ions for the SO − surface sites of γ-Al 2O 3. (b) At low concentrations of electrolyte ions, Cu 2+ uptake by γ-Al 2O 3 can be enhanced. This is due to synergistic coadsorption of Cu 2+ and electrolyte anions, SO 4 2 - and PO 4 3 - . This results in the formation of ternary surface species ( SOH 2SO 4Cu) +, ( SOH 2PO 4Cu), and ( SOH 2HPO 4Cu) + which enhance Cu 2+ uptake at pH < 6. The effect of phosphate ions may be particularly strong resulting in a 100% Cu uptake by the oxide surface. (c) EPR spectroscopy shows that at pH ≪ pH PZC, Cu 2+ coordinates to one SO − group. Phosphate anions form stronger, binary or ternary, surface species than sulfate anions. At pH ≫ pH PZC Cu 2+ may coordinate to two SO − groups. At pH ≪ pH PZC electrolyte ions SO 4 2 - and PO 4 3 - are bridging one O-atom from the γ-Al 2O 3 surface and one Cu 2+ ion forming ternary [ γ-Al 2O 3/elecrolyte/Cu 2+] species.