Ternary [Al2O3–electrolyte–Cu] species: EPR spectroscopy and surface complexation modeling

Cu²⁺ binding on γ-Al₂O₃ is modulated by common electrolyte ions such as Mg²⁺, , and in a complex manner: (a) At high concentrations of electrolyte ions, Cu²⁺ uptake by γ-Al₂O₃ is inhibited. This is partially due to bulk ionic strength effects and, mostly, due to direct competition between Mg²⁺ and Cu²⁺ ions for the SO⁻ surface sites of γ-Al₂O₃. (b) At low concentrations of electrolyte ions, Cu²⁺ uptake by γ-Al₂O₃ can be enhanced. This is due to synergistic coadsorption of Cu²⁺ and electrolyte anions, and _. This results in the formation of ternary surface species (SOH₂SO₄Cu)⁺, (SOH₂PO₄Cu), and (SOH₂HPO₄Cu)⁺ which enhance Cu²⁺ 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, Cu²⁺ coordinates to one SO⁻ group. Phosphate anions form stronger, binary or ternary, surface species than sulfate anions. At pH  pH_PZC Cu²⁺ may coordinate to two SO⁻ groups. At pH  pH_PZC electrolyte ions and are bridging one O-atom from the γ-Al₂O₃ surface and one Cu²⁺ ion forming ternary γ-Al₂O₃/elecrolyte/Cu²⁺] species.