Cu2+ binding on gamma-Al2O3 is modulated by common electrolyte ions such as Mg2+, SO42-, and PO43- in a complex manner: (a) At high concentrations of electrolyte ions, Cu2+ uptake by gamma-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 gamma-Al2O3. (b) At low concentrations of electrolyte ions, Cu2+ uptake by gamma-Al2O3 can be enhanced. This is due to synergistic coadsorption of Cu2+ and electrolyte anions, SO42-and PO43-. 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 SO42- and PO43- are bridging one O-atom from the gamma-Al2O3 surface and one Cu2+ ion forming ternary [gamma-Al2O3/elecrolyte/Cu2+] species. (C) 2009 Published by Elsevier Inc.