Ternary [Al2O3–electrolyte–Cu2+] species: EPR spectroscopy and surface complexation modeling

Cu2+ binding on γ-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 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, 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 ≪ pHPZC, Cu2+ coordinates to one SO− group. Phosphate anions form stronger, binary or ternary, surface species than sulfate anions. At pH ≫ pHPZC Cu2+ may coordinate to two SO− groups. At pH ≪ pHPZC electrolyte ions SO42- and PO43- are bridging one O-atom from the γ-Al2O3 surface and one Cu2+ ion forming ternary [γ-Al2O3/elecrolyte/Cu2+] species.

Graphical abstractEPR spectroscopy of ternary [Al2O3–electrolyte–Cu2+] species.Figure optionsDownload full-size imageDownload as PowerPoint slide