Application of Surface Complexation Modeling to Investigate the Mechanism of Cu2+ Adsorption on TiO2, Al2O3, and SiO2 Under High Surface Coverage

We have shown that the adsorption of Cu²⁺ ions on various metal oxides, depending on the pH of the solution, can be described assuming the formation of only two surface complexes with surface hydroxyl groups SOH: SOCu(OH) and SOCu⁺. Using an ion-selective electrode for Cu²⁺, we determined the adsorption edges, i.e., the dependence of the amount of adsorbed metal expressed as a percentage depending on the solution pH for three oxides: TiO₂, Al₂O₃, and SiO₂. The measurements were carried out with high surface coverage where the ratio of the adsorption sites/copper ions in the system were from 2 to 3, depending on the oxide. Simultaneously, with the adsorption edge, the hydrogen surface charge density and the electrokinetic potential of the oxide particles were measured as a function of pH. These three types of experimental data were fitted all together using the surface complexation model (2-pK TLM). In modeling, it was not necessary to consider the precipitation of Cu(OH)₂ on the oxide surface to obtain good agreement with the data. Additionally, it was shown that the presence of charged surface species SOCu⁺ (about 10% of total adsorbed copper) was crucial to fit the data for zeta potential.