Research on Ca²⁺ adsorption onto the mineral surface is of significant importance with regard to geochemical processes. Sverjensky (Geochim Cosmochim Acta 70(10), 2427–2453, 2006) assumed that alkaline earths form two types of surface species on oxides: tetranuclear (> SOH)₂(> SO⁻)₂_M(OH)⁺ and mononuclear > SO⁻_M(OH)⁺. To look into the above assumption we investigated calcium adsorption on SiO₂ and Al₂O₃ because they are the most widespread minerals in the environment. We have determined the proton surface charge, electrokinetic potential and metal adsorption as a function of pH. The Ca²⁺ uptake and concentration in the system were monitored by the calcium ion-selective electrode (Ca-ISE). The Ca-ISE measurements indicated a similar affinity of Ca²⁺ for both materials despite their differently charged surface, negative for silica and mainly positive for alumina. This may suggest that simple electrostatic interactions are not the primary driving force for calcium adsorption, and that solvation of calcium ions at the surface may be crucial. We have analyzed our experimental data using the 2-pK triple-layer model (2-pK TLM). Three calcium complexes on the mineral surface were reported. Two of them were the same for both oxides, i.e. the tetranuclear ( $$>$$> SOH)₂( $$>$$> SO⁻)₂_Ca²⁺ and mononuclear complexes > SO⁻_CaOH⁺. Additionally, minor contribution from >SOH…Ca²⁺ for silica was assumed. In the case of Al₂O₃ the hydrolyzed tetranuclear complexes ( $$>$$> SOH)₂( $$>$$> SO⁻)₂_CaOH⁺ at pH > 7.5 occurred based on the modeling results. Two types of surface complexes suggested by Sverjensky allowed for the correct description of proton and calcium uptake for alumina. However, the electrokinetic data excluded hydrolyzed tetranuclear surface species for this oxide.

We have demonstrated that the redox potential of the solution containing an inert electrolyte, ferrous ions, and metal oxide allows a much better understanding of the process of Fe(II) adsorption on oxide with the increase of pH. We tested two oxides: γ-Fe₂O₃ (maghemite) and TiO₂ (the mixture of anatase and rutile). For both of them, we determined proton surface charge, Fe(II) uptake curves, electrokinetic potential, and redox potential in solution. The all measured quantities except the last, behaved in an almost identical manner for ferric oxide and titanium dioxide. The redox potential strongly depends on the pH of the solution and stabilizes when the adsorption process is finished (pH above 6). We observed that for the solution consisting only of 0.2 mM Fe(II) and 0.1 KCl the redox potential in the pH range 3–6 can be expressed by the formula: \(Eh[mV]=1120 - 3 \times 59 \times {\text{pH}} - 59 \times \log (a\_{\text{Fe}}({\text{II}}))\).

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.