A study on the mechanism of Ca adsorption on TiO2 and Fe2O3 with the usage of calcium ion-selective electrode.

The paper presents the quantitative characterization of the solid/water interface applying both experimental and theoretical approaches for the system of TiO2 (mixture of anatase and rutile) and Fe2O3 (maghemite) with calcium ions in the pH function. The aim of the study was also to find a bonding mechanism between Ca and metal oxides surface based on the calculations from the surface complexation modeling code (GEOSURF by Sahai and Sverjensky, 1998). In order to obtain adsorption edges, a calcium ion-selective electrode (Ca-ISE) was applied for determination of Ca concentration in the suspensions. The results of both the Ca-ISE and parallel spectrophotometric determination were similar. The adsorption data showed that TiO exhibited stronger calcium binding than Fe2O3 at pH > 8. Using 2-pK TLM (triple-layer model) it was demonstrated that mechanism of the calcium adsorption onto the metal oxides surface involved different reactions. In the case of TiO it involved formation of >SO_CaOH predominately on the β-plane and at pH > 9 also on the 0-plane. In the case of Fe2O3 one could observe the existence of (>SO)_Ca on the β-plane in the whole studied pH range. At pH above 7 the tetranuclear complexes (>SOH)(>SO)_Ca(OH) were found, and at pH > 9 also >SO_CaOH could be observed. On the other hand, the analysis of the ζ-potential data suggested the absence of the tetra-species on the maghemite surface. The study indicated that the properly validated calcium ion-selective electrode can be an attractive instrument for monitoring Ca adsorption on metal oxides in the environment.