A study on the mechanism of Ca2+ 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 Ca2+ 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 Ca2+ concentration in the suspensions. The results of both the Ca-ISE and parallel spectrophotometric determination were similar. The adsorption data showed that TiO2 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 TiO2 it involved formation of > SO-_CaOH+ predominately on the beta-plane and at pH > 9 also on the 0-plane. In the case of Fe2O3 one could observe the existence of (> SO-)(2-)Ca2+ on the beta-plane in the whole studied pH range. At pH above 7 the tetranuclear complexes (> SOH)(2)(> SO-)(2-)Ca(OH)(+) were found, and at pH > 9 also > SO-_CaOH+ could be observed. On the other hand, the analysis of the zeta-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 Ca2+ adsorption on metal oxides in the environment. (C) 2019 Elsevier Ltd. All rights reserved.