Advances in the Mechanism of Deposition of the CrO42-, HCrO4-, and Cr2O72- Species on the Surface of Titania Consisting of Anatase and Rutile

Following a methodology based on the "2pK/one site" and "triple layer" models, we further investigated the mechanism of deposition of the CrO42-, HCrO4-, and Cr2O72- species on the titania/electrolyte solution interface. The surface of the titania used is consisted of anatase and rutile patches. This methodology involved the following steps: (i) The assumption of tentative deposition equilibria. (ii) The derivation of the corresponding equations. (iii) The calculation of the concentration of each of the deposited CrVI−oxo species as well as the total concentration of the deposited CrVI at various pH's and bulk solution concentrations. (iv) The calculation of the variation with pH of the difference in the hydrogen ions consumption by the titania surface in the presence and absence of the CrVI−oxo species in the impregnating solution. (v) The calculation of the variation with pH of the ζ-potentials developed above the anatase and rutile patches. (vi) The comparison of the calculated parameters and variations mentioned above with the corresponding ones achieved from deposition experiments, potentiometric titrations, and microelectrophoresis. It was found that the deposition takes place following the equilibria: TiOH2+ + CrO42- ↔ TiOH2+···CrO42-; TiOH2+ + HCrO4- ↔ TiOH2+···HCrO4-; TiOH2+ + Cr2O72- ↔ TiOH2+···Cr2O72-; TiOH + CrO42- ↔ Ti−O−(CrO3-) + OH-; TiOH + HCrO4- ↔ Ti−O−(CrO2)−OH + OH-; TiOH + Cr2O72- ↔ Ti−O−(Cr2O6-) + OH-. At pH's 7.5 and 8.0 the deposition occurs exclusively by surface reaction (fourth and fifth equilibria). In the pH range 6.0−4.0 all the above equilibria contribute to the deposition. The only exception is that the fourth equilibria does not practically occur at pH ≤ 4.5. However, in the pH range 6.0−4.0 the deposition by adsorption of the CrVI species, mainly of the HCrO4- species, is the predominant process. It should be noted that the extent of the deposition in the anatase region is much larger than that on the rutile region. Lateral, attractive, interactions are exerted between the deposited CrVI−oxo species through water molecules being in the inner Helmholtz plane (IHP). The intensity of these interactions is proportional to the charge of the deposited CrVI−oxo species.