Characterization of Cr sorption and reduction on TiO₂: Batch and XPS studies

Cr interactions with TiO₂ were systematically studied using batch and spectroscopic investigations. Sorption of chromium on TiO₂ at pH 4.5 increases with increasing Cr concentration. The sorption of Cr(III) is in good agreement with Langmuir isotherm model, whereas that of Cr(VI) is better accounted for by the Freundlich model. At pH 7.0, however, the uptake of Cr(III) by TiO₂ is over 95 %, while the extent of Cr(VI) sorption on TiO₂ is much less than that of pH 4.5. These results are consistent with SEM observations showing that precipitates of Cr(III) are dominant under neutral pH. The sorption of Cr(VI) on TiO₂ decreases with increasing pH. However, Cr(VI) sorption decreases with increasing ionic strength below pH 4.5 whereas the sorption increases with ionic strength above pH 4.5. These observations suggest that Cr(VI) sorption is sensitive to ionic strength, and Cr(VI) could form weakly bound adsorption complexes at the TiO₂-water interface. Phosphate competes with Cr(VI) for TiO₂ surface sites during sorption processes, and Cr(VI) desorption accelerates and increases in the presence of phosphate. It is noted that the reduction of Cr(VI) is induced by sunlight on the TiO₂ surface, but not detected in acidic solution throughout batch experiments at pH≥4.5 for 24 h.