Characterization of Cr Sorption and Reduction on TiO2: Batch and XPS Studies

Abstract

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.