Removal of Crystal Violet and Hexavalent Chromium using TiO2-Bentonite under Sunlight: Effect of TiO2 Content

The main objective of this study was to investigate the correlation between TiO2 content in photoactive bentonite (B-TiO2) and the pathway by which crystal violet (CV) and hexavalent chromium (Cr (VI)) are removed from water under sunlight. B-TiO2 samples were prepared by impregnation with TiCl4 with different weight ratios (g/g) (namely, 5, 10, 20 and 30%). Materials were characterized using different techniques, among which: SEM, FT-IR, XRD, HRTEM, EDX and Zeta potential measurements. Results show that, only the anatase TiO2 polymorph was formed in the bentonite and the porosity of materials decreases with the increase of TiO2 content. Furthermore, zeta potential measurements indicate that, when TiO2 content increases, the negative charge of materials decreases. On the other hand, experimental results show that these materials combine both adsorption and photocatalytic reactions to remove CV molecules from water. As the TiO2 content increases, the adsorption capacity decreases, while the photocatalytic activity is more important. In the case of Cr (VI) species, all samples show a few adsorption because of the repulsion effect between these species and the negative charge of the bentonite. Therefore, under sunlight, the Cr (VI) removal occurred mainly by the photoreduction reaction that is more efficient when the TiO2 content increases.