An innovative green process for the depollution of Cr(VI)-contaminated surfaces using TiO2-based photocatalytic gels

In this study, we propose a process based on self-drying gels for the depollution of chromium contaminated surfaces. A colloidal gel has been formulated with TiO2 as a thickener. TiO2 is widely used for the treatment of organic compounds contained in liquid waste because of its photocatalytic properties under UV irradiation. This type of gel was chosen because it should reduce the toxicity of the final waste by reducing the oxidation state of Cr(VI). The drying and cracking of the gel is first studied in terms of the thickness of the applied gel and of the presence or not of ethanol. The results highlight the importance of the initial water content in the gel and of the ambient parameters on these phenomena. The efficiency of the gel for the surface depollution and reduction of Cr(VI) is then evaluated. These gels extract Cr pollution better than classical gels because the positive TiO2 particles at pH 5.5 attract the Cr2O72− ions from the contaminated surface. The impact of irradiation on the performance of the gel shows that TiO2 is activated by UV-A light and promotes the formation of electron-hole pairs that completely reduce Cr(VI). The effects of ethanol are also described. Since, in basic solution, ethanol inhibits electron-hole pairs recombination and photoinduced free radicals from ethanol partially offset the electrostatic repulsion between the TiO2 particles and the dichromate ions, this enhances the reduction of Cr(VI).