A novel visible light-driven TiO2 photocatalytic reduction for hexavalent chromium wastewater and mechanism

Titanium dioxide (TiO2) photocatalyst was prepared with a sol-gel method and its characterizations were analyzed TiO2 photocatalytic reduction of Cr6+ was investigated in visible light irradiation and reduction mechanisms were calculated. Prepared TiO2 is anatase with a bandgap of about 2.95 eV. Experimental results display that almost 100% of Cr6+ is removed by visible light-driven TiO2 photocatalytic reduction after 120 min when Cr2O72− initial concentration is 1.0 mg·L−1, TiO2 dosage is 1.0 g·L−1, and pH value is 3. In acidic aqueous solution, HCrO4− is the dominant existing form of Cr6+ and is adsorbed by TiO2, forming a complex catalyst HCrO4−/TiO2 with an increase in wavelength to the visible light zone, demonstrated by UV–Vis diffuse reflection spectroscopy. Based on X-ray photoelectron spectroscopy data, it can be deduced that Cr6+ is adsorbed on the surface of TiO2 and then reduced to Cr3+ in situ by photoelectrons. Self-assembly of HCrO4−/TiO2 complex catalyst and self-reduction of Cr6+ in situ are the key steps to start the visible light-driven TiO2 photocatalytic reduction. Furthermore, TiO2 photocatalytic reduction of Cr6+ fits well with pseudo-first-order kinetics and has the potential application to treat chemical industrial wastewater.