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 Cr₂O₇^2- 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.