Photocatalytic reduction of Cr(VI) over TiO2 catalysts was investigated in both the absence and presence of organic compounds (Papadama et al., 2007 and Wang et al., 2008). A marked synergistic effect between the photocatalytic reduction of Cr(VI) and organic compounds was observed over the photocatalyst with the largest specific surface area. These results demonstrated that the photocatalytic reduction of Cr(VI) alone was dependent on both the specific surface area and crystalline structure of the photocatalyst in the absence of any organic compounds, but was dominated by the specific surface area of the photocatalyst in the presence of organic compounds because of the synergistic effect between the photocatalytic reduction of Cr(IV) and the photocatalytic oxidation of organic compounds.
A novel photocatalyst, titanium dioxide (TiO2) doped with neodymium (Nd), was prepared by the sol–gel method and used for the photocatalytic reduction of Cr(VI) under UV illumination (Rengaraj et al., 2007). The results indicated that the presence of Nd(III) in TiO2 catalysts substantially enhances the photocatalytic reaction of Cr(VI) reduction. The neodymium ions deposited on the TiO2 surface behave as sites at which electrons accumulate. The improved separation of electrons and holes on the modified TiO2 surface allows more efficient channeling of the charge carriers into useful reduction and oxidation reactions rather than recombination reactions. The presence of sacrificial electron donors such as formic acid enhances the photocatalytic reduction. The Cr(VI) adsorbed on the surface of the TiO2 particles was observed to be almost completely photoreduced to Cr(III). To overcome the limitation of powder TiO2, a novel technique of immobilization based on anodization was applied and investigated (Yoona et al., 2009). Immobilized TiO2 electrode was applied to reduce toxic Cr(VI) to non-toxic Cr(III) in aqueous solution under UV irradiation. The anodization was performed with 0.5% hydrofluoric acid, and then the anodized samples were annealed under oxygen stream in the range 450–850 °C. The photocatalytic Cr(VI) reduction was favorable in acidic conditions, with 98% of the Cr(VI) being reduced within 2 h at pH 3.
Heterogeneous photocatalytic oxidation of arsenite to arsenate in aqueous TiO2 suspensions has also been proved recently to be an effective and environmentally acceptable technique for the remediation of arsenite contaminated water. The process was performed using an adsorbent developed by loading iron oxide and TiO2 on municipal solid waste melted slag (Zhang and Itoh, 2006). A concentration of 100 mg/L arsenite could be entirely oxidized to arsenate within 3 h in the presence of the adsorbent and under UV-light irradiation (Fig. 18).