Photocatalytic reduction of Cr(VI) over different TiO2 photocatalysts and the effects of dissolved organic species

The toxic Cr(VI) in industrial wastewaters can be removed by a reduction from Cr(VI) to Cr(III) and a followed precipitation treatment. The reduction of Cr(VI) to Cr(III) is able to be achieved by a photocatalytic process. Thus, photocatalytic reduction of Cr(VI) over TiO2 catalysts was investigated in both the absence and presence of organic compounds. The TiO2 catalyst was pre-calcined at different temperatures to tune the photocatalytic activity and surface area of the photocatalyst. Under the tested conditions, the photocatalytic reduction of Cr(VI) behaved as a pseudo-first-order reaction in kinetics. In the absence of any organic species, the rate constant (kCr) for the photocatalytic reduction of Cr(VI) was found to be increased initially, passing a maximum, and then decreased, as calcination temperature was increased. In the presence of organic compounds, however, kCr was decreased with the increase of calcination temperature. 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 of 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.