Removal of Cr(VI) and humic acid by using TiO2 photocatalysis

The removal efficiencies of Cr(VI) and HA, using a TiO2-mediated photocatalytic process, were investigated with variations in the pH, TiO2 dosage and Cr(VI)/HA ratio. During the photocatalytic reaction, the total removal of Cr(VI) occurred through adsorption onto TiO2, as well as its reduction to Cr(III). However, oxidation and adsorption were identified as important removal processes for the treatment of HA. Due to the anionic type adsorption onto TiO2 and its acid-catalyzed photocatalytic reduction, the removal of Cr(VI) decreased with increasing pH, while that of HA increased with increasing pH. The TiO2 dosage was also an important parameter for the removal of Cr(VI). As the TiO2 dosage was increased to 2.5 g l−1, the removal of Cr(VI) was continuously enhanced, but decreased at dosages above 3 g l−1 due to the increased blockage of the incident UV light used for the photocatalytic reaction. The removal of Cr(VI) was greatly enhanced when the system contained both HA and Cr(VI) compared to Cr(VI) alone. Also, the removal of HA was greatly enhanced when the system contained both HA and Cr(VI) compared to HA alone. The removal of Cr(VI) was continuously enhanced as the HA concentration gradually increased; however, no further increase was observed above 20 mg l−1 HA due to the increased absorption of the UV light. This result supports that the photocatalytic reaction, with illuminated TiO2, could be applied to more effectively treat wastewater containing both Cr(VI) and HA than that containing a single species only.