Effect of different types of organic compounds on the photocatalytic reduction of Cr(VI).

The effect of different types of organic compounds (humic acid, oxalate, ethylenediaminetetraacetic acid, nitrilotriacetic acid, phenol) on the photocatalytic reduction of Cr(VI) with illuminated TiO2 was studied in this work with variation of the solution pH, contact time, initial Cr(VI) concentration and type of organic compounds. As the pH increased, the removal efficiency for Cr(VI) decreased. The increase in Cr(VI) removal by UV/TiO2 with decreasing solution pH was due to the increased potential difference between the conduction band of TiO2 and Cr(VI)/Cr(III) as well as the anionic-type adsorption of Cr(VI) on to the TiO2 surface. Removal efficiency for Cr(VI) increased in the presence of organic compound compared with that without organic compound because positive holes in the TiO2 were scavenged by organic additives. The reduction pattern of Cr(VI) was better described by the first-order kinetic model. Finally photocatalytic reaction with illuminated TiO2 can be effectively applied to treat wastewater contaminated with Cr(VI).