Visible light-induced reactions of humic acids on TiO 2

Abstract

We investigated the sensitized degradation of humic acids on TiO 2 under visible light ( λ>420 nm) irradiation. The photolysis rates were strongly dependent on pH due to the pH-dependent adsorption of humic acids on TiO 2 with the maximum rate observed at pH∼3. Reduction in the UV–visible absorbance and fluorescence emission ( λ ex=350 nm) of humic acids was observed during the irradiation. Humic acids acted as a sensitizer for injecting electrons from their excited state to the conduction band of TiO 2, which were subsequently transformed and decolorized through a series of electron transfer reactions. However, the dissolved organic carbon (DOC) of humic acid solutions remained almost unchanged under visible light while the UV-irradiation was able to remove part of the total DOC. The trihalomethane formation potential (THMFP) on the contrary increased with visible light irradiation. The macromolecular humic acids on visible light-illuminated TiO 2 degraded with the reduction of aromatic character but not to the complete mineralization. Humic acids that can both donate and accept electrons to and from TiO 2 conduction band seem to withstand many electron transfers without undergoing mineralization. Humic acids also successfully acted as a sensitizer for the reductive degradation of CCl 4 on platinized TiO 2 under visible light irradiation, while no dechlorination was observed in the absence of either humic acids or TiO 2.