Visible light-induced reactions of humic acids on TiO2

We investigated the sensitized degradation of humic acids on TiO2 under visible light (lambda > 420 nm) irradiation, The photolysis rates were strongly dependent on pH due to the pH-dependent adsorption of humic acids on TiO2 with the maximum rate observed at pH similar to 3. Reduction in the UV-visible absorbance and fluorescence emission (lambda(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 TiO2, 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 TiO2 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 TiO2 conduction band seem to withstand many electron transfers without undergoing mineralization. Humic acids also successfully acted as a sensitizer for the reductive degradation Of CCl4 on platinized TiO2 under visible light irradiation, while no dechlorination was observed in the absence of either humic acids or TiO2. (C) 2002 Elsevier Science B.V. All rights reserved.