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The effect of Cl^- on photocatalytic degradation (PCD) of pollutants is an important factor since it is ubiquitous in nature. In general, Cl^- showed an inhibition on photodegradation due to its scavenging HO^{·} radicals. In this paper, experiments were carried out to examine the effects of Cl^- on the PCD of Methylene Blue (MB) and Orange II (Oll) in aqueous TiO_2 suspensions under UV light illumination. It was found that low concentration of Cl^- (< 0.01 mol/L) showed little influence on both dyes, however, high concentration of Cl^- (> 0.10 mol/L) had a very different influence on the decolorization of dyes: a significant inhibition for MB but a great promotion for Oll. In the presence of 0.50 mol/L Cl^-, the rate decreased by 70% for MB while increased 7.5-fold for Oll. Furthermore, two bands in the ultraviolet region of Oll were rapidly broken down. The proposed mechanism was discussed in detail. [ABSTRACT FROM AUTHOR]

For TiO_2 heterogeneous reaction, the reaction site and the detailed mechanism are interesting and controversy topics. In this paper, effects of surface fluorination of TiO_2 on the photocatalytic degradation of an azo dye, Orange G(OG) under UV or visible light irradiation were investigated, and the possible reaction site and mechanism were elucidated. The adsorption of OG on TiO_2 was nearly inhibited by fluoride but its UV light induced photodegradation rate was greatly increased by a factor of about 2.7, which was due to the more generated free hydroxyl radicals. It supported the views that fluoride could desorb the oxidant species from surface and that the reaction sites could move to the bulk solution. In TiO_2/Vis system, the observed inhibition effects of fluorination could be interpreted by the competitive adsorption, which provided additional evidences that the visible light sensitized photodegradation of dye pollutants on the catalyst surface. [ABSTRACT FROM AUTHOR]