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(OII) in aqueous TiO-2 suspensions under UV light illumination. It was found that low 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 OII. In the presence of 0.50 mol/L Cl^-, the rate decreased by 70% for MB while increased 7.5-fold for OII. Furthermore, two bands in the ultraviolet region of OII were rapidly broken down. The proposed mechanism was discussed in detail.
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