Adsorption and photocatalytic bleaching of acid orange 7 on P25 titania

The observed adsorption of acid orange 7, AO7−, on P25 titania over a range of pH values (pH 2–8) gives a good fit to data generated using a charge distribution, multisite complexation, i.e. CD-MUSIC, model, modified for aggregated dye adsorption. For this system the model predicts that both the apparent dark Langmuir adsorption constant, KL, and the number of adsorption sites, no, increase with decreasing pH, and are negligible above pH 6. At pH 2 the CD-MUSIC model predicts the fraction of singly co-ordinated sites occupied by the dye, fAO7, is ca. 32% under the in situ monitoring experimental conditions used in this work to study the photocatalytic bleaching of AO7− under UV light illumination ([TiO2] = 20 mg dm−3; [AO7−]total = 4.86 × 10−5 M). Although AO7− adsorption on P25 titania is insignificant above pH 6 and increases almost linearly and markedly below this pH, the measured initial rate of bleaching of AO7−, photocatalysed by titania using UV appears to only increase modestly (