UV- or visible-light-induced degradation of X3B on TiO2 nanoparticles:: The influence of adsorption

Photodegradation of a textile dye X3B using either UV (lambda greater than or equal to 320 nm) or visible light (lambda greater than or equal to 450 nm) over three catalysts of highly adsorptive TiO2 nanoparticles in water has been examined. All the adsorption isotherms demonstrated-the Langmuir type behavior. The common observation was confirmed that for all the reactions induced by UV or Visible light, the apparent initial rate of X3B loss in the aqueous phase increased with the initial equilibrated concentration of X3B. However, this correlation was changed when the rate was determined by the decreased concentration both in the aqueous phase and on the catalyst surface. This increase of real initial rate with the initial equilibrated concentration was observed only in the visible-light-induced reaction over TiO2 of Degussa P25. For all the other reactions, especially under UV irradiation, the real initial rate was found to increase only initially and then decrease with the initial equilibrated concentration. The result suggests that there is a screening effect by the adsorbed dye in the TiO2 photocatalytic reaction and a solution filter effect in the photosensitized reaction. Moreover, the photosensitized photodegradation of X3B was found to be also dependent on the physical properties of TiO2, but interestingly the relative activity among the catalysts was similar to that demonstrated in the photocatalytic reaction.