Ferric ions present in an acidic aqueous suspension of TiO₂ can promote significantly the photocatalytic degradation of a textile dye X3B upon UV light irradiation (λ≥320 nm).The initial rate of X3B degradation was observed to increase exponentially with the initial concentration of Fe(Ⅲ).However, this enhancement disappeared as all ferric species were transferred into ferrous ions, and after that the degradation of X3B proceeded via a regular pathway of photocatalysis.Several control experiments revealed that the beneficial effect of Fe(Ⅲ) was attributed mainly to electron trapping at the semiconductor surface, whereas the contribution from homogeneous Fe(Ⅲ) photolysis, capable of producing hydroxyl radicals, was not significant.In the liquid phase, more than 98 2% of Fe(II) was detected, implying that the involvement of (Photo)-Fenton reaction was less possible.On the other hand, the ferrous ion, once produced, exhibited no obvious effect on X3B degradation, but its formation became faster in the presence of X3B, suggesting that hole oxidation of X3B might be involved as well.Both Fe(Ⅲ) and X3B were adsorbed strongly on the catalyst surface, allowing efficient separation and utilization of the electron-hole pairs, whereas Fe(II) was hardly adsorbed, which probably was one of the reasons that Fe(II) could not be re-oxidized in situ by hole or other reactive species generated in the system.Moreover, the increase in the initial pH from 1 6 to 3 7 did not affect obviously the initiate rate of X3B degradation, but led to significant decrease in the initial rate of Fe(II) formation.Together with the exponential increase of Fe(II) formation rate with initial Fe(Ⅲ) concentration, the primary evidence suggests that the efficiency of electron trapping by Fe(Ⅲ), similar to Fe(Ⅲ) photolysis, is also dependent of ferric species polymerization on the surface.The results obtained in this work will be relevant to the study on Fe-doped photocatalyst.

Key words: titanium dioxide, ferric ion, reactive dye, photocatalysis, mechanism