Effects of iron species in the photocatalytic degradation of an azo dye in TiO2 aqueous suspensions

The effects of Fe(III) species on the photocatalytic degradation of azo dye Acid Red 1 (AR1) have been studied in titanium dioxide aqueous suspensions under irradiation in the 315- 400 nm range. The initial increase of the photocatalytic degradation rate observed in water suspensions containing Fe(III) aquo ions (10-5 to 10-4 M) was attributed to the increased amount of dye adsorbed on the iron(III)-modified semiconductor surface. This was confirmed by the fact that iron species not adsorbed on the semiconductor, such as ferrioxalate complexes and Fe(II) species, had no kinetic effects. The mineralization kinetic profiles obtained under simultaneous sonication further confirmed the role of AR1 adsorption. The accumulation of hydrogen peroxide during the photocatalytic degradation of the dye was completely suppressed in the presence of all iron species, mainly due to the Fenton reactions consuming H2O2 in the aqueous phase, although a decrease in the rate of H2O2 formation cannot be excluded, due to the competition between adsorbed Fe(III) species and adsorbed oxygen for photo-promoted conduction band electrons.