Photocatalytic degradation of three azo dyes using immobilized TiO₂ nanoparticles on glass plates activated by UV light irradiation: Influence of dye molecular structure

In order to discuss the effect of chemical structure on photocatalysis efficiency, the photocatalytic degradation of three commercial textile dyes (C.I. Acid Orange 10 (AO10), C.I. Acid Orange 12 (AO12) and C.I. Acid Orange 8 (AO8)) with different structure and different substitute groups has been investigated using supported TiO₂ photocatalyst under UV light irradiation. All the experiments were performed in a circulation photochemical reactor equipped with a 15-W UV lamp emitted around 365 nm. The investigated photocatalyst was industrial Millennium PC-500 (crystallites mean size 5-10 nm) immobilized on glass plates by a heat attachment method. SEM images of the immobilized TiO₂ nanoparticles showed the good coating on the plates, after repeating the deposition procedure three times. Our results indicated that the photocatalytic decolorization kinetics of the dyes were in the order of AO10 > AO12 > AO8. Photocatalytic mineralization of the dyes was monitored by total organic carbon (TOC) decrease, changes in UV-vis spectra and ammonium ion formation. The dye solutions could be completely decolorized and effectively mineralized, with an average overall TOC removal larger than 94% for a photocatalytic reaction time of 6 h. The nitrogen-to-nitrogen double bond of the azo dyes was transformed predominantly into NH₄⁺ ion. The kinetic of photocatalytic decolorization of the dyes was found to follow a first-order rate law. The photocatalysis efficiency was evaluated by figure-of-merit electrical energy per order (E_EO).