Photocatalytic degradation of the Acid Blue 113 textile azo dye in aqueous suspensions of four commercialized TiO₂ samples

Abstract

The apparent first-order rate constant, k_app, of the Acid Blue 113 textile azo dye photocatalytic disappearance in aqueous suspensions of 4 commercialized TiO₂ samples (50 m² g⁻¹-Degussa P25; 54 m² g^{− 1}-Millennium Chemicals PC50; 250–270 m² g⁻¹-Sachtleben Chemie Hombikat UV100; 320 m² g⁻¹-Ishihara ST01) was determined from the decrease in the calibrated 566 nm-absorbance of the supernatant. Under the conditions used (1 liter-batch reactor; water-filtered irradiation from a Philips HPK 125-W high-pressure mercury lamp; [TiO₂] = 1 g/L; [dye] = 70 mg/L), the k_app ranking was: UV100 > PC50 > P25 > ST01. The corresponding maximal difference in k_app was roughly equivalent to that caused by adding an optimal dose of H₂O₂ to the P25 suspension. Taking into account the dye adsorbed amount, q_ads, in the dark, it is deduced that the recombination rate k_r of photogenerated charges was higher for poorly crystallized TiO₂ samples, as expected; additionally, a low content of impurities is suggested to be at the origin of the low k_r of UV100 relative to ST01. It is also argued that differences in the values of q_ads divided by the surface area, S, was less due to pores inaccessible to the dye than to thicker layers of surface tightly bound water for the samples with high S. A FTIR study showed that the dye weakly interacted with TiO₂ and, however, remained the dominant adsorbed species during the degradation. This latter conclusion was in accord with the fact that the dye initial disappearance rates—directly measured or calculated from k_app—were almost equal, indicating that the intermediate products of degradation did not significantly compete with the remaining dye for active species.

Keywords:

• Photocatalysis
• TiO₂
• azo dye
• water purification
• FTIR

Acknowledgment

GZ thanks the Région Rhône-Alpes for a MIRA scholarship which has allowed him to carry out experiments in France and the Tunisian Ministry of Research and Technology for supporting this study in the framework of the “contrat programme” titled “Traitement photocatalytique des effluents textiles: mécanismes de dégradation.” The authors warmly thank Dr N. Negishi (AIST, Japan) for the measurement of the Ishihara ST01 porosity.