Investigation of the photocatalytic degradation kinetics of reactive azo dye with TiO2 nanoparticules

Azo dyes constitute an important class of pollutants in wastewater released from textile industries. Due to their ability to bind to fibers through formation of covalent bonds, they are the most widely used group of reactive dyes. Azo dyes are characterized by one or more (?N=N?) azo bonds attached to two aromatic rings combined with reactive groups making them hard to degrade. Reactive azo dyes are resistant to aerobic degradation and their degradation products are highly carcinogenic. Conventional treatment methods are either ineffective to remove them from wastewater or they only transfer them to another phase causing a secondary pollution. TiO2 photocatalysis, which is an alternative approach for decolorization of azo dyes in waste water, is based on the generation of highly reactive hydroxyl radicals and photogenerated valence holes when TiO2 particles are irradiated by UV light. This study deals with the photocatalytic decolorization-degradation of Reactive Blue 221, a reactive synthetic dye chosen as the representative member of the azo dyes. The photocatalytic degradation reactions were carried out in a constant temperature batch-type photoreactor using commercial TiO2 Degussa P25 as the photocatalyst under UV-A light irradiation. Adsorption of the dye on TiO2 particles was examined. The effect of various parameters such as catalyst loading, pH and initial concentration of the dye on decolorization and degradation were determined. Samples from the reaction mixture were analyzed by UV-vis spectroscopy, FTIR and GC-MS techniques. Conceptual Density Functional Theory was applied to the degradation reaction of the target molecule and reactivity descriptors were calculated by means of DFT/B3LYP/6-31G* level of theory. Eventually, the reactive sites of the molecule for ?OH radical attack were determined and the reaction mechanism was predicted by combining the results of the DFT calculations with the experimental FTIR and GC-MS analyses. These results of the study suggest that TiO2/UV photocatalysis may be used as a method for treatment of diluted wastewaters in textile industries. The irradiation of titania with UV-A light did produce a photoinduced decolorization of the dye. There is a significant difference in adsorption of the dye on TiO2 surface with the change in the solution pH. Maximum adsorption was obtained at pH=3. A pseudo-first order kinetic model was used to describe the results of the photocatalytic degradation reaction. A linear dependence of the rate constant upon the reciprocal of the initial dye concentration was obtained. The FTIR analysis indicate that adsorption occurs through sulfo and carbonyl groups of the dye molecule, while decolorization by the breaking of the ?N=N? double bond of the chromophore group. Keywords: Reactive Blue 221, adsorption, heterogeneous photocatalytic degradation, TiO2, Conceptual Density Functional Theory (DFT), DFT reactivity descriptors