A kinetic study of the adsorption and decolorization reactions of azo dyes

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 bons, 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. Thus, there is a need to develop new methods to remove such hazardous dyes from water, converting them to harmless compounds. This study deals with the adsorption and photocatalytic degradation of Reactive Red 195 (RR195), a reactive synthetic dye chosen as the representative member of azo dyes. The reactions were carried out in a constant temprature batch-type photoreactor using commercial TiO2 as the photocatalyst under UV-A light irradiation. The effects of various parameters such as initial pH, concentration of dye and concentration of adsorbent were examined in order to find the optimum conditions. The data obtained from adsorption experiments were analyzed by the Langmuir and Freundlich adsorption models. 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 FT-IR and GC-MS analyses. The results of the study suggest that TiO2/UV photocatalysis may be used as a method for treatment of diluted wastewaters in textile industries. The effects of various parameters such as catalyst loading, pH and initial concentration of the azo-dye on decolorisation-degradation experiments were determined. 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.