Comparative study on the adsorption, kinetics, and thermodynamics of the photocatalytic degradation of six different synthetic dyes on TiO₂ nanoparticles

Abstract

Organic effluents have become a serious global environmental concern. Therefore, the removal of pollutive compounds from water systems is essential, and one of potential methods is photocatalysis, employing metal oxides as catalyst. The present study, for the first time, compared the photocatalytic degradation of six different synthetic dyes, including methylene blue (MB), rhodamine B (RhB), crystal violet (CV), methyl violet (MV), malachite green (MG), and brilliant green (BG) as models of organic effluents in aqueous solutions, on anatase titanium dioxide (TiO2) nanoparticles under UV light illumination. The results show the complete degradation of the dyes within 30 min of irradiation. The effects of initial concentration, catalyst dosage, and medium temperature on the photocatalytic degradation of the dyes suggested that the six different dyes in this study were adsorbed in the same way and undergo an oxidation reaction with photo-generated OH· radical on the surface of TiO2 catalyst. The photo-induced degradation of all the dyes follows pseudo first-order kinetics, which can be explained using the Langmuir–Hinshelwood model. The photocatalytic degradation of the dyes was found to be a diffusion-controlled reaction, and its degradation rate was enhanced at higher temperature. Interesting finding is that, under the same experimental conditions, the photodegradation rate was estimated to be 0.77, 0.71, 0.44, 0.30, 0.23, and 0.15 min−1 for CV, BG, MB, MV, MG, and RhB, respectively, suggesting the different potential barriers of demethylation or deethylation reaction of the dyes, which is related to their molecular structure, size, and reactivity. From thermodynamic viewpoint, the activation energy attributed to the potential barrier of the photocatalytic degradation is related to diffusion-controlled reaction between the dyes and generated hydroxyl radical on the TiO2 catalyst surface.