TiO2-assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations

The photocatalytic degradation of azo dyes containing different functionalities has been reviewed using TiO2 as photocatalyst in aqueous solution under solar and UV irradiation. The mechanism of the photodegradation depends on the radiation used. Charge injection mechanism takes place under visible radiation whereas charge separation occurred under UV light radiation. The process is monitored by following either the decolorization rate and the formation of its end-products. Kinetic analyses indicate that the photodegradation rates of azo dyes can usually be approximated as pseudo-first-order kinetics for both degradation mechanisms, according to the Langmuir–Hinshelwood model. The degradation of dyes depend on several parameters such as pH, catalyst concentration, substrate concentration and the presence of electron acceptors such as hydrogen peroxide and ammonium persulphate besides molecular oxygen. The presence of other substances such as inorganic ions, humic acids and solvents commonly found in textile effluents is also discussed. The photocatalyzed degradation of pesticides does not occur instantaneously to form carbon dioxide, but through the formation of long-lived intermediate species. Thus, the study focuses also on the determination of the nature of the principal organic intermediates and the evolution of the mineralization as well as on the degradation pathways followed during the process. Major identified intermediates are hydroxylated derivatives, aromatic amines, naphthoquinone, phenolic compounds and several organic acids. By-products evaluation and toxicity measurements are the key-actions in order to assess the overall process.