Φωτοκαταλυτική διάσπαση αζωχρωμάτων από βιομηχανικά απόβλητα

Abstract

The photocatalytic degradation of aqueous solutions of Acid Orange 7 (AO7), a model non-biodegradable textile azo-dye, is investigated with the use of TiO2 photocatalyst and artificial solar- or visible (>400 nm)-light irradiation. It is shown that when the full solar spectrum is utilized, the dye molecules adsorbed on TiO2 surface undergo a series of oxidation steps, which lead to rapid decolorization and formation of a number of intermediates, mainly aromatic and aliphatic acids, which are further oxidized toward compounds of progressively lower molecular weight and, eventually, to CO2 and inorganic ions. Under these conditions, complete decolorization and mineralization of the solution is achieved with appreciable reaction rates. When the solution is irradiated with visible light (λ> 400 nm), reaction intermediates are similar to those observed under artificial solar-light irradiation but decolorization takes place with much lower rates. In addition, when the solution is bleached, the COD does not further decrease with time of irradiation and reaction intermediates remain unchanged, indicating that the visible-light induced photocatalytic degradation of AO7 is governed by the “photosensitization mechanism”. Kinetic results show that the initial rate of decolorization depends on the intensity and the wavelength of incident light irradiation, the surface coverage, the initial concentration of the azo-dye, the concentration of the photocatalyst and the initial pH of the solution. From the kinetic investigation, a kinetic model is obtained which fits well the experimental data. The influence of the physicochemical properties of the photocatalyst such as crystal structure, specific surface area, average crystallite size, absorption in the UV/vis region and morphology, as well as the preparation method of TiO2 on the rate of decolorization is investigated. The improvement of the photocatalytic activity of semiconductor photocatalysts by doping with different altervalent cations and by deposition of noble metals onto the TiO2 surface is studied.