Photocatalytic and photochemical degradation of mono-, di- and tri-azo dyes in aqueous solution under UV irradiation

The degradation of three commercially available textile azo dyes, Solophenyl Green BLE 155% (SG), Erionyl Red B (ER) and Chromotrope 2R (C2R), has been studied by using photochemical and photocatalytic processes under UV irradiation. The adequacy of each process depends on the concentration of dye. At low dye concentration, the neat photochemical degradation is very efficient. The photocatalytic process, using either slurry of Degussa P-25 TiO2 or a biphasic mixture of TiO2 and activated carbon (AC), more effectively bleaches heavier colored solutions. The heterogeneous photocatalytic regime was characterized in terms of catalyst load, TiO2 to AC mass ratio, initial dye concentration and oxygen partial pressure. Catalyst recovery and reuse was also analyzed. Based on the Langmuir–Hinshelwood approach, reaction rates and adsorption equilibrium constants were calculated. A positive effect on the photocatalytic degradation was observed by the addition of AC to TiO2 catalyst, which was quantified in terms of a synergy factor (R). The efficiencies of different photo-induced degradation processes were compared based on the experimentally determined apparent rate constants, mineralization degrees and initial quantum yields.