Ultrasound and UV Stimulated Heterogeneous Catalytic Oxidation of an Azo Dye: A Synergistic Effect

This research deals with the decolourisation of direct azo dye, Direct Blue 71 (DB71), in aqueous solution by means of sonocatalytic, photocatalytic and sonophotocatalytic heterogeneous oxidation processes in the presence of a zinc oxide (ZnO) catalyst. The effect of these oxidation processes under visible light and 20 kHz ultrasound was investigated to study their influence on degradation rates by varying the initial dye concentration, pH and catalyst load and to understand the effect of synergy on the degradation process. The synergistic effect is quantified in terms of a synergy factor which has already been defined in the literature. The highest value of the factor was obtained for a low DB71 dye concentration at pH 5.5, 20 °C temperature and 400 mg L −1 optimum ZnO dosage. Pseudo first-order kinetics were followed for the removal of the azo dye according to the Langmuir–Hinshelwood model. For comparison, the Langmuir–Hinshelwood model was also used for experimental values obtained from the sonocatalytic process, the photocatalytic process and the combination of these processes. The Langmuir–Hinshelwood kinetic model demonstrated consistency in all calculations of the initial rates of degradation with the appropriate values for the reaction rate constant and DB71 dye absorption constant.