An attempt was made to investigate the potential of two widely used commercial nano and micro TiO2 photocatalysts in degradation and mineralization of a typical azo dye “direct blue 71” in water. A special circulating up-flow reactor with an UV-C- 150 W lamp was employed and the influence of the operational parameters was investigated. The results showed that both the photolysis and photocatalysis cause decomposition simultaneously. Despite a half concentration, the nano TiO2 particles provide a nearly twice efficiency, and for the vital case of mineralization 98% reduction in COD after 120 min irradiation. The influence of turbidity of catalyst particles was determined with a novel method and was taken into account in kinetic studies. It was found that the bulk degradation by the OH radicals has a major role in the photocatalysis process.
