A conic body packed bed reactor, internally irradiated with a UV-C lamp and equipped with circulating upflow stream was employed to investigate the decomposition of a widely used azo dye, direct red 16, in water. The synthesized nanostructure TiO2 photocatalyst particles were immobilized on the surface of transparent Raschig ring packings. Solutions with initial concentration of 30 mg L−1 of dye, within the range of typical concentration in textile waste waters, were treated under the mild operating conditions of natural pH of 6.75 and temperature of 25 °C. Investigations on the active species showed that hydroxyl radicals play the major role in the process, providing a perfect degradation in 90 min of irradiation and hence 93% in about 60 min; and also about 71% desired decomposition of aromatic groups in 120 min. For kinetic investigations, the rate of degradation of the dye was expressed as the sum of the rates of individual photolysis and photocatalysis process branches in power law model. Meanwhile, the Langmuir–Hinshelwood kinetic model describes the variations in pure photocatalytic branch in consistent with a first order power law model.
