Photocatalytic Decolorization of Direct Red16 from an Aqueous Solution Using B-ZnO/TiO₂ Nano Photocatalyst: Synthesis, Characterization, Process Modeling, and Optimization

The aim of this study was to modify the TiO2 photocatalyst with different amounts of ZnO (0.25, 0.5, and 1 wt. %) and B (1, 5, and 10 wt. %), as B-ZnO/TiO2, for mineralization and photodegradation of direct red 16 (DR16). B-ZnO/TiO2 was synthesized by the sol-gel method and the composite with 5 wt. % of B and 0.5 wt. % of ZnO was selected as the optimal composition, based on DR16 removal experiments. Th results showed that the removal efficiencies for optimum amounts of B and ZnO were 47 and 87 % in B-TiO2 and B-ZnO/TiO2 composition, respectively. The structural and chemical characteristics, modeling and optimization of the operating variables, adsorptive behavior, and reusability of the synthesized photocatalyst were evaluated. The acquired findings confirmed the generation of an amorphous phase with a low recombination rate and an improvement of photodegradation efficiency under visible light irradiation. The effects of (NH4)2S2O8, H2O2, KCl, and KHCO3 salts on dye photocatalytic removal were evaluated, and the maximum positive effect was observed using (NH4)2S2O8. The results of optimization of the operational variables and their optimum values proved that an increase in B-ZnO/TiO2 loading, reaction time, LED intensity, and a decrease in DR16 concentration and initial pH, improved the removal efficiency. The maximum DR16 degradation (100%) was obtained in the presence of 10 ppm DR16 and 1 g/L B5%-ZnO0.5%/TiO2, at pH 3, under visible light irradiation, after 200 min. The DR16 adsorption process by the B-ZnO/TiO2 followed a pseudo-second-order model. The mechanism of the photodegradation of DR16 dye was ascribed to the absorbed h+ and OH− active species. According to the results, the B-ZnO/TiO2 photocatalyst can be considered as a promising candidate for actual dye removal under visible light irradiation.