Synthetic dyes that are widely used in the textile and paper industries and being discharged into the environment have caused serious environmental pollution due to their toxicity and recalcitrant nature. The first part of this study was based on the photodegradation of three single azo dyes (diazo dye RG19, mono azo dye; AO7 and mono azo dye; MO) and two binary azo dyes (MO and RG19 mixture; AO7 and RG19 mixture) using commercial TiO2 under solar light irradiation for 6 h reaction time. Some operating parameters such as the effect of solar light irradiation, initial azo dye concentrations (10, 30, 50 mg/L), TiO2 loading (600, 1200, 2000, 3000, 4000 mg/L), pH (1, 3, 6.8, 9, 11) and aeration (using air pump) were studied in the single and binary azo dye aqueous solutions. In the single dye system, the decolourization efficiency of RG19, AO7 and MO was 22 %, 12 % and 8 %, respectively, under dark condition after 6 h reaction time following the sequence of molecular weight of RG19 > AO7 > MO (1418.92 g/mol > 350.32 g/mol > 327.33 g/mol). Besides, the photodegradation of RG19 was 100 % after 2 h reaction time, AO7 was 100 % and MO was 60 % after 6 h reaction time under solar light irradiation. In the binary system, the decolourization efficiency of RG19 and AO7 was 18 % and 4%, respectively, under dark condition, while the photodegradation efficiency was 98 % and 90 % under solar light irradiation after 6 h reaction time for AO7 and RG19 mixture. Besides, the decolourization efficiency of RG19 and MO was 16 % and 2 % under dark condition and that was 92 % and 56 % under solar light irradiation after 6 h reaction time for MO and RG19 mixture, respectively. The second part of this study was focused on the preparation of synthesized TiO2, metal (zinc, chromium, nickel and iron) doped TiO2 and non-metal (nitrogen) doped TiO2 by sol-gel assisted reflux method. After that, the synthesized photocatalysts were characterized by using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and nitrogen sorption analysis, respectively. Besides, the performances of synthesized photocatalysts on photodegradation of azo dyes were studied in two operating parameters such as the effect of solar light irradiation and different azo dyes. Among the performance of synthesized photocatalysts, the fastest photodegradation of azo dyes was investigated by using Zn-doped TiO2. Therefore, the performance of commercial TiO2 and synthesized Zn-doped TiO2 was compared for photodegradation of azo dyes under solar light irradiation. Overall, all water samples were analysed by using UV-Vis spectrophotometer. The kinetics study of photodegradation of azo dye was evaluated by using Langmuir-Hinshelwood model. Chemical oxygen demands (COD) analysis was used to evaluate the mineralization of azo dye under solar light irradiation. Finally, the photodegradation and mineralization of azo dye using commercial TiO2 and synthesized photocatalysts were compared under solar light irradiation. The mineralization of azo dye was slower than the photodegradation of azo dye in this study.
