Photocatalytic degradation of three dyes by UV/H2O2, UV/TiO2/H2O2, and UV/TiO2 processes

This study investigates the photocatalytic decolourisation of three direct dyes commonly used in textile effluents, Solophenyl Red 3BL (DR), Solophenyl Orange TGL (DO), and Solophenyl Brilliant Blue BL (DB), using TiO2 (P25) and ZnO under UV irradiation. The effects of adsorption, initial dye concentration, pH, and hydrogen peroxide dosage were systematically examined to determine their influence on the degradation process. TiO2 (P25) exhibited higher adsorption capacities and superior photocatalytic activity for DR and DB, whereas ZnO was more effective for DO. The addition of hydrogen peroxide enhanced degradation efficiency up to an optimal concentration of 250 mg/L by promoting hydroxyl radical generation, beyond which an inhibitory effect was observed. pH strongly influenced dye removal, with acidic conditions favouring DB degradation and both acidic and basic conditions being optimal for DR and DO. Kinetic analysis confirmed that the photocatalytic degradation of all three dyes over TiO2 (P25) follows the Langmuir–Hinshelwood model, indicating a surface-controlled mechanism. The results highlight the crucial role of dye structure, adsorption behaviour, and operating parameters in optimising photocatalytic treatment for textile wastewater decolourisation.