Photochemical degradation of azo textile dyes by UV/H2O2 advanced oxidation process: kinetic, parametric and mechanistic insights.

Water contamination by textile effluents poses a critical environmental challenge, with azo dyes accounting for 60-70% of commercial dyes and exhibiting high resistance to biodegradation due to their extended conjugated π-systems. In this study, the degradation of two representative azo dyes, Solophenyl Blue (SB) and Cibacron Blue (CB), was investigated using a UV/H2O2 advanced oxidation process at 254 nm, generating hydroxyl radicals in situ. The influence of initial dye concentration (1-5 ppm), H2O2 dosage (17.6-35.2 mM), and solution pH (3-10) was systematically evaluated. Degradation efficiency strongly depended on these parameters, with acidic conditions (pH 3) and an optimized H2O2 concentration of 35.2 mM providing the highest removal rates, reaching up to 99.8% after 180 min of irradiation. Degradation kinetics followed a pseudo-first-order model with high correlation coefficients (R2 > 0.95 for all conditions, and >0.99 for most experiments), with apparent rate constants ranging from 0.293 to 0.686 h⁻1 for SB and 0.018 to 0.044 h⁻1 for CB, depending on initial dye concentration. UV-visible spectral analysis confirmed progressive azo chromophore cleavage, while density functional theory calculations revealed that SB exhibits a slightly higher electronic chemical potential (μ = -0.162 a.u.) and nucleophilicity index (N = 3.66 eV) compared to CB (μ = -0.164 a.u.; N = 3.48 eV), indicating greater electron-donating capacity and susceptibility to electrophilic HO• attack. From an environmental engineering perspective, the UV/H2O2 process offers advantages including ambient temperature operation, the absence of solid catalysts, and water as the reaction medium. Although further investigations using real wastewater matrices and mineralization analyses (COD/TOC) are required, the results demonstrate that UV/H2O2 is a promising technology for dye-contaminated wastewater treatment.