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

Abstract

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.