Photocatalytic Degradation of Azo Dyes over Magnetite-Based Catalyst: Kinetic, Thermodynamic and Structure-Activity Studies

Textile wastewater containing dyes pose significant environmental hazards. Advanced oxidative processes, especially the heterogeneous photo-Fenton process, are effective in degrading a wide range of contaminants due to high conversion rates and ease of catalyst recovery. This study evaluates the photocatalytic degradation of azo dyes Acid Red 18 (AR18), Acid Red 66 (AR66) and Orange 2 (OR2) using magnetite as a catalyst. The magnetic catalyst was synthesized via a hydrothermal process at 150°C. Experiments were conducted at room temperature, investigating the effect of catalyst dosage, pH, and initial concentrations of H2O2 and AR18 dye. Kinetic and thermodynamic studies were performed at 25, 40 and 60°C for the three azo dyes (AR18, AR66 and OR2) and the effect of the dye structures on the photocatalytic efficiency was investigated. At 25 °C for 0.33 mmol.L-1 of dyes at pH 3.0, using 1.4 g.L-1 of catalyst and 60 mg.L-1 of H2O2 under a UV radiation of 16.7 mW.cm-2 the catalyst showed a 62,3% degradation for AR18, 79.6% for AR66, and 83.8% for OR2 in 180 minutes of reaction. The oxidation of azo dyes under these conditions is spontaneous and endothermic. The pseudo-first order kinetic constants indicated a strong temperature dependence with an order of reactivity of the type: OR2 > AR66 > AR18, which is associated to the characteristics of molecular complexity (size, steric hindrance, aromatic conjugation), electrostatic repulsion and the nature of the acid-base interactions on the catalytic surface.