Decolourization of Azo Dyes Reactive Black 5, Acid Orange 7 and Acid Orange 20 from Synthetic Textile Effluent Using Soybean Peroxidase: Optimization and Kinetic Study

This study investigates the application of soybean peroxidase (SBP), an enzyme extracted from a soybean processing byproduct, for the decolourization and oxidative treatment of three industrial azo dyes: Acid Orange 7 (AO7), Acid Orange 20 (AO20), and Reactive Black 5 (RB5), each at a concentration of 50 µM. These dyes are widely used in textile, paper, and leather industries and persist in wastewater. Optimization experiments were conducted at room temperature (approximately 22°C) to examine the effects of pH, SBP activity, and hydrogen peroxide (H2O2) concentration. Optimal degradation conditions were identified as: for RB5, pH 3.5, 0.075 U/mL SBP, and 0.0375 mM H2O2; for AO7, pH 3.0, 0.5 U/mL SBP, and 0.0375 mM H2O2; and for AO20, pH 3.0, 0.0025 U/mL SBP (200-fold less than for the isomeric AO7) and 0.0625 mM H2O2. Under these conditions, dye removal kinetics followed pseudo-first-order behaviour during the initial stages, assuming constant SBP and H2O2 concentrations. However, accumulation of oligomeric products, depletion of H2O2 over time, and enzyme inactivation caused deviation from first-order kinetics. The initial rate constants and half-lives were 10.7 min-1 and 0.065 min (AO7), 7.3 min-1 and 0.095 min (AO20), and 8.5 min-1 and 0.081 min (RB5). When normalized to enzyme activity, AO7 showed the highest catalytic efficiency. More than 95% decolourization was achieved in under 30 seconds. These findings support the use of SBP as a low-cost, eco-friendly, and effective biocatalyst for the rapid treatment of dye-containing industrial wastewater.