Optimization of aqueous p-aminophenol degradation by external-loop airlift sonophotoreactor using response surface methodology

The combination of sonolysis and photolysis in the presence of hydrogen peroxide (H₂O₂) in a 7-L external-loop airlift sonophotoreactor was used to treat the aqueous solution of p-aminophenol. The central composite design (CCD) and response surface methodology (RSM) were employed to evaluate the interaction effects of the initial H₂O₂ concentration (x₁ = 100–900 mg/L), the ultrasonic power (x₂ = 25–65 W), the air flow rate (x₃ = 1–5 L/min), and the initial concentration of p-aminophenol (x₄ = 10–50 mg/L) on the p-aminophenol degradation and total organic carbon (TOC) reduction efficiencies as well as to optimize operating conditions. The coefficients of determination (R²) and adjusted-R² obtained from the analysis of variance (ANOVA) were 0.9900 and 0.9812 for the p-aminophenol degradation; and 0.9742 and 0.9516 for the TOC removal, respectively, ensuring a satisfactory adjustment of the quadratic regression model with experimental results. The linear, square, and interaction effects of x₁, x₂, x₃, and x₄ were also calculated. Genetic algorithm optimization was employed to maximize the mineralization efficiency. 79% TOC reduction efficiency after 90 min and 86.5% p-aminophenol removal efficiency after 30 min were achieved under recirculating batch mode at operating conditions of x₁ = 740 mg/L, x₂ = 65 W, x₃ = 5 L/min, and x₄ = 24 mg/L.