Photocatalytic degradation of sulfamethoxazole by hierarchical magnetic ZnO@g-C3N4: RSM optimization, kinetic study, reaction pathway and toxicity evaluation

Highlights: • In situ growth of g-C₃N₄ on hierarchical magnetic ZnO obtained nanocomposite. • Operating parameters were optimized by response surface methodology. • Partial mineralization and high loss of antibacterial activity follow SMX removal. • Degradation pathways include cleavage of S-N bound and sulfone moiety abstraction. • Hierarchical photocatalysts are easily separated by external magnetic force. - Abstract: The degradation of sulfamethoxazole (SMX) by a synthesized hierarchical magnetic zinc oxide based composite ZnO@g-C₃N₄ (FZG) was examined. Hierarchical FZG was synthesized by using Fe₃O₄ nanoparticle as the magnetic core and urea as the precursor for in situ growth of g-C₃N₄ on the surface of petal-like ZnO. The effect of catalyst dosage (0.4-0.8 g/L), solution pH (3–11) and airflow rate (0.5-2.5 L/min) on the SMX removal efficiency and the optimization of process was studied by response surface methodology (RSM) based on central composite design (CCD). The obtained RSM model with R² = 0.9896 showed a satisfactory correlation between the predicted values and experimental results of SMX removal. Under the optimum conditions, i.e. 0.65 g/L photocatalyst concentration, pH = 5.6 and airflow rate = 1.89 L/min, 90.4% SMX removal was achieved after 60 min reaction. The first-order kinetic rate constant for SMX removal by using FZG was 0.0384 min⁻¹ while the rate constant by commercial ZnO was 0.0165 min⁻¹. Moreover, under the optimum conditions, about 64% COD removal and 45% TOC removal and a considerable reduction in toxicity were observed. The analysis of generated intermediates during the photocatalytic degradation of SMX was conducted by LC-HR-MS/MS method and a degradation pathway was proposed.