Cu₂O-promoted degradation of sulfamethoxazole by <i>α</i>-Fe₂O₃-catalyzed peroxymonosulfate under circumneutral conditions: synergistic effect, Cu/Fe ratios, and mechanisms

Abstract

To promote the application of iron oxides in sulfate radical-based advanced oxidation processes, a convenient approach using Cu₂O as a catalyst additive was proposed. Composite catalysts based on α-Fe₂O₃ (CTX%Cu₂O, X = 1, 2.5, 5, and 10) were prepared for peroxymonosulfate (PMS) activation, and sulfamethoxazole was used as a model pollutant to probe the catalytic reactivity. The results show that a synergistic catalytic effect exists between Cu₂O and α-Fe₂O₃, which was explained by the promoted reduction of Fe(III) by Cu(I). Iron K-edge X-ray absorption spectroscopy investigations indicated that the promoted reduction probably occurred with PMS acting as a ligand that bridges the redox centers of Cu(I) and Fe(III). The weight ratio between Cu₂O and α-Fe₂O₃ influenced the degradation of sulfamethoxazole, and the optimal ratio depended on the dosage of PMS and catalysts. With 40 mg L^-1 PMS and 0.6 g L^-1 catalyst, a pseudo-first-order constant of ∼0.019 min^-1 was achieved for CT2.5%Cu₂O, whereas only 0.004 min^-1 was realized for α-Fe₂O₃. Nearly complete degradation of the sulfamethoxazole was achieved within 180 min under the conditions of 40 mg L^-1 PMS, 0.4 g L^-1 CT2.5%Cu₂O, and pH 6.8. In contrast, less than 20% degradation was realized with α-Fe₂O₃ under similar conditions. The CT2.5%Cu₂O catalyst had the best stoichiometric efficiency of PMS (0.317), which was 4.5 and 5.8 times higher than those of Cu₂O (0.070) and α-Fe₂O₃ (0.054), respectively. On the basis of the products identified, the cleavage of the S-N bond was proposed as a major pathway for the degradation of sulfamethoxazole.