An enhanced Bi/nZVI activated molecular oxygen process for the degradation of sulfonamide antibiotics in a citrate buffering system†

Citric acid (CA) and sodium citrate (NaCA) have been effectively employed to synergize with bismuth-doped nanoscale zero-valent iron (Bi/nZVI) to degrade sulfonamide antibiotics (SAs) without the need for additional H₂O₂. In the integrated Bi/nZVI-CA/NaCA system, excellent oxidation activity of sulfamethazine (SM2), sulfadiazine (SD) and sulfamethoxazole (SMX) in the mixed solution was obtained. The bimetallic enhancement alongside ligand complexation significantly promoted Bi/nZVI to catalyze molecular oxygen and was conducive to the spontaneous generation of H₂O₂. Fe(II)[Cit]⁻ was formed in the CA/NaCA system, and then underwent a Fenton-like reaction with spontaneously produced H₂O₂ to achieve the oxidation of SAs. Long service life was confirmed by the results of characterization, electrochemical analysis, utilization rate (UR), electronic efficiency (EE) and cycling degradation experiments. In the Bi/nZVI-CA/NaCA system, two comparable degradation pathways (hydroxylation and SO₂ extrusion) for SM2, SMX and SD were obtained, while another degradation pathway for SMX was reflected in the opening of the N–O bond on the benzene ring. Additionally, post-reactive solution toxicity was assessed to ensure environmental safety. Overall, our findings provide a theoretical research basis for the effective elimination of SAs from contaminated environments.