A low-cost iron mining residue catalyst employed in antibiotic degradation: Parameter optimization, influence of photogenerated species and the identification of main transformation products

The ubiquitous presence of antibiotics in aquatic ecosystems due to their frequent and indiscriminate use has become a major global health concern, leading to high environmental bacteria and resistance gene development. In this context, this study aimed to evaluate the degradation of three antibiotics, ciprofloxacin (CIP), sulfamethoxazole (SMX) and tetracycline (TET), employing a mining residue as the photocatalyst. The mining residue was characterized by the Brunauer-Emmett-Teller (BET), X ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X ray diffraction (XRD) techniques, and exhibited high iron content (∼70%) predominantly Fe+2. A rotatable central composite design (RCCD) employing catalyst concentrations, H2O2 and pH as variables was applied, resulting in optimal catalyst and H2O2 concentrations of 277.01 mg L−1 and 65.37 mg L−1, respectively, at pH 3.4. These conditions achieved 89% and 83% SMX and CIP degradation rates and the TET concentration at 60 min of treatment was lower than the instrumental LOD, confirming the predominance of catalyst photo-Fenton reactions. When applied to a real effluent sample, 71% and 79% SMX and CIP degradation rates were observed, respectively, while no degradation value changes were noted for TET at 60 min. The main reactive species involved in the antibiotic degradation reactions were HO• for SMX, O2•- and h+ for CIP, and O2•- for TET. In total, 12 antibiotic transformation products (TPs) were identified by LC-QTOF MS. The most frequent TPs observed during antibiotic degradation were formed by hydroxylation, dealkylation, defluorination and hydrolysis of the sulfonamide bond. The investigated mining residue is, therefore, a low-cost and efficient alternative for the photocatalytic degradation of CIP, SMX and TET, contributing to the United Nations Sustainable Development Goals (SDG 6) and follows circular economy principles.