Tailoring Fe 3d spin states for enhanced adsorption/activation of in situ generated H2O2 on S-doped magnetite@resorcinol-formaldehyde resins core-shell catalysts toward boosted cascade self-Fenton oxidation

Although photocatalytic self-Fenton oxidation technology enables sustainable utilization of oxidants through in situ H₂O₂ generation, the low concentration and sluggish diffusion of such H₂O₂ on the surface of self-Fenton catalysts substantially restrict the organic pollutant oxidation efficiency in self-Fenton systems. Herein, this study developed a core-shell sulfur-doped magnetite (S-Fe₃O₄) @ resorcinol-formaldehyde (RF) resins photo-assisted self-Fenton catalyst by tuning the electronic structure of iron-based catalysts through a sulfidation strategy. The introduction of sulfur species promoted the Fe(II)/Fe(III) redox cycle and significantly enhanced the adsorption and activation of H₂O₂, which is in situ generated from the visible-light-responsible RF resins thin shell. With all the merits above, this catalyst achieved efficient degradation of fluoroquinolone antibiotics, represented by ciprofloxacin (CIP). Radical quenching experiments and electron spin resonance (ESR) analysis identified •OH and •O₂⁻ as the dominant reactive oxygen species in the S-Fe₃O₄@RF-mediated self-Fenton reactions. Systematic characterizations and density functional theory (DFT) calculations revealed that the sulfidation treatment reduced the activation energy barrier of H₂O₂ to produce •OH by regulating the electronic state density of the iron species in the composite catalysts and the free radical-dominated degradation pathway of the pollutants was also clarified. The unique core-shell structure of the catalyst not only increased the exposure of active sites but also endowed it with a favorable magnetic recovery feature, providing an innovative strategy for the development of efficient and stable heterogeneous photocatalytic self-Fenton systems for the next generation of wastewater treatment.