Synergistic enhancement of electron transfer in the photocatalysis/persulfate-oxidation system for highly efficient norfloxacin degradation

The photocatalysis/persulfate-oxidation hybrid (PPOH) system is recognized as an effective approach for eliminating organic contaminants. Nevertheless, designing highly stable, high-performance catalysts remains an ongoing challenge. In this study, graphene quantum dots (GQDs)-loaded Fe-doped NH₂-MIL-125 (Ti) (G_xNM (Fe_0.2Ti_0.8)) are synthesized via a co-modification strategy for norfloxacin (NOR) degradation in PPOH. The introduction of Fe doping not only enhances visible light responsiveness but also provides activation sites for peroxydisulfate (PDS), facilitating the photogenerated carrier transfer in the metal-to-metal charge transfer (MMCT) process between Ti-O-Fe. GQDs incorporation further suppresses photogenerated carrier recombination. In the PPOH system, NOR degradation by G₁₀NM (Fe_0.2Ti_0.8) achieves a rate of 80.1 % within 120 min, surpassing that of NH₂-MIL-125 (Ti) by a significant factor of 5.24. Free radical trapping and electron paramagnetic resonance (EPR) experiments confirm that •O₂^-, h⁺ and SO₄•^- are the primary active species responsible for NOR degradation. The synergy of Fe doping and the unique electron transfer properties of GQDs accelerates electron transfer, thereby enhancing the catalytic efficiency. Moreover, the developed composite exhibits outstanding stability and recyclability, maintaining a NOR removal rate of over 70.3 % even after five cycles. This work opens up new avenues for applying modified metal-organic frameworks (MOFs) in antibiotic contaminants removal through the PPOH approach.