Degradation difference of fluoroquinolones by vacuum ultraviolet (VUV) and VUV/Fe²⁺ processes: Performance, mechanism, and influencing factors

Vacuum UV (VUV) has attracted much attention because it effectively splits water to produce hydrogen peroxide (H₂O₂) and reactive oxygen species (ROS) in situ. So far, the formation and consumption pathways of H₂O₂ and its role in the generation of ROS in VUV and VUV/Fe²⁺ processes are not clear. Herein, added Fe²⁺ utilized in-situ generated H₂O₂ by VUV irradiation to accelerate the degradation and mineralization of three fluoroquinolones. The H₂O₂ and hydroxyl radical (HO^•) generated in situ under VUV irradiation were in a dynamic equilibrium state of mutual transformation, and H₂O₂ played an important role in VUV and VUV/Fe²⁺ processes. The fluorescent molecular probe experiments proved that both HO^• and hydroperoxyl radical (HO₂^•) existed in VUV and VUV/Fe²⁺ processes. Radical quenching experiments confirmed that HO^• and HO₂^• were the main ROS, and their contributions to norfloxacin (NOR) degradation were also quantified. Besides, organic and inorganic intermediate by-products of NOR degradation by VUV and VUV/Fe²⁺ were detected, and degradation pathways of NOR were also proposed. Added Fe²⁺ accelerated defluorination effect and the conversion of organic nitrogen to inorganic nitrogen during NOR degradation under VUV irradiation. Effects of initial pH, dissolved oxygen, dissolved organic matter and coexisting ions on VUV and VUV/Fe²⁺ were also compared. Moreover, energy consumption values of VUV/Fe²⁺ were always lower than that of VUV, and the average energy-saving rate of VUV/Fe²⁺ was 37.12%. Based on the satisfactory results achieved by VUV and VUV/Fe²⁺ in real waters, application strategies for VUV and VUV/Fe²⁺ were also proposed.