Oxalate regulate the redox cycle of iron in heterogeneous UV-Fenton system with FeO nanoparticles as catalyst: Critical role of homogeneous reaction.

The redox cycle of iron is a well-known rate-determining step for hydroxyl radical generation in photo-Fenton system. In this study, oxalate was employed as regulator to enhance the degradation of Orange II in FeO magnetic nanoparticles (NPs)-catalyzed heterogeneous UV-Fenton system. Results showed that the oxalate could interact with the surface ≡Fe species of catalyst, which weakened the bond of ≡Fe-O and promoted the leaching of iron ions. Then the redox cycle of iron and generation of HO· would be accelerated via the homogeneous UV-Fenton reaction. The degradation rate constant of Orange II reached 0.220 min when additional oxalate concentration was 0.4 mM, which was 2.5 times as high as that without oxalate in heterogeneous UV-Fenton system. In this case, the removal efficiencies of color and TOC were 99.3% and 92.0% after 30 and 120 min treatment, respectively. In addition, based on the results of XRD and XPS characterization, it could be deduced that the crystal structure and elemental configuration of FeO magnetic nanoparticles could be maintained after reaction. Besides, the results of FTIR and magnetization characterization indicated that the CO on surface of catalyst could be degraded and the catalyst could be easily separated from aqueous by applying an external magnetic field. The FeO magnetic nanoparticles showed high catalytic stability and reusability under the regulation of oxalate due to the fact that the leached iron ions could be re-adsorbed on the catalyst after treatment.