Activation of peroxydisulfate by natural titanomagnetite for atrazine removal via free radicals and high-valent iron-oxo species

Natural titanomagnetite, a potential catalyst, was used as a peroxydisulfate (PDS) activator for the removal of atrazine (ATZ) in water for the first time. The surface Fe(II) and Fe(III) of titanomagnetite was certified as the active components by XRD and XPS analyses. ATZ removal showed a strong dependence on the titanomagnetite concentration (0-10 g/L), PDS concentration (0-10 mM) and the initial pH (3.0-11.0) under experimental conditions. Quite different from the traditional heterogeneous PDS-activated processes, titanomagnetite/PDS system could remove ATZ via both radical and nonradical pathways. Here, both free radicals (SO4 center dot- and HO center dot) and high-valent iron-oxo species were proved to be the main reactive oxygen species (ROS) by quenching experiments, EPR tests and UPLC/ESI-MS/MS analysis. The high-valent iron-oxo species, Fe-IV=O and Fe-V=O, could be generated from the Fe(II) and Fe(III) of the titanomagnetite via a nonradical pathway. In addition, the generated passivation layer after reaction was found via XPS analysis, which could block the function of Fe(II) and Fe(III), hindering the generation of ROS. However, the catalytic activity of titanomagnetite could recover if passivation layer was removed by ultrasonic cleaning. Finally, ten intermediates of ATZ were measured by UPLC-QTOF-MS/MS and the degradation pathways of ATZ in titanomagnetite/PDS system were proposed with the Gaussian 09 results. This work helps to better understand the generation of high-valent iron-oxo species and re-evaluate the effect of high-valent iron-oxo species in the heterogeneous iron-based PDS-activated processes.