Visible-light activation of sulfite by ZnFe2O4@PANI photocatalyst for As (III) removal: The role of radicals and Fe(IV)

Inorganic arsenic (As) exists widely in industrial wastewater and poses a serious risk to human health. Here, sulfite was employed to enhance the photocatalytic oxidation ability, and As(III) containing wastewater was rapidly purified within 30 min. Furthermore, this photocatalytic system maintain excellent performance at wide pH range of 3-10. By measuring intermediate radicals and ions, pH-dependent mechanism of sulfite activation was proposed. Under acid conditions, sulfite radicals (center dot SO3-) were detected in photocatalytic system, and quickly convert into oxidizing radicals (center dot SO4- and center dot OH), which were responsible for As(III) oxidation. While at alkaline conditions, only tetravalent iron [Fe(IV)] was detected as oxidizing substance contributing to As(III) removal during photo reaction process. Moreover, Fe(IV) mainly existed on catalyst surface rather than solution, the limited Fe leaching could extend the service life of catalyst, which should be attributed to the core-shell structure of ZnFe2O4@PANI. These findings in this work advances the fundamental understanding of radicals conversion and Fe(IV) formation during sulfite activation process.