Visible Light Photocatalytic Ozonation of Oxalic Acid by ${\mathrm{MnO}}_{\mathrm{x}}\text{-}\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ Composite

${\mathrm{MnO}}_{\mathrm{x}}\text{-}\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ composite is prepared and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray fluorescence (XRF), ultraviolet–visible (UV-vis) spectroscopy, and surface area analysis. The prepared ${\mathrm{MnO}}_{\mathrm{x}}\text{-}\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ composite is used as a catalyst to examine the performance of visible light photocatalytic ozonation. The results prove that the ${\mathrm{MnO}}_{\mathrm{x}}\text{-}\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ composite has good catalytic activity for oxalic acid removal during the photocatalytic ozonation process. Compared with bare $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$, the $1:20{\mathrm{MnO}}_{\mathrm{x}}\text{-}\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ increases the removal of oxalic acid from 24.8 to 91.2% in 60 min. The manganese combined in the $\mathrm{g}\text{-}{\mathrm{C}}_3{\mathrm{N}}_4$ mainly exists in the form of oxide with valences of III and IV. The oxalic acid removal is partly through the oxidation by reactive oxygen species such as ${}^{·}{\mathrm{O}}_2^{-}$ and partly through the charge transfer between different valences of manganese species, whereas ${}^{·}\mathrm{OH}$ accounts for only a negligible part of the oxalic acid removal.