Mn3O4-g-C3N4 composite to activate peroxymonosulfate for organic pollutants degradation: Electron transfer and structure-dependence

A novel heterogeneous manganese/graphitic carbon nitride (Mn3O4-CN) catalyst for activating peroxymonosulfate (PMS) was successfully assembled using alkali precipitation. The g-C3N4 improved the composite's surface morphology, micro-porous structure, surface area, and particle size distribution, and an electron-rich center with Mn site was created. The Mn3O4-CN/PMS system exhibited high efficiency and stability when the solution pH varied from 3.0 to 9.0, with more than 90% of p-acetaminophen (ACT) removal in 30 min under experimental conditions. A possible reaction mechanism was proposed, primarily involving electron transfer from Mn (II) and Mn (III) to PMS along with the generation of·O2- and 1O2, and the degradation of ACT was attributed to the 1O2. Specifically, the degradation rate of phenolic compounds varied with their molecular structure in the following order: ACT > bisphenol A (BPA) > p-cresol (MP) > p-chlorophenol (CP) > phenol (Ph) > p-nitrophenol (NP). Further, the density functional theory (DFT) calculations indicated that the phenols' degradation efficiency was related to their adsorption energy and Bader charge value. These results improved our understanding of the manganese-based PMS non-radical dominated process and provided a method for predicting the degradation performance of phenols for the first time.