Multi-dimensional micro-nano scale manganese oxide catalysts induced chemical-based advanced oxidation processes (AOPs) in environmental applications: A critical review

As transition metal oxides, manganese oxides (MnOx) have emerged as a potent activator in various advanced oxidation processes (AOPs) for the removal of recalcitrant pollutants from the environment. This article systematically reviewed the activation performance and mechanisms of various MnOx catalysts in typical chemical-based AOPs, and summarized the practical applications. This review covers a wide range of MnOx with different origins (natural, synthetic and biogenic MnOx), dimensions (1D to 3D), sizes (nm to mm), crystallinity, structures (mainly α-MnO2, β-MnO2, δ-MnO2, γ-MnO2 and λ-MnO2), Mn valences (mainly + 2, +3 and + 4) and contents. The relation between MnOx properties and their activation roles was attempted to explore. The activation mechanisms and resulting radical and non-radical pathways in different MnOx-based AOPs for pollutant removal were investigated and compared in detail. The effects of crucial experimental parameters, e.g., pH, temperature, co-existing anions and organic matter, on contaminant degradation were assessed and explained. The practical applications of MnOx-induced AOPs are currently limited and focused on groundwater and soil remediation, and wastewater treatment. Finally, the conclusions were drawn and future perspectives were critically put forward.