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 sys-tematically 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 & alpha;-MnO2, & beta;-MnO2, & delta;-MnO2, & gamma;-MnO2 and & lambda;-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.