Nanostructured manganese oxides: natural/artificial formation and their induced catalysis for wastewater remediation

Manganese oxides, with low toxicity and wide adaptability, have been demonstrated as promising catalysts for substituting noble metals/oxides in a diversity of chemical reactions. In environmental remediation, manganese oxides can catalyze peroxides to produce reactive oxygen species (ROS) in an aqueous phase for in situ chemical oxidation (ISCO) and advanced oxidation processes (AOPs). The manganese oxides stand out among the transition metal oxides due to their inherent dissimilarity in redox properties, crystal structure, and surface nano-architectures. In this paper, a comprehensive review is presented on the formation of nanostructured manganese oxides in nature (abiotic oxidation and biogenic evolution) as well as their artificial synthesis with rationally controlled tunnels and layers, crystal structures, exposed facet orientations, dimensional architecture and oxidation states. We further overview the applications of nanostructured manganese oxides in activation of various peroxides for catalytic oxidation to destroy organic contaminants during water purification. The roles of manganese oxides are emphasized in catalytic activation of hydrogen peroxide (H2O2), ozone (O-3), and persulfates (peroxymonosulfate and peroxydisulfate). The mechanisms of the interactions between manganese oxides with the diverse peroxides and structure-dependent ROS production will be illustrated. The regulating rules of compositional alien-metal doping, formation of mixed metal oxides and hybrid materials are further discussed regarding the promoted catalytic activity. More importantly, both radical oxidation and nonradical pathways involved in manganese-based AOPs will be illustrated. Lastly, we will propose several prospects for future development of manganese oxides in practical applications.