Progress in MnO₂/MnO₂-based materials catalytic ozonation process for water and wastewater treatment

Heterogeneous catalytic ozonation (HCO) utilizes catalysts to enhance the adsorption and decomposition of ozone (O₃), promote the formation of reactive oxygen species (ROS), and improve the removal of organic compounds, thereby overcoming some disadvantages of ozonation. MnO₂/MnO₂-based materials are widely used as catalysts for HCO due to their multi-valent Mn species, environment friendliness, abundant resources, and high efficiency. This review aims to provide an overview of the advancements in HCO using MnO₂/MnO₂-based materials, focusing on their preparation, structural characteristics, catalytic performance, and proposed mechanisms. In particular, the effects of MnO₂ synthesis methods on the crystalline structure and morphology of catalysts are discussed. Then, the catalytic performances of various catalysts involving different phases, morphologies, and facets are compared. Subsequently, the enhanced applications of MnO₂-based catalysts in HCO for water treatment are described, including metals doping, metal oxides combination, and MnO₂-carrier. Furthermore, approaches of ROS identification are clarified, and the mechanisms of strengthening catalytic ozonation efficiency by MnO₂/MnO₂-based catalysts are summarized, containing redox couple theory, oxygen vacancy theory, complexation theory, and surface hydroxyl theory. Finally, the potential applications and perspectives of MnO₂/MnO₂-based catalysts are proposed. This review plans to bridge the gap between research and practical applications, providing new insights into the application of HCO technologies in water treatment.