Manganese oxides for electrochemical adsorption of metal ions in aqueous environments: A comprehensive review from fundamentals to applications

The removal and recovery of metal ions from wastewater are crucial for environmental sustainability and resource management. Electrochemical adsorption emerges as a promising technology due to its simplicity, controllability, and eco-friendliness. Manganese oxides (Mn oxides), naturally abundant and electrochemically active, exhibit substantial adsorption capacities and selectivity for various metal ions in aqueous solutions, making them excellent candidates for this technology. This comprehensive review synthesizes the latest developments in Mn oxides-based electrochemical adsorption from fundamentals to applications. This review finds that the superior performance of Mn oxides stems from a synergy of multiple mechanisms. Beyond conventional electric double-layer adsorption, pseudocapacitive ion storage and surface redox reactions play a dominant role, offering a significant advantage in terms of both capacity and selectivity. The analysis reveals that this performance is intricately dependent on a delicate interplay between the material’s intrinsic properties (such as crystal structure, morphology, and average oxidation state), solution chemistry (including pH and the presence of co-existing ions), and operational parameters. Furthermore, this review provides a detailed overview of the diverse applications of Mn oxides electrodes, spanning not only wastewater treatment for heavy metals and radioactive ions but also crucial resource recovery endeavors such as seawater desalination, water softening, and lithium extraction. By offering a critical framework for understanding these complex mechanisms and identifying key influencing factors, this work provides a roadmap for the rational design of next-generation Mn oxides adsorbents and the future development of electrochemical metal ion recovery technologies.