Reactions of Mn(II) with common oxidants: Kinetics, pathways, and key role of Mn(III)/Mn(IV) products in enhanced contaminants degradation

Transition metal activation represents a flexible strategy to enhance oxidant reactivity for aqueous contaminants abatement. Soluble manganese ions (Mn(II)) have garnered increasing attention owing to the redox activity, low toxicity, and high natural abundance. Although Mn(II) reacts with multiple oxidants with a broad spectrum of rate constants, its activation capability is relatively limited. Generally, colloidal manganese dioxide (cMnO2) is generated as the stable oxidation product of Mn(II), exhibiting pronounced catalytic promotion of pollutant oxidation via its surface interactions with oxidants and/or organics. Introduction of complexing ligands not only influences the reactivity of Mn(II) and its reaction kinetics with oxidants but also affects the reaction pathway, typically shifting the mechanism from one-electron to two-electron transfer. This converts Mn(II) to ligand-stabilized Mn(III), which is the key intermediate for high-valent Mn(V)-oxo species generation. Alternative pathways involving the Mn(IV)-oxo species are also proposed. These reactive manganese species enhance pollutant removal efficiency in Mn(II)/ligand/oxidant systems. Collectively, Mn(II) and its derivatives Mn(III)/Mn(IV) engage in intricate reactions with diverse oxidants. Elucidating the reaction chemistry is of paramount importance for advancing the scientific understanding and practical applications of Mn-based oxidation technologies. So, this work provides an integrated review on this topic. Reaction pathways and kinetics between Mn(II) and common oxidants (i.e. H2O2, persulfates, HOCl, ClO2, periodate (PI), peracetic acid (PAA), Mn(VII), Fe(VI), and O3) were first summarized. Mechanisms of cMnO2 surface-catalyzed pollutant degradation by oxidants were interpreted. The pivotal role of Mn(III) in ligand-assisted Mn(II)/oxidant systems was discussed. Finally, knowledge gaps in this field were identified.