Unexpected Mn(V) Generation in Mn(II)/Sulfite System for Efficient Water Decontamination: Critical Role of Complexing Ligands in Regulating Mn(II) Activity

In recent times, advanced oxidation processes (AOPs) based on sulfite activation via transition metal ions have gained significant attention for water decontamination. In this work, we unexpectedly discovered that Mn(II) was inefficient in sulfite activation for water treatment. Intriguingly, the introduction of amino ligands such as nitrilotriacetic acid and picolinic acid significantly enhanced the performance of Mn(II) in sulfite activation, enabling the effective abatement of contaminants. By combining quenching, chemical probing, ¹⁸O isotope tracing, and electrochemical experiments, this study addressed why the Mn(II)/sulfite system was sluggish in contaminants degradation, how Mn(V) was generated in the Mn(II)/sulfite/amino ligand system, and why different complexing ligands exhibited distinct performances. We demonstrated that the amino ligand notably enhanced the activity of Mn(II) with the production of stabilized Mn(III), which underwent further conversion to Mn(V) species, resulting in the rapid degradation of contaminants. This study represents the first discovery of the unexpected Mn(V) formation from low-valence manganese in a sulfite-based system. Furthermore, the spectral characteristics of Mn(V) species under environmental pH conditions were identified for the first time. These findings introduce a novel oxidation process for water decontamination and will broaden our understanding of sulfite-activation-based AOPs as well as the application of manganese chemistry in water treatment for decontamination and beyond.