Oriented redox pathways of Fe(VI) and Mn(II) by ligand engineering towards different reactive intermediate generation and micropollutant degradation

Ferrate (Fe(VI)) activation by Mn(II) has been proven as a powerful technology due to the high catalytic activity of Mn(II) and the generation of multiple intermediates. However, the reactivity and stability of these intermediates greatly differ, thus necessitating the precise regulation of redox pathways towards the generation of the most desirable intermediates. In this study, we utilized ligand engineering to finely tune the redox pathways between Fe(VI) and Mn(II). Mn(II) complexing with ethylenediaminetetraacetic acid (EDTA) and nitrotriacetic acid (NTA) switched the redox pathways from generating Mn(IV) to more robust intermediates and enhanced the decontamination ability of Fe(VI). Ligand-to-metal charge transfer promoted the electron migration from Mn(II) complexes to Fe(VI), generating Fe(V) and Mn(III) intermediates. Then, complexing regulated the transformation of Mn(III) to generate different reactive species. Specifically, Mn(II)-EDTA activated Fe(VI) through Fe(IV)/Fe(V) and Mn(V), whereas the Fe(VI)-Mn(II)-NTA system relied on Mn(VII). Owing to the weaker chelating effect of NTA, unstable Mn(III)-NTA was successively oxidized via Mn(V) to Mn(VII). In contrast, the stronger stability of EDTA suppressed the excessive oxidation of Mn(V)-EDTA, boosting pollutant interception of reactive species. Accordingly, the Fe(VI)-Mn(II)-EDTA process delivered stronger activation and wider applicability, diminishing contaminant toxicity and resisting background interference. This study provides valuable insights into Fe(VI) activation induced by transition metal complexes and serves as a reference for selecting efficient strategies to avoid excessive consumption of oxidative capacity.