MnAOS-dependent catechol interfacial oxidation on birnessite: Dissolved Mn release, persistent organic radical formation and surface evolution

Mn AOS controlled the partitioning of catechol interfacial redox pathways. • Low-Mn AOS birnessite sustained late-stage catechol depletion and Mn release. • Persistent organic radicals revealed single-electron transfer during catechol oxidation. • Radical accumulation was decoupled from catechol depletion and Mn release. • Anions and organic acids reshaped Mn AOS -dependent interfacial reactivity. Phenolic compounds are reactive components involved in organic carbon transformation and contaminant fate in soils and sediments. Manganese oxides are important natural oxidants, yet how the Mn average oxidation state (Mn AOS ) regulates phenolic interfacial oxidation on birnessite remains unclear. Here, four birnessite samples with different Mn AOS values were synthesized, and catechol was used as a representative o-diphenolic compound to probe Mn AOS -dependent interfacial redox processes. All samples retained the layered δ-MnO 2 phase, whereas decreasing Mn AOS from 3.39 to 3.28 altered layer stacking, pore structure, local Mn–O environments, and initial electrochemical responses. Catechol was rapidly depleted in all systems with simultaneous dissolved Mn release. HB3 and HB4 showed stronger sustained transformation in the middle-to-late stage, with C t /C 0 decreasing to 0.09 and 0.07 at 360 min, respectively, and dissolved Mn reaching 3.02 and 3.13 mg L −1 . EPR detected oxygen-centered semiquinone-like persistent organic radicals near g = 2.0046, but radical intensities were not fully synchronized with terminal catechol depletion or Mn release. Post-reaction Raman, FTIR, SEM, and XPS further indicated organic deposition, Mn valence reduction, and changes in the surface morphology and oxygen environment. Weakly alkaline conditions enhanced catechol transformation, whereas HCO 3 - , HPO 4 2- , and citric acid suppressed the reaction. Overall, Mn AOS -dependent differences were observed in the stage-dependent coupling among catechol transformation, dissolved Mn release, persistent organic radical accumulation, and surface evolution. These findings identify Mn AOS as an important descriptor of birnessite interfacial reactivity toward phenolic compounds. Download: Download high-res image (195KB) Download: Download full-size image