Iron Enhancing\nSuperoxide-Mediated Mn(II) Oxidation\nby Peroxymonosulfate: Elucidating the Role of Superoxide Radicals

The effective removal of soluble Fe(II) and Mn(II) is\nproblematic\nin water supply utilities. This study explored the oxidation behavior,\nkinetics, and reaction mechanisms of using peroxymonosulfate (PMS)\nto mediate the co-oxidation of Fe(II) and Mn(II) in natural water.\nAt [Fe(II)] and [Mn(II)] of 1 mg/L, PMS oxidized all Fe(II) spontaneously\nwithin 15 s, irrespective of the oxidant concentration (50–500\nμM) and solution pH (6–9), while it required 7–30\nmin for complete Mn(II) oxidation, indicating its distinctive behavior\nin reacting with Fe(II) and Mn(II). Scavenging assays and electron\nparamagnetic resonance (EPR) analysis revealed the dominant presence\nof O<sub>2</sub><sup>•–</sup> in the system. EPR analysis\ncombined with chemical probing experiments using nitroblue tetrazolium\nchloride suggested that O<sub>2</sub><sup>•–</sup> was\nproduced exclusively via surface reactions of ferric oxide with PMS.\n PMS co-oxidation eventually yielded amorphous hydrous manganese-bearing\nferric co-oxides (hMnFeO<sub><i>x</i></sub>), with increasing\nMn:Fe compositional ratios over time and pH, i.e., Mn<sub>0.31</sub>Fe<sub>0.69</sub>, Mn<sub>0.67</sub>Fe<sub>0.33</sub>, Mn<sub>0.93</sub>Fe<sub>0.07</sub> at pH 7 and Mn<sub>0.68</sub>Fe<sub>0.32</sub>,\nMn<sub>0.89</sub>Fe<sub>0.11</sub>, Mn<sub>0.90</sub>Fe<sub>0.10</sub> at pH 9. The co-occurrence of Fe(II) provided hydrous FeO<sub><i>x</i></sub> surfaces enriched with chemisorbed oxygen (∼60%),\nacting as nucleation sites for the heterogeneous MnO<sub><i>x</i></sub> oxidation through enhanced electron transfer and surface complexation\npathways. This co-occurrence thus reduced the half-life time of PMS-induced\nMn(II) oxidation, by 5.3–18.7 times compared to the Mn(II)\noxidation alone. This study provides fresh evidence, underscoring\nthe significance of O<sub>2</sub><sup>•–</sup> in PMS-mediated\nmetal oxidation systems.