Oxidative removal of dissolved manganese(II) by peroxymonosulfate: Non-radical mechanism and influencing factors

In this study, the oxidative removal of Mn(II) by peroxymonosulfate (PMS) was investigated. PMS was found to be a more efficient oxidant than chlorine for Mn(II) oxidation. Weak autocatalysis kinetics were observed in PMS/Mn(II) system suggesting that the Mn(II) oxidation consisted of both homogeneous (direct oxidation) and heterogeneous (amorphous MnO2 catalytic oxidation) processes. Homogeneous oxidation of Mn(II) by PMS showed strong pH dependency with apparent second-order rate constants of 3.1–249.9 M−1s−1 at pH 7.0–9.0. Stoichiometry of PMS to Mn(II) was about 1:1 suggesting a two-electron transfer pathway (O transfer), which was further confirmed by the XPS analysis results. However, the change in the coordination structure of Mn(II) was found to greatly affect the oxidation rate and mechanism of Mn(II). Complexion with inorganic or organic ligands could inhibit the oxidation rate of Mn(II) and alter the oxidation mechanism from two-electron to one-electron pathway. Furthermore, coexisting metal ions such as Ca(II), Zn(II), and Fe(II)/Fe(III) ions could greatly accelerate the oxidation of Mn(II) by PMS and chlorine by promoting the heterogeneous oxidation process. Oxidative removal of Mn(II) by PMS was more efficient than chlorine in real waters (56 %–100 % versus 12 %–17 %). The combination of PMS oxidation with coagulation could further increase the removal efficiency from 56 % to 92 %. These findings highlight that PMS oxidation is an attractive alternative technology for Mn(II) removal and may also provide a fundamental understanding of the Mn materials-based advanced oxidation processes.