Permanganate Reduction by Hydrogen Peroxide: Formation of Reactive Manganese Species and Superoxide and Enhanced Micropollutant Abatement

Permanganate (MnO4-, Mn(VII)) is widely applied at the initial stage of water treatment to, e.g., abate taste and odor compounds, Mn(II) and Fe(II). However, its selectivity limits its application for micropollutant abatement. Hydrogen peroxide (H2O2), which is commonly applied in advanced oxidation processes, was found to quickly react with Mn(VII) in the pH-range 6.0-8.5. A lag-phase was followed by a rapid reduction of Mn(VII) to Mn(VI), with a proposed catalysis by the deprotonated form of Mn(VI), for the electron transfer of the [H2O2-OMnO3](-) complex. At molar [H2O2](0):[Mn(VII)](0) ratios <= 1 and pH 7.5, a one-electron reduction of Mn(VII) by H2O2 potentially formed Mn(VI) and superoxide radical (O-2(center dot-)), with O(2)(center dot- )yields approaching 100% based on the consumed Mn(VII). Mn(VII)-H2O2 significantly enhanced the abatement of the Mn(VII)-resistant compound ciprofloxacin and its transformation products, and quenching of Mn(VI) by Ba-2(+) completely inhibited this enhancement effect. At molar [H2O2](0):[Mn(VII)](0) ratios of 2-10, O-2(center dot-) yields of 145-245% indicate that other H2O2-reactive manganese species might also be formed. Among them, Mn(II) was found to produce O-2(center dot-) by reaction with H2O2 at a relatively low rate with overstoichiometric yields. Overall, this study provides evidence that a combination of Mn(VII) with H2O2 might be a novel option for enhanced micropollutant abatement in water treatment.