New insight into the reactivity of Mn(III) in bisulfite/permanganate for organic compounds oxidation: The catalytic role of bisulfite and oxygen

A recently discovered bisulfite(HSO₃⁻)/permanganate(MnO₄⁻) system was reported to produce highly reactive free Mn(III) that can oxidize organic compounds in milliseconds. However, this characteristic reactivity was not found in all other known reaction systems that can also produce free Mn(III). Why can Mn(III) in NaHSO₃/KMnO₄ be so active? Here, we found NaHSO₃ and O₂ acted as catalysts for the reaction between Mn(III) and organic compounds. Without O₂, 0% of organic compounds were oxidized in NaHSO₃/KMnO₄, indicating the absence of O₂ inactivated Mn(III) reactivity. When the reaction between NaHSO₃ and KMnO₄ was monitored in air, Mn(III) catalyzed rapid oxidation of NaHSO₃ by O₂. Then, the Mn(III) that could oxidize organic compounds was found to be the ones involved in the catalytic reaction between NaHSO₃ and O₂, thus the link between O₂ and Mn(III) reactivity was established. Finally, NaHSO₃/O₂ can be viewed as catalysts for the reaction between Mn(III) and organic compounds because 1) when Mn(III) was involved in oxidizing organic compounds, it stopped being the catalyst for the reaction between NaHSO₃ and O₂ so that they were consumed to a much smaller extent; and 2) without NaHSO₃ and O₂, Mn(III) lost its oxidation ability. To the best of our knowledge, this is the first report on “catalytic role exchange” where Mn(III) is the catalyst for NaHSO₃/O₂ reaction while NaHSO₃/O₂ are the catalysts for Mn(III)/organic compounds reaction. Understanding the critical role of oxygen in NaHSO₃/KMnO₄ will enable us to apply this technology more efficiently toward contaminant removal.