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

Abstract

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.