Formation and Oxidation Reactivity of MnO₂⁺(HCO₃^-) in the Mn^II(HCO₃^-)-H₂O₂ System

The Mn^II(HCO₃^-)-H₂O₂ (Mn^II-BAP) system shows high reactivity toward oxidation of electron-rich organic substrates; however, the predominant oxidizing species and its formation pathways involved in the Mn^II-BAP system are still under debate. In this study, we used the Mn^II-BAP system to oxidize As(III) in that As(III), Mn²⁺, and HCO₃^- are common components in As(III)-contaminated groundwater. Kinetic results show that Mn^II(HCO₃^-) [including Mn^II(HCO₃)⁺ and Mn^II(HCO₃)₂] is a key factor in the Mn^II-BAP system to oxidize As(III). Quenching experiments rule out contributions of OH^• and ¹O₂ to As(III) oxidation and reveal that O₂^•- and the oxidizing species generated from O₂^•- play predominant roles in the oxidation of As(III). We further reveal that the MnO₂⁺(HCO₃^-) intermediate generated in the reaction between Mn^II(HCO₃^-) and O₂^•-, instead of O₂^•-, is the predominant oxidizing species. Although CO₃^•- also contributes to As(III) oxidation, the high reaction rate constant between CO₃^•- and O₂^•- indicates that CO₃^•- is not the predominant oxidizing species in the As(III)-Mn^II-BAP system. In addition, the presence of Mn(III) further indicates the important Mn(II)-Mn(III) cycling in the Mn^II-BAP system. We therefore suggest two important roles of Mn^II(HCO₃^-) in the Mn^II-BAP system: (i) Mn^II(HCO₃^-) reacts with H₂O₂ to form the Mn^III(HCO₃)₃ intermediate, followed by a subsequent reaction between Mn^III(HCO₃)₃ and H₂O₂ to produce O₂^•-; (ii) Mn^II(HCO₃^-) can also stabilize O₂^•- with the formation of MnO₂⁺(HCO₃^-). MnO₂⁺(HCO₃^-) is an electrophilic reagent and plays the predominant role in the oxidation of As(III) to As(V).