Formation and Oxidation Reactivity of MnO<sub>2</sub><sup>+</sup>(HCO<sub>3</sub><sup>–</sup>)<sub><i>n</i></sub> in the Mn<sup>II</sup>(HCO<sub>3</sub><sup>–</sup>)–H<sub>2</sub>O<sub>2</sub> System
Zi-He Meng, Song‐Hai Wu, Shiwei Sun, Zhi Xu, Xiaocong Zhang, Xiangming Wang, Yong Liu, Hai‐Tao Ren, Shao‐Yi Jia, He Bai, Xu Han
Abstract
The MnII(HCO3–)–H2O2 (MnII-BAP) system shows high reactivity toward oxidation of electron-rich organic substrates; however, the predominant oxidizing species and its formation pathways involved in the MnII-BAP system are still under debate. In this study, we used the MnII-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 MnII(HCO3–)n [including MnII(HCO3)+ and MnII(HCO3)2] is a key factor in the MnII-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–)n intermediate generated in the reaction between MnII(HCO3–)n 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)-MnII-BAP system. In addition, the presence of Mn(III) further indicates the important Mn(II)–Mn(III) cycling in the MnII-BAP system. We therefore suggest two important roles of MnII(HCO3–)n in the MnII-BAP system: (i) MnII(HCO3–)n reacts with H2O2 to form the MnIII(HCO3)3 intermediate, followed by a subsequent reaction between MnIII(HCO3)3 and H2O2 to produce O2•–; (ii) MnII(HCO3–)n can also stabilize O2•– with the formation of MnO2+(HCO3–)n. MnO2+(HCO3–)n is an electrophilic reagent and plays the predominant role in the oxidation of As(III) to As(V).