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Bi2+xWO6 is a cost-effective and environmentally friendly photocatalyst that shows high reactivity in the oxidation of various contaminants under visible light. However, under alkaline conditions, the reactive oxidative species in the Bi2+xWO6 system are still not clear yet. In this study, it is observed that the oxidation rates of As(iii) increase with increasing pH values in the Bi2.15WO6 system. Photoluminescence and the Mott-Schottky analyses confirm that OH- promotes the separation and transfer of photogenerated electron-hole pairs over Bi2.15WO6, thus facilitating the oxidation of As(iii). Electron spin resonance spectra analysis and quenching experiments rule out contributions of •OH, O2˙-, 1O2 and superoxo species to As(iii) oxidation and indicate that surface -OOH and/or H2O2 are indeed the predominant species under alkaline conditions. The improved production of H2O2 by H-donors such as glucose and phenol, as well as the UV-vis diffuse reflectance and Raman analyses, further confirms the formation of surface -OOH on Bi2.15WO6 under alkaline conditions. In the dark, the significant higher oxidation rate of As(iii) by H2O2-Bi2.15WO6 than that by H2O2 alone reveals that surface -OOH, instead of H2O2, plays an important role in As(iii) oxidation. This study enriches our understanding of the diversity of reactive oxygen species (ROS) in the Bi2.15WO6 system and gives new insight into the mechanism involved in the oxidation of As(iii) under alkaline conditions.

The Mn II (HCO 3 - )-H 2 O 2 (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), Mn 2+ , and HCO 3 - are common components in As(III)-contaminated groundwater. Kinetic results show that Mn II (HCO 3 - ) n [including Mn II (HCO 3 ) + and Mn II (HCO 3 ) 2 ] is a key factor in the Mn II -BAP system to oxidize As(III). Quenching experiments rule out contributions of OH • and 1 O 2 to As(III) oxidation and reveal that O 2 •- and the oxidizing species generated from O 2 •- play predominant roles in the oxidation of As(III). We further reveal that the MnO 2 + (HCO 3 - ) n intermediate generated in the reaction between Mn II (HCO 3 - ) n and O 2 •- , instead of O 2 •- , is the predominant oxidizing species. Although CO 3 •- also contributes to As(III) oxidation, the high reaction rate constant between CO 3 •- and O 2 •- indicates that CO 3 •- 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 (HCO 3 - ) n in the Mn II -BAP system: (i) Mn II (HCO 3 - ) n reacts with H 2 O 2 to form the Mn III (HCO 3 ) 3 intermediate, followed by a subsequent reaction between Mn III (HCO 3 ) 3 and H 2 O 2 to produce O 2 •- ; (ii) Mn II (HCO 3 - ) n can also stabilize O 2 •- with the formation of MnO 2 + (HCO 3 - ) n . MnO 2 + (HCO 3 - ) n is an electrophilic reagent and plays the predominant role in the oxidation of As(III) to As(V).