Roles of intrinsic Mn3+ sites and lattice oxygen in mechanochemical debromination and mineralization of decabromodiphenyl ether with manganese dioxide

Abstract

Commercial β-MnO2 with a chemical formula of approximate Mn0.774+Mn0.233+O1.88 was used for mechanochemical (MC) oxidative degradation of decabromodiphenyl ether (BDE209). The ball milling process initiated the degradation of BDE209 on β-MnO2, yielding a nearly complete degradation and debromination of BDE209 within 2 h. The use of β-MnO2 exhibited much higher MC debromination efficiency than that by using birnessite (δ-MnO2, 40.2%), Bi2O3 (45.6%), CaO (65.3%), and persulfate (81.9%). It was demonstrated that the oxidative degradation of BDE209 was promoted by the redox half reactions of both Mn4+→ Mn3+ and Mn3+→ Mn2+, but naturally existed Mn3+ centers on the surface of β-MnO2 functioned as dominant reactive species at the initial stage of the MC degradation (often before the degradation efficiency of BDE209 achieved 50%). Moreover, the surface lattice oxygen of MnO2, rather than O2, played a key role in the debromination and mineralization of BDE209. The Mn3+ sites on β-MnO2 not only easily accepted the electron of BDE209, but also promoted the mobility of lattice oxygen from the bulk to the surface for mineralizing BDE209. These results firstly highlighted the importance of Mn3+ availability and oxygen mobility on the reactivity of manganese oxide for the MC oxidative degradation of organic pollutants.