Boron vacancies of mesoporous MnO2 with strong acid sites, free Mn3+ species and macropore decoration for efficiently decontaminating organic and heavy metal pollutants in black-odorous waterbodies

Abstract

Oxygen vacancy (OV), one frequently designed structural anion defect, can transport active oxygen species towards multi-phase pollutant oxidation by activating oxygen molecules or chemical oxidants. However, dissolved oxygen (DO), a desirable oxidant source for water organics degradation, is very difficult to be directly utilized by OV-abundant metal oxides without extra heat or light input. To solve this problem, in this study another anion defect, i.e., boron vacancy (BV), was successfully incorporated into ɑ-MnO2 via a two-step method, i.e., thermal NaBH4 treatment for boron doping followed by in situ wash-away of boron during organic dye elimination process. The NaBH4-modified ɑ-MnO2 exhibited much higher Rhodamine B (RhB) elimination than the original one. BV is capable of transforming DO into 1O2, OH and O2− radicals at 30 °C and without extra energy input, completely mineralizing RhB into inorganic carbon. Besides, NaBH4-induced strong acid sites, free Mn3+ species and large pore size contributed to adsorptive RhB removal. For the original ɑ-MnO2, surface lattice oxygen was the key oxidant for dye degradation. Malachite Green, Congo Red, Orange I and Methyl Blue were further tested as target substrates. The NaBH4-treated ɑ-MnO2 presented superiority over the pristine sample in adsorptive Pb2+ removal as well.