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

Highlights: • Thermal NaBH₄ treatment is used to update the surface properties of α-MnO₂. • BV is in situ formed via the wash-away of B dopant during dye elimination process. • BV transforms DO into ROS without extra energy input. • Strong acidity, free Mn³⁺ site and large pore contribute to adsorptive dye removal. • The treated MnO₂ shows excellent Pb²⁺ adsorption capacity especially in acid matrix. 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 α-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 α-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 α-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 α-MnO₂ presented superiority over the pristine sample in adsorptive Pb²⁺ removal as well.