Facile synthesis of 3D hierarchical MnO₂ microspheres and their ultrahigh removal capacity for organic pollutants

Well-defined 3D hierarchical MnO₂ microspheres have been successfully prepared by a simple microwave heating method in just 10 minutes without using any templates, catalysts or organic reagents. The as-synthesized MnO₂ microspheres were characterized using XRD, XPS, BET, SEM, TEM and HRTEM methods. The 3D hierarchical MnO₂ microspheres have porous structures with high specific surface area (184.32 m² g^-1) and large pore volume (0.416 cm³ g^-1). The MnO₂ microspheres are composed of ultrathin nanosheets with thicknesses of around 2 nm. The MnO₂ microspheres exhibit exceptional catalytic performance for the degradation of methyl blue (MB) in aqueous solution, indicating their strong potential for wastewater treatment. The microspheres degrade 95% of MB (400 mg L^-1) within 25 minutes in the presence of H₂O₂via a Fenton-like reaction. The weight ratio of degraded MB to MnO₂ microspheres (40 mg mg^-1) is the highest among all reported catalysts for MB. The MnO₂ microspheres also demonstrate efficient performance as a reactive sorbent for organophosphate pesticides, with 90% of parathion methyl degraded within 100 minutes in non-polar organic solvent (n-heptane). The ultrahigh removal capacity can be ascribed to the high specific surface area, porous structure and ultrathin nanosheets that are the building blocks of the MnO₂ microspheres.