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 m(2) g(-1)) and large pore volume (0.416 cm(3) 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 H2O2 via 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.