Hierarchically porous MnO2 microspheres with enhanced adsorption performance

Hierarchically porous manganese dioxide (MnO2) microspheres were fabricated by a facile hydrothermal method using potassium permanganate as the precursor at different hydrothermal temperatures. The as-prepared samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption isotherms and Fourier transform infrared (FTIR) spectroscopy. Adsorption of methyl blue (MB) onto the as-prepared samples from aqueous solutions was investigated and discussed. It was found that MnO2 microspheres were composed of two levels of hierarchical porous organization, viz., mesopores (2–50 nm) and macropores (>50 nm). The equilibrium adsorption data of MB on the as-prepared samples was well fitted with the Freundlich isotherm model. The sample obtained at 80 °C displayed the highest adsorption capacity with 259.2 mg g−1. In addition, adsorption data were fitted using the pseudo-second-order kinetics equation, suggesting that pseudo-second-order kinetics could well represent the adsorption kinetics. The adsorption between MB and MnO2 was mainly attributed to the strong electrostatic attraction force. The as-prepared hierarchically porous MnO2 microspheres turned out to be an effective adsorbent for the removal of MB from effluent because of their unique hierarchical porous microstructure and high specific surface areas.