Hierarchically porous MnO2 microspheres with enhanced adsorption performance

Abstract

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.