Hierarchical MnO₂ nanostructures: synthesis and their application in water treatment

Hierarchical MnO₂ nanostructures were prepared through the reaction between KMnO₄ and oleic acid at room temperature in the surfactant-free microemulsion system. The obtained samples were characterized by powder X-ray diffraction, N₂ adsorption, scanning electron microscopy, and transmission electron microscopy. The results indicated that the flowerlike nanospheres were three-dimensional (3D) porous microstructures consisting of nanoplates. The surface area of the sample was 171.5 m(2)/g and the distribution of pore diameter lay within the range of 5-15 nm. The prepared hierarchically structured MnO₂ showed excellent adsorption capacity and rapid adsorption rate for methylene blue ions in water. The maximum adsorption capacity of methylene blue was as high as 273.9 mg/g and 97.5% of the dye was removed within initial 5 min of contact time. Compared with other adsorbents, the synthesized hierarchical MnO₂ nanostructures displayed a faster adsorption rate and higher adsorption capacity, which implied potential application for removing dye pollutants from waste water.