Highly efficient removal of methylene blue from aqueous solution by a novel fishing-net effect of manganese oxide nano-sheets

Manganese (IV) oxide (MnO₂) nano-sheets were synthesized using tetramethylamine, H₂O₂, and MnCl₂·4H₂O and used in suspension for removal of methylene blue (MB). Up to 95 % removal was achieved for MB concentrations less than 500 mg/L. The adsorption data fitted better with the Freundlich isotherm model than the Langmuir model, indicating a heterogeneous adsorption of MB on MnO₂ nano-sheets. The maximum adsorption capacity of MnO₂ nano-sheets was approximately 4300 mg/g, which is the highest yet reported. Suspensions of MnO₂ nano-sheets aggregated as precipitates after adsorption of MB. Using a suite of analyses, including X-ray diffraction and X-ray photoelectron spectroscopy, the high adsorption capacity was mainly attributed to ion exchange, coordination, and a fishing-net effect. The term, ‘fishing-net effect,’ is proposed here for the first time, to explain the extremely high adsorption capacity of nano-sheets, which increased with increasing ionic strength and sorbate concentration. Our results demonstrate that dispersive MnO₂ nano-sheets have a potential application in adsorption of organics from strongly basic solutions with high salinity.