Core–shell magnetic manganese dioxide nanocomposites modified with citric acid for enhanced adsorption of basic dyes

Chemically surface modified core–shell magnetic manganese dioxide nanocomposites were prepared by redox reaction of KMnO4 to MnO2 on the surface of hydrophilic carbon coated magnetite (Fe3O4/C) microspheres in alkaline media, coupling with the modification by citric acid (CA). Properties of the synthesized adsorbent were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and vibration sample magnetometer (VSM) analysis and Brunauer–Emmett–Teller (BET) technique. Characterization results showed that prepared material had well-defined core–shell structure composed of inner magnetite core with superparamagnetic property and outer shell of nano-manganese dioxides modified with citric acid. The adsorption kinetics and isotherm were studied to examine the adsorption efficiency of citric acid modified magnetic manganese dioxide (Fe3O4/C/MnO2-CA) nanocomposites towards methylene blue (MB) as basic dye. The kinetic data were found to fit pseudo-second-order model as compared to pseudo-first-order model. Langmuir isotherm model fitted the experimental data better than the Freundlich isotherm model. The adsorption mechanism study suggested that the hydrogen bonding, electrostatic and electron acceptor-donor interactions were responsible for the adsorption of MB on Fe3O4/C/MnO2-CA nanocomposites.