Manganese dioxide/iron oxide/acid oxidized multi-walled carbon nanotube magnetic nanocomposite for enhanced hexavalent chromium removal

• MnO2/Fe3O4/o-MWCNTs were prepared and well characterized.
• Chromium removal using MnO2/Fe3O4/o-MWCNTs was studied for the first time.
• The adsorption process for chromium removal was studied in detail.
• The maximum adsorption capacity was 186.9 mg g−1.
• The reusability of MnO2/Fe3O4/o-MWCNTs was high.

This study presents a simple route for the fabrication of manganese dioxide/iron oxide/acid oxidized multi-walled carbon nanotube magnetic nanocomposites (MnO2/Fe3O4/o-MWCNTs). The materials were characterized by transmission electron microscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, vibrating sample magnetometry, and Brunauer, Emmett, and Teller surface area measurement. MnO2/Fe3O4/o-MWCNTs are more effective for hexavalent chromium ion adsorption compared with other materials. Hexavalent chromium adsorption by MnO2/Fe3O4/o-MWCNTs is strongly pH dependent. The Langmuir isotherm model is consistent with the experimental data at different temperatures. The maximum adsorption capacity was determined to be qmax = 186.9 mg g−1. A contact time of different initial concentrations was about 150 min to attain adsorption equilibrium. The kinetic adsorption of different initial concentrations can be described by the pseudo-second-order rate equation. The overall rate process was apparently influenced by external mass transfer and intraparticle diffusion. Moreover, the thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic, and that the adsorption mechanism included both the physical and the chemical adsorption mechanisms. After adsorption, MnO2/Fe3O4/o-MWCNTs can be conveniently and quickly separated from the media by an external magnetic field, and adsorption capacity can remain up to 85% after five times of usage. Thus, MnO2/Fe3O4/o-MWCNTs are good candidate for efficient hexavalent chromium removal from wastewater and for the deep-purification of polluted water.