Adsorption behavior of MnO2 functionalized multi-walled carbon nanotubes for the removal of cadmium from aqueous solutions

• The removal of cadmium using MnO2/o-MWCNTs was studied for the first time.
• The MnO2/o-MWCNTs which synthesized at pH = 5 is the most effective adsorbent.
• A detailed study for the adsorption process on cadmium removal was obtained.
• The maximum adsorption capacity was 41.6 mg g−1.
• Adsorption mechanisms were revealed and elucidated in detail.

Manganese dioxide (MnO2) was coated on acid oxidized multi-walled carbon nanotubes (MnO2/o-MWCNTs) by immersing o-MWCNTs into a KMnO4 aqueous solution. The materials were characterized by different techniques, such as Fourier-transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis, and BET surface area measurement. MnO2/o-MWCNTs were the most effective materials for cadmium ion adsorption among other adsorption materials synthesized at different conditions. Cadmium ion adsorption by MnO2/o-MWCNTs was strongly pH dependent. The Langmuir isotherm model is consistent with the experimental data at different temperatures; the maximum adsorption capacity was determined (qmax = 41.6 mg g−1). A contact time of at least 150 min was required to attain adsorption equilibrium, and kinetic adsorption can be described by the pseudo-second-order rate equation. The overall rate process was apparently influenced by external mass transfer, intraparticle diffusion, and chemical adsorption. Moreover, the thermodynamic parameters indicated that the adsorption process was spontaneous and endothermic, and that the adsorption mechanism included both physical and chemical adsorption mechanisms.