Experimental study on mercury ions removal from aqueous solution by MnO2/CNTs nanocomposite adsorbent

• MnO2/CNT introduced itself as a highly effective adsorbent for Hg(II) removal.
• MnO2/CNT was synthesized, characterized and used for Hg(II) removal from aqueous phase.
• Experimental data followed by Freundlich isotherm and pseudo-second-order rate.
• pH, temperature and initial concentration had a great effect on Hg(II) removal.
• Sorption of Hg(II) by MnO2/CNT was an endothermic physico-chemical process.

MnO2-coated carbon nanotubes (MnO2/CNT) were prepared, characterized and used to remove Hg(II) from aqueous solution. Characterization of the prepared hybrid adsorbent was carried out using transmission electron microscopy (TEM), N2 adsorption–desorption, X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The synthesized MnO2/CNT sorbent possessed specific area of 110.38 (m2 g−1), mesopore size (2.70 nm) and total pore volume of 0.522 (cm3 g−1). XRD diffraction analysis revealed the birnessite-type MnO2 is produced through the hydrothermal synthesis and was coated on CNTs surface. A series of batch adsorption experiments were conducted to study the effect of temperature, pH, initial concentration, the presence of other heavy metals and contact time on Hg(II) uptake by the adsorbent. Optimum conditions obtained were 50 °C, pH 5–7 and contact time of 80 min. Hg adsorption on MnO2/CNT was a fast process and the kinetics followed a pseudo-second-order rate equation. The adsorption isotherms were analyzed using Langmuir and Freundlich models, and the Freundlich equation best describes the interaction between mercury species and MnO2/CNT. Thermodynamic parameters of ΔH°, ΔS° and ΔG° were calculated and it was found that the Hg(II) species adsorption on the MnO2/CNT is endothermic and spontaneous. From the D–R isotherm, the mean free energy was calculated as 7.07 kJ mol−1 indicating that the sorption of Hg(II) was taken place by physico-chemical mechanism.