Efficient adsorption of Hg(II) from aqueous solution by N, S co-doped MnFe₂O₄@C magnetic nanoparticles

N, S co-doped MnFe₂O₄@C magnetic nanoparticles were successfully synthesized by a simple method involving the preparation of MnFe₂O₄ nanoparticles and subsequent pyrolysis treatment. The physical and chemical properties of MnFe₂O₄, MnFe₂O₄@C and MnFe₂O₄@C-NS nanoparticles were characterized by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), transmission electron microscopy (TEM), N₂ adsorption-desorption and the pH at the point of zero charge. Their performances in the adsorption of Hg(II) from water were investigated. The adsorption process followed pseudo-second-order kinetics and the experimental data of equilibrium isotherms fitted well with the Langmuir model. MnFe₂O₄@C-NS showed the highest adsorption capacity of 108.56 mg/g, increasing more than 1.7 times compared to MnFe₂O₄. The enhanced adsorption performance was attributed to the larger specific surface area as well as the complexation of N and S ligands on the surface. The thermodynamic parameters of ΔH°, ΔS° and ΔG° at 30 °C were -24.39 kJ/mol, -0.046 kJ/mol K and -10.45 kJ/mol, respectively, which indicated that the adsorption of Hg(II) on MnFe₂O₄@C-NS was exothermic and spontaneous in nature. Moreover, MnFe₂O₄@C-NS showed superior selectivity towards Hg(II) compared with other metal ions generally present in mercury-containing industrial wastewater.