Core–shell superparamagnetic monodisperse nanospheres based on amino-functionalized CoFe2O4@SiO2 for removal of heavy metals from aqueous solutions

Abstract

To remove heavy metals from aqueous solutions, amino-functionalized superparamagnetic CoFe₂O₄@SiO₂ (CoFe₂O₄@SiO₂–NH₂) core–shell nanospheres were designed and constructed. In particular, well-defined CoFe₂O₄ nanoparticles (NPs) were synthesized by reverse co-precipitation. The shell of the CoFe₂O₄ NPs was composed of amorphous silica (SiO₂), which had a thickness of ∼35 nm. Monodisperse CoFe₂O₄@SiO₂ nanospheres grafted with more amino groups had a greater adsorption capacity and higher removal efficiency for heavy metal ions (Cd(II): 199.9 mg g⁻¹, 99.96%; Cu(II): 177.8 mg g⁻¹, 88.05%; Pb(II): 181.6 mg g⁻¹, 90.79%). The effects of the pH, initial concentrations, reaction temperature and time on the adsorption of heavy metal ions by CoFe₂O₄@SiO₂–NH₂ were analyzed systematically. The adsorption process on the nanospheres was well described by the Langmuir model. The adsorption kinetics can be best fitted by the pseudo-second-order kinetics model. Analysis of a thermodynamic study of Cu(II) showed that the process of adsorption is spontaneous and endothermic in nature. Owing to the superparamagnetic properties with a high saturation magnetization value (32.92 emu g⁻¹) of CoFe₂O₄@SiO₂–NH₂, the metal-loaded nanospheres can be quickly removed from an aqueous solution (30 s) by magnetic separation. Moreover, the nanospheres exhibited good reusability for up to five cycles. The results confirm that the monodisperse amino-functionalized CoFe₂O₄@SiO₂ magnetic nanospheres could be a potential adsorbent for the effective and regenerable removal of heavy metals from aqueous solutions.