Adsorption and Removal of Hg²⁺ by Nano-Silica and Magnetic γ-Fe₂O₃@SiO₂

The traditional St ber method was employed to prepare core-shell structured γ-Fe₂O₃@SiO₂ particles by coating silica shell on the γ-Fe₂O₃ nano-particles, which was made by microemulsion method. The obtained magnetic core-shell γ-Fe₂O₃@SiO₂ microspheres with a thin silica layer about 15 nm in thickness were characterized by wide-angle X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The specific saturation magnetization, Ms of the core-shell γ-Fe₂O₃@SiO₂, is 14.5 emu/g, less than the Ms of γ-Fe₂O₃ at 47.2 emu/g. Using dithizone chromometer method at 551 nm, the adsorption isotherms of Hg²⁺ on the surface of three kinds of nanometer materials including nano-silica, γ-Fe₂O₃ and γ-Fe₂O₃@SiO₂ were determined, the changes of adsorption isotherms of Hg²⁺ are found that adsorption isotherms of Hg²⁺ on nano-silica and γ-Fe₂O₃ belong to type Ⅱ and type Ⅲ, respectively. However adsorption isotherms of Hg²⁺ on γ-Fe₂O₃@SiO₂ changes into type Ⅰ, because the formed Fe-O-Si bond on the surface of γ-Fe₂O₃@SiO₂ may play a leading role in the action with Hg²⁺. Adsorption isotherms of Hg²⁺ are related to the dispersibility, surface silicon hydroxyl, ion vacancy and magnetic properties of the nanomaterials. After fitting of the experimental results by double adsorption isotherm method, the pseudo-first-order kinetic model and the pseudo-second-order kinetic model, the adsorption of Hg²⁺ on nano-silica and γ-Fe₂O₃@SiO₂ are found to be well fitted by Langmuir isotherm, while adsorption of Hg²⁺ on γ-Fe₂O₃ nano-particles is well fitted by Freundlich isotherm. The removal of Hg²⁺ by these three kinds of nanometer materials is pH-dependent, and all well fitted by the pseudo-second-order kinetic model.