Synthesis of magnetic core-shell amino adsorbent by using uniform design and response surface analysis (RSM) and its application for the removal of Cu2+, Zn2+, and Pb2+

The magnetic Fe₃O₄ was synthesized by using a one-step solvothermal method. Then, anhydrous ethanol as a solvent, tetramethyl ammonium hydroxide (TMAOH) as an auxiliary agent, tetraethyl orthosilicate (TEOS) as a silicon source, and (3-aminopropyl) triethoxysilane (APTES) as amino source were used to prepare Fe₃O₄@mSiO₂-NH₂ by using the sol-gel method. Uniform design U14*(14⁵) and the response surface method (RSM) were used to optimize the synthesis ratio. According to the results of TEM, SEM, N₂ adsorption–desorption test, VSM, and XRD, it found that the best coating effect obtained when the relative molar ratio of TMAOH:TEOS:APTES:Fe₃O₄ was 5:4:6:0.45. The results of EDS and elemental analysis confirmed the success of amino group coating; VSM magnetization after surface modification was 32 emu/g; BET results show that specific surface area is 236 m²/g, size 5 nm, and the pore volume is 0.126 cm³/g. The removal of Cu²⁺, Zn²⁺, and Pb²⁺ by Fe₃O₄@mSiO₂-NH₂ was studied at the optimal initial pH value 6 of the adsorption test system. The isothermal adsorption results show that the Langmuir model and Redlich–Peterson model are more suitable than the Freundlich model to describe the adsorption behavior, and Cu²⁺, Zn²⁺, and Pb²⁺ adsorption is mainly single molecular layer. The maximum adsorption capacity qm of the Langmuir model for Cu²⁺, Zn²⁺, and Pb²⁺ removal was 48.04 mg/g, 41.31 mg/g, and 62.17 mg/g, respectively. The adsorption kinetic rates of Cu²⁺, Zn²⁺, and Pb²⁺ on Fe₃O₄@mSiO₂-NH₂ relatively more suitable for pseudo-second-order kinetic model, i.e., R², were ranged between 0.995 and 0.999, and the suitable reaction time was 60 min. These results proved that Fe₃O₄@m-SiO₂-NH₂ prepared by using this method is easy to synthesize, has easy recovery, is ecofriendly, and can be potential adsorbent for Cu²⁺, Zn²⁺, and Pb²⁺ removal.