Fe₃O₄-doped lithium ion-sieves for lithium adsorption and magnetic separation

Spinel lithium manganese oxide ion-sieve is considered the most promising adsorbents to extract lithium from brine. Here, we report a Fe₃O₄-doped magnetic lithium ion-sieve prepared by a facile hydrothermal method. The Fe₃O₄-doped lithium manganese oxide (LMO/FO) was first synthesized as the magnetic ion-sieve precursor. The chemical and morphological properties of LMO/FO were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that Fe₃O₄ was uniformly dispersed in the precursor. Doping Fe₃O₄ into LMO led to the increase of the average valence for Mn in the precursor from +3.48 to +3.53, which helps improve its structural stability. The equilibrium adsorption capacity of acid-treated LMO/FO (i.e. HMO/FO) was 29.33 mg/g which was greater than that of the undoped (i.e. HMO). Moreover, the adsorption behavior of the both ion-sieves well fitted with pseudo-second-order kinetic model. In 0.05 mol/L HCl solution, the desorption equilibrium could be achieved after 30 min. The dissolution loss of manganese and iron were 6.22% and 4.14%, respectively. The saturation magnetization value (Ms) of LMO/FO is 2.5 emu/g and the solid-liquid separation can be achieved by a magnet. In addition, the magnetic ion-sieve demonstrated excellent adsorption selectivity to Li⁺ in a mixed solution containing Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺.