Structural Insights Into Aluminum-Doped Manganese Dioxides as Promising Materials for Direct Lithium Extraction: Modeling and Mechanism Study

This study aimed to provide new insights into lithium (Li) sorption and extraction mechanisms by systematically investigating the structure, surface properties, and defect vacancies of both doped and undoped manganese oxide sorbents. HMnO and Al-doped HMnO sorbents are successfully synthesized, and their formulas and vacancy ratios are determined using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) characterization. Among the materials tested, HMO-2.5Al exhibited the best performance in batch sorption experiments, enhancing Li⁺ sorption to 44.49 mg g⁻¹ and reducing Mn loss to 3.38%. HMO-2.5Al also demonstrated exceptional lithium selectivity in the simulated brine test, with separation factors of α^Li_Ca, α^Li_Na, α^Li_K and α^Li_Mg being 3.33, 353.08, 1327.44, and 6552.76, respectively. The sorbent displayed sustained durability before and after five cycles of sorption-desorption. The ion exchange-surface complexation model is employed to investigate the titration behavior, pH effects, and sorption isotherms, providing insights into the mechanism underlying Li⁺ sorption. A two-stage sorption mechanism is proposed, involving a surface ion exchange reaction and a surface complexation reaction. Overall, the synthesized HMO-2.5Al sorbents demonstrate significant potential for direct lithium extraction from solutions with high concentrations of coexisting ions and contribute a novel mechanism to the field of lithium extraction study.