Selective Lithium–Magnesium Separation from Brines by Ionic-Liquid Solvent Extraction: Molecular Mechanisms, Extractant Design, and Process Optimization

The growing global demand for lithium (Li+), driven by its strategic role in energy storage technologies, faces a critical challenge in its recovery from brines: the selective separation of Li+ and magnesium (Mg2+) ions. Because Li+ (ionic radius ≈ 76 pm) and Mg2+ (≈ 72 pm) have almost identical ionic radii despite their different charge, their coordination and hydration behaviour are difficult to discriminate at the molecular level. The high Mg2+ content in natural brines, such as those from the Atacama Salar, delays the efficiency of conventional extraction processes, which also result in significant Li+ losses. In this context ionic liquids (ILs) are emerging as alternative solvents with tunable properties, low volatility, and high thermal stability, although their environmental performance depends strongly on their chemical structure, toxicity, aqueous solubility, and recyclability, offering advantages over traditional organic extractants. This study evaluates the use of ionic liquids in liquid–liquid extraction systems for the selective removal of Mg2+, analysing operational parameters such as pH, organic-to-aqueous phase ratio (O/A), and extractant concentration. This review critically compares ionic-liquid-based extraction systems by relating Mg2+/Li+ selectivity to extractant structure, coordination chemistry, and key operational variables, while also considering the main limitations associated with their application to high-salinity brines. By linking macroscopic extraction performance to the underlying coordination chemistry, hydration-shell disruption, and computational screening tools such as quantum chemical calculations and COSMO-RS, this approach supports the rational, molecular-level design of environmentally responsible extractants for brines with a high Mg/Li ratio.