Water Molecule Transfer Equilibrium between Li+ and Mg2+ to Reveal the Lithium Separation Mechanism

The neutral organophosphate ester extraction system, particularly tri - butyl - phosphate ester (TBP), has demonstrated remarkable efficiency in lithium recovery from high - magnesium solutions. However, the underlying mechanism governing the separation process remains incompletely understood. To elucidate this, the hydration of ions and the water transfer equilibrium are considered pivotal factors. Given the scarcity of comprehensive theoretical studies on the water transfer equilibrium between lithium and magnesium in solution, this investigation employed a novel approach. Structural optimizations of the clusters , and were conducted at the wB97X - D4/def2 - TZVPPD theoretical level, followed by analyses of the formation electron energy changes and Gibbs free energy variations. The results indicated that the ion - water interaction within the first hydration sphere is significantly stronger than the water - water interaction in the second hydration sphere. According to the equilibrium analysis, the stable hydration states of the ions in high - magnesium solutions are proposed to be and . The hypothesis of as the stable hydration state of magnesium provides a plausible explanation for the selective separation of Li+ from Mg2+ by the TBP extraction system in high - magnesium solutions. The water transfer equilibrium method and the proposed hydration state hypothesis not only enhance our understanding of the magnesium hydration state and the mechanism of lithium recovery via solvent extraction but also offer innovative perspectives for elucidating various metal separation mechanisms and the salting - out effect in related processes.