Selective extraction of group I metals using organophosphorus ligands: Mechanisms, trends, and emerging strategies

Alkali metal ions from Group I, such as lithium (Li + ), sodium (Na + ), potassium (K + ), rubidium (Rb + ), and cesium (Cs + ), play a crucial role in modern electronics, energy storage, and nuclear technology. However, selectively recovering these ions from complex aqueous solutions is difficult due to their identical charges and systematic similarities in ionic radii and hydration behavior across the series. This review offers an in-depth, mechanism-based analysis of alkali metal ion extraction using organophosphorus ligands, going beyond empirical performance to uncover the molecular basis of selectivity. Extractants are organized into acidic systems (including phosphoric, phosphonic, and phosphinic acids) and neutral phosphoryl donors (like TBP and TOPO), highlighting how factors such as ligand acidity, donor strength, steric structure, and coordination geometry influence ion selectivity across the alkali metal series. Four main mechanistic pathways are identified: cation exchange, neutral solvation, synergistic extraction, and ion-pair or multinuclear cluster formation, with a focus on the role of phosphoryl (P=O) and deprotonated P–O − donor sites in offsetting the high hydration energies of alkali ions, with emphasis on systematic trends from strongly hydrated Li + to weakly hydrated Cs + . Although lithium-selective systems are prevalent in the literature, the mechanistic principles discussed are placed within a broader comparative framework and apply across the entire alkali-metal series. Emerging technologies, including phosphonium-based ionic liquids, phosphonate-functionalized metal–organic frameworks, and hybrid membrane architectures, are evaluated for their improved tunability, stability, and process relevance. By combining classical extraction theory with spectroscopic, thermodynamic, and mechanistic insights, this review establishes structure–selectivity relationships that provide a rational basis for ligand design and the development of sustainable separation technologies for alkali metal recovery.