Two-Dimensional Membranes for High-Value Ion Separation from Seawater and Brines

Seawater and brines contain substantial inventories of high-value ions, yet their extraction is constrained by dilute targets, concentrated competitors and feed-dependent speciation. Two-dimensional (2D) membranes offer short, tunable pathways that can amplify subtle differences in ion size, hydration, charge, and coordination. This review integrates ionic properties and aqueous speciation with membrane structure to establish a mechanistic framework for ion selectivity in 2D nanochannels. It critically examines size exclusion, dehydration, electrostatic partitioning, chemical recognition, and coupled non-ideal transport and evaluates their relevance to Li, U, Cs, Sr, Rb, and other underexplored ions. Major membrane platforms, including graphene-based materials, MXenes, transition-metal dichalcogenides, 2D metal–organic and covalent organic frameworks, layered inorganic materials and heterolamellar composites, are compared in terms of channel architecture, chemical tunability, stability, and manufacturability. The available evidence indicates that selectivity is not an intrinsic property of a nominal pore or material class, but an operating-state response of the coupled ion–solvent–membrane–electrolyte system. Finally, this review critically examines rational membrane-design strategies, outstanding challenges and future research priorities to guide the development of high-performance 2D membranes for the selective separation of high-value ions.