Adsorption and Ion-Exchange Materials for Direct Lithium Extraction from Aqueous Resources: Progress, Intrinsic Trade-Offs, and Engineering Perspectives

Direct lithium extraction (DLE) from complex aqueous resources, including salt lake brines, geothermal waters, seawater, and other low-concentration lithium-containing streams, has become an important technological route for securing a sustainable lithium supply. However, these resources are typically characterized by low lithium concentrations, high Mg/Li ratios, and complex ionic compositions, which pose substantial challenges to the selective separation and efficient enrichment of Li. Among various DLE technologies, adsorption- and ion-exchange-based separation has attracted increasing attention because of its simple process configuration, tunable active sites, and adaptability to low-concentration lithium resources. This review critically summarizes the structural features, recognition mechanisms, and performance boundaries of titanium-based, aluminum-based, and manganese-based lithium ion-sieves, organic recognition and organic–inorganic hybrid adsorbents, as well as emerging crystalline porous materials including metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). Particular emphasis is placed on the intrinsic trade-offs among adsorption capacity, selectivity, kinetics, and structural stability across different material systems. Beyond summarizing material categories, this review further emphasizes how powder-level adsorption performance can be translated, weakened, or reshaped during immobilization, dynamic mass transfer, regeneration, and validation in real aqueous resources. Manganese-based lithium ion-sieves are highlighted as a representative high-capacity yet stability-limited system, for which interfacial stabilization and sustainable precursor design remain critical. Finally, municipal sludge-derived humic acid is discussed as a hypothesis-driven multifunctional precursor platform rather than a conventional inert carrier, with potential roles in redox-mediated MnOx formation, interfacial coordination, carbonaceous confinement, and engineering recovery. Future research directions are proposed to guide the development of high-performance, stable, and sustainable adsorbent materials for practical DLE applications.