Ion-Imprinted Superabsorbent Polymers for Rapid Lithium Capture

Ion-imprinted superabsorbent polymers (ISAPs) incorporating 2-hydroxymethyl-12-crown-4 were developed to integrate lithium-selective ion recognition into a rapidly swelling hydrogel matrix. Lithium imprinting was achieved by introducing crown ether-Li⁺ complexes into the acrylic acid–acrylamide (AA-AAm) precursor solution prior to polymerization, enabling non-covalent confinement of lithium-selective crown-ether coordination environments within a crosslinked superabsorbent polymer network. Single-particle swelling measurements showed that ISAP microparticles reached water absorption equilibrium within ~5 min, consistent with the fast transport behavior of AA-AAm SAPs. The rapid fluid uptake associated with SAP swelling promotes enhanced solution ingress and convective transport of Li⁺ into the particle interior, where selective capture at imprinted crown-ether-based recognition cavities, yielding a coupled transport–recognition mechanism that is not accessible in conventional rigid ion-imprinted polymers (IIPs). In batch adsorption experiments, ISAPs achieved a lithium adsorption capacity of ~1200 µmol g⁻¹ at an initial Li⁺ concentration of 20 ppm (pH 7), substantially exceeding the uptake of non-imprinted SAPs and conventional rigid IIPs under identical conditions. ISAPs exhibited preferential Li⁺ uptake over Mg²⁺ and Co²⁺ in both single-ion and competitive multi-ion systems and retained higher adsorption capacity than SAPs and IIPs at pH 4.5, indicating that crown-ether-based imprinted sites remain accessible and functionally active under acidic conditions. These results demonstrate that embedding ion-imprinted crown ether recognition sites within a fast-swelling SAP matrix creates a synergistic capture mechanism in which swelling-driven solution uptake enhances access to selective imprinted sites, enabling high-capacity and selective lithium recovery from dilute aqueous solutions.