Surface-Localized Hydrogen Manganese Oxide Polyethersulfone Membranes for Selective Lithium Capture under Recirculating Flow

Selective lithium recovery from multication aqueous streams requires translating the high selectivity of lithium-ion-sieve powders into macroscopic materials without compromising the accessibility of their ion-exchange sites. Here, hydrogen manganese oxide (HMO) was localized at the surface of a commercial polyethersulfone (PES) membrane through an adhesive polydopamine (PDA) interlayer, providing direct access to the active phase while preserving the porous membrane architecture. Microscopy and surface analyses confirmed the formation of an HMO-rich interfacial layer, and the resulting PES-PDA-HMO membrane exhibited a Langmuir maximum uptake of 35.6 ± 0.7 mg Li g-1 HMO, corresponding to approximately 91% of the experimentally measured capacity of the free HMO powder (39.1 ± 0.2 mg g-1). Under closed-loop recirculating flow, Li+ uptake approached equilibrium within 6 h and was accompanied by proton release, while X-ray photoelectron spectroscopy directly confirmed Li incorporation into the HMO-containing surface. Lithium removal remained similar across recirculation flow rates of 10-150 mL min-1 under the closed-loop configuration, whereas increasing the HMO surface loading from 5.8 to 8.7 mg cm-2 increased Li+ removal from 68 ± 2 to 99 ± 3% at an initial concentration of 50 ppm. After five adsorption-regeneration cycles using lactic acid at pH 2.1, the membrane retained 90 and 85% of its initial removal performance at 25 and 250 ppm Li+, respectively, with no detectable Mn release. Importantly, selectivity was preserved under severe multication competition: in a solution containing 50 ppm Li+ and 250 ppm each of Co2+, Ni2+, and Mn2+, Li+ removal remained 65 ± 3%, whereas removal of the competing divalent ions remained near 1%. These results demonstrate that interfacial localization of HMO can preserve its ion-exchange functionality after macroscopic immobilization while enabling selective, regenerable lithium capture under dynamic aqueous conditions.