Highly positively-charged membrane enabled by a competitive reaction for efficient Li+/Mg2+ separation

Advanced monovalent cation exchange membranes (MCEMs) have been applied to extract key monovalent cations (Li+ or Na+) from nature, but it is challenging to design high-performance MCEMs with low membrane area resistance and specific charge properties. Herein, a positively-charged Fe3+-bridged pyrogallic/polyethyleneimine membrane (Fe3+-bridged PY/PEIM) for efficient Li+/Mg2+ separation is fabricated through a covalent bond and coordination bond (CB/COB) competitive reaction. Various physicochemical characterizations elucidate that the formation of COBs reduces the reaction sites of PY and PEI molecules for Michael-addition and Schiff-base reactions, which reduces the thickness of the selective layers to reduce the membrane area resistance from 6.58 to 4.83 Ω·cm2. A highly positively-charged selective layer caused by unreacted NH2 groups and Fe3+ makes the optimal Fe3+-bridged PY/PEIM exhibit a superior separation performance (perm-selectivity (Li+/Mg2+) of 45.57 with a Li+ flux of 4.61 × 10−8 mol·cm−2·s−1). Moreover, the optimal Fe3+-bridged PY/PEIM also has excellent separation performance (high Li+ flux and perm-selectivity) and operational stability when treating three simulated solutions with different Mg/Li mass ratios and salinities. The new strategy for constructing a thin, defect-free and positively-charged selective layer through a CB/COB competitive reaction can provide new insights for designing high-performance MCEMs towards environmental remediation and energy harvesting.