Electro-nanofiltration membranes with high Li+/Mg2+ selectivity prepared via sequential interfacial polymerization

With the rapid development of batteries and other industries, the demand for lithium has exploded. As about 70 wt% of the lithium resources exist in brines, the extraction of lithium by selective electrodialysis (S-ED) has attracted extensive attention. However, the similar hydrated ionic radius of Li+ and Mg2+ caused many challenges. Herein, a simple sequential interfacial polymerization (SIP) method was developed to adjust the surface charge and pore size of the electro-nanofiltration membranes (ENFMs), and the ENFMs were endowed with excellent selectivity due to the electrostatic repulsion and pore-size sieving effect. Polyethyleneimine (PEI) with different molecular weights was used for the SIP process based on the nascent polyamide separation layer prepared by interfacial polymerization. With the increase of the PEI molecular weight, the zeta potential of the membrane surface changed from negative to positive, while the pore size of the membranes also decreased. The optimal ENFMs achieved outstanding selectivity for Li+/Mg2+ (16.55) and high Li+ flux (3.08 × 10−8 mol·cm−2·s−1) at a current density of 10 mA·cm−2. Moreover, the optimal ENFMs exhibited high limiting current density (50.6 mA·cm−2) and low membrane electrical resistance (2.42 Ω·cm2), which has a great potential to be used for practical applications.