Engineering surface charges of nanofiltration membranes to maximize Li⁺/Mg²⁺ separation properties

Polyamide-based nanofiltration (NF) membranes are attractive for Li⁺/Mg²⁺ separation for lithium recovery from salt brine (containing mainly Mg²⁺). However, Li⁺ and Mg²⁺ have similar hydration radii, resulting in a low separation factor (SF_Li/Mg). Herein, we demonstrate that SF_Li/Mg can be significantly enhanced by optimizing the membrane surface positive charges. This results in an unexpected maximum SF_Li/Mg at a solution pH slightly below its isoelectric point (IEP). Specifically, NF270 membrane was surface-grafted with 2-(methacryloyloxy)ethyltrimethylammonium chloride (META) or polyethylenimine (PEI) using bio-adhesive dopamine, forming a thin, stable, charged layer (20–40 nm) on the surface. The effects of solution pH and surface modification on the surface zeta potential (ZP) and single- and mixed-salt Li⁺/Mg²⁺ separation properties are thoroughly investigated. For example, the META grafting increases the ZP from 9.2 to 16 mV and SF_Li/Mg by 130 % from 35 to 80 at pH 4, superior to the state-of-the-art commercial NF membranes. This surface modification occurs at ≈22 °C in aqueous solutions and can be utilized to enhance commercial modules for practical applications.