(35b) 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 brines (containing mainly Mg²⁺). However, Li⁺ and Mg²⁺ have similar hydration radii, leading to a low separation factor (SF_Li/Mg) in state-of-the-art commercial polyamide-based nanofiltration (NF) membranes. Herein, we show that the SF_Li/Mg can be dramatically enhanced by optimizing the membrane surface positive charges, achieving a maximum SF_Li/Mg at a solution pH slightly below its isoelectric point (IEP). Specifically, NF270 membrane was surface-grafted by 2-(methacryloyloxy)ethyltrimethylammonium chloride (META) or polyethylenimine (PEI), assisted by bio-adhesive dopamine, forming a thin, stable, charged layer (20 – 40 nm) on the surface. The effect of the surface modification and solution pH values on the surface zeta potential (ZP) and single- and mixed-salts SF_Li/Mg properties is thoroughly investigated. For instance, co-depositing META with PDA increases the ZP from 9.2 to 16 mV and increases SF_Li/Mg by 130% from 34 (for NF270) to 80 at pH = 4 when treated by a salt solution (2000 ppm) with a Mg²⁺:Li⁺ mass ratio of 5.0, surpassing the state-of-the-art NF membranes. Mixed salt SF_Li/Mg values are slightly higher than the single-salt values because the unbalanced Cl^- ions pull smaller Li⁺ ions in the permeate side, further reducing their rejection, while Mg²⁺ rejection remains high. For instance, Li⁺ rejection drops from 40% to 20%, while Mg²⁺ rejection remains >99% at pH=4. The membrane exhibits stable separation properties over 30-h with mixed salt solutions, showcasing its potential for practical applications. This surface modification occurs at ≈22 ℃ in aqueous solutions, and it can be used to enhance commercial membrane modules for practical applications.