Charge-enhanced polyamide nanofiltration membranes via carboxyl doping for Li⁺-Mg²⁺ separation

Polyamide (PA) membranes are promising nanofiltration (NF) materials for Li⁺-Mg²⁺ separation in lithium extraction from salt-lake brines. However, their limited water permeance and reliance on acidic environments to maintain protonated electropositive surfaces hinder overall performances. Herein, we introduced a carboxyl doped PA NF membrane, hereafter referred to as GLY/PA-PDA (GLY = glycine; PDA = polydopamine). The hydrophilic ultrathin rough GLY/PA rejection layer and PDA interlayer enabled an exceptional water permeance of 17.2 L m⁻² h⁻¹ bar⁻¹. More importantly, carboxyl groups acted as unique Mg²⁺ catchers to functionalize surfaces, resulting in enhanced surface positive charge intensity and sustained ion selectivity. The GLY/PA-PDA membranes achieved an impressive Li⁺-Mg²⁺ separation coefficient of 30.5 in mixed-cation feeds. During a 3-stage separation process, the membrane demonstrated continuously increasing separation coefficients of each stage, obtaining an ultra-low-Mg²⁺ extraction solution of 0.06 ppm. Our work proposes a direction for improving selectivity by utilizing guest metal cations within NF systems.