Narrowed pore size distribution of positively charged nanofiltration membranes via interlayer-tailored interfacial polymerization for lithium extraction

Positively charged polyamide (PA) nanofiltration (NF) membranes fabricated via interfacial polymerization of polyethyleneimine (PEI) and trimesoyl chloride (TMC) hold great potential for lithium extraction from salt-lake brines, but still face challenges in precise Li⁺/Mg²⁺ separation under high-salinity conditions due to broad pore size distribution and electrostatic shielding. Herein, we designed a PEI/TMC-based PA membrane with enhanced positive charge and narrowed pore size distribution. These properties were regulated by an interlayer fabricated via a Schiff base reaction between polyallylamine (PAA) and glutaraldehyde (GA) on a polysulfone (PSf) substrate. The enrichment and diffusion of amine monomers were regulated by the interlayer, as confirmed through experiments and molecular dynamics simulations. As a result, a PA layer with narrowed pore size distribution (σ_p reduced from 1.36 to 1.21), increased positive surface charge, and reduced thickness was obtained. The optimized interlayer polyamide (iPA) membrane demonstrated a remarkable Li⁺/Mg²⁺ separation factor up to 174.1 in simulated brine, a two-fold increase in water permeance of 5.5 L m⁻² h⁻¹ bar⁻¹, as well as excellent long-term separation stability under high-salinity conditions. This work highlights the potential of iPA membranes for achieving both high lithium purity and recovery, offering a promising avenue for lithium extraction from high-concentration salt-lake brines.