Multifunctional role of surfactant in fabricating polyamide nanofiltration membranes for Li+/Mg2+ separation

Extracting Lithium from salt-lake brines can effectively alleviate global lithium scarcity. Separating the co-existing Mg²⁺ ions from Li⁺ ions in the brines is essential. While thin-film composite (TFC) nanofiltration (NF) membrane show potential for this separation, commercial NF membranes with negatively charged surfaces fail to meet the high rejection requirement for Mg²⁺ ions due to the electrostatic attractions between membranes and cations. Positively charged NF membranes fabricated by interfacial polymerization (IP) between aqueous phase polyethylenimine (PEI) and hexane phase trimesoyl chloride (TMC) have shown promise for Li⁺/Mg²⁺ separation. However, lithium extraction efficiency is greatly limited by the relatively low permeance and high lithium rejection of the membrane caused by an excessively cross-linked structure. Therefore, optimizing the pore size while maintaining the positive charge of PEI/TMC-based TFC membranes is necessary. We propose adding anionic surfactants to the aqueous PEI solution to modulate PEI/TMC-based NF membrane formation. Surfactants control PEI diffusion in IP through their interactions and improve reaction uniformity at the water-hexane interface. This results in a narrow pore size distribution of the PA network. In this study, three sulfate surfactants with varying alkyl chain lengths were used to control membrane formation. Results showed that sodium n-decyl sulfate (SDES), the shortest sulfate surfactant, improved membrane performance most effectively. The optimized membrane exhibited a crumpled surface and relatively loose pore structure with narrow pore size distribution. It demonstrated a pure water permanence of 5.65 L m⁻² h⁻¹ bar⁻¹, high MgCl₂ rejection of 92.6 %, and low LiCl rejection of 21.5 %. After filtering a Mg²⁺ and Li⁺ binary mixture solution, the Mg²⁺/Li⁺ ratio decreased significantly from 40 (feed) to 3.08 (permeate). This study provides an efficient strategy for preparing PEI/TMC-based NF membranes with favorable Li⁺/Mg²⁺ separation performance.