Fine-tuning polyamide membrane pores and charge for enhanced Li+/Mg2+ separation

Abstract

Positively charged polyethyleneimine (PEI) nanofiltration (NF) membranes exhibit great potential for Li⁺/Mg²⁺ separation. However, the dense nature of the polyamide (PA) layer presents a challenge in breaking through the permeance bottleneck, significantly hindering its further application. In this study, 1-(2-aminoethyl)piperazine (AEP) was first incorporated into the PEI solution as a structural regulator to adjust the microstructure of the PA separation layer. The structure of the PA selective layer was fine-tuned with the incorporation of AEP, enlarging the spacing between the long-chain molecules of PEI, and established hydrogen bonding interactions with PEI. This led to the generation of Turing structures on the membrane surface, which enhanced permeance. Meanwhile, the primary amine groups in AEP were protonated in water, aiding in boosting positive membrane charge and subsequently elevating its rejection capability for divalent cations. The ingenious combination of large molecule PEI and small molecule AEP served a dual purpose in promoting the membrane construction and performance. The optimized PEI/AEP-TMC NF membrane exhibited high pure water permeance (11.24 L m⁻² h⁻¹ bar⁻¹) and excellent Li⁺/Mg²⁺ selectivity (S_{Li, Mg} = 24.59), while also achieving higher Li purity and Li recovery. Insights into the remarkable performance of the modified membranes were gained through the transition state theory. Additionally, the internal stress generated in the NF process was determined by utilizing the principles of thin plate theory, revealing that the NF membrane possesses good pressure resistance and maintains stable operation at pressures up to 9 bar. This study demonstrates the capability of PEI/AEP-TMC NF membranes for Li⁺/Mg²⁺ separation and offers a novel aqueous phase monomer for the preparation of NF membranes specifically for Li⁺/Mg²⁺ separation.