Regulating structure of nanofiltration membrane via bi-directional interfacial polymerization for enhanced Li+/Mg2+ separation

To address the membrane challenges such as the trade-off effect between water permeance and separation selectivity, development of a desirable nanofiltration membrane structure is crucial. And, it is important to design a positively charged surface to enhance charge repulsion force between membrane surface and ions due to lithium (Li) extraction problem as the close hydration diameter of magnesium (Mg) and lithium ions. Herein, a strategy that involves regulating membrane structure to create a loose and dense double-layer structure via bidirectional interfacial polymerization has been proposed to form selectivity-enhanced nanofiltration membranes for Li+/Mg2+ separation. The incorporation of dopamine (DA) and trimesoyl chloride (TMC) in first loose layer could improve the membrane stability, while the amine groups from polyethyleneimine (PEI) or polyvinylamine (PVAm) in the second dense layer surface, provided by rich amine polymers, could improve the rejection of divalent magnesium ion. The as-fabricated membrane DA-TMC-PEI shows enhanced permeance (around 15.4 L m- 2 h- 1 bar- 1) and the membrane DA-TMC-PVAm demonstrates outstanding ion selectivity with Li+/Mg2+ separation factor of 30. Moreover, the DA-TMC-PEI and DA-TMC-PVAm membranes could efficiently purify the ratio of Mg2+/Li+ solution from 50 to 0.43. We believe the strategy could provide a promising potential in lithium extraction.