Tailoring MOF nanoparticle size in thin-film nanocomposite (TFN) membrane selective layers for enhanced Mg2+/Li+ selectivity

Nanofiltration (NF) has been recognized as a sustainable approach for lithium extraction from salt lakes. However, conventional thin-film nanocomposite (TFN) membranes face persistent challenges in overcoming the “trade-off”. This study reports a tunable growth strategy of MOF nanocrystals within the selective layer, enabling enhanced permeability and ion selectivity for lithium recovery from brine. Based on a “De novo synthesis” strategy, ultrasound-assisted in situ growth of smaller-sized NH 2 -ZIF-8 nanocrystals is achieved through controlled ligand substitution and nanocrystal size modulation within the separation layer. The optimized membrane exhibited outstanding performance, with a high Mg 2+ /Li + selectivity ( S Li/Mg  = 113.97), high permeability (18.52 L m −2  h −1  bar −1 (LMH/bar)), and long-term stability over a 336-hour separation test. Furthermore, the mass transfer behavior during nanofilm formation has been elucidated by theoretically analyzing monomer diffusion kinetics and the variation of monomer concentration at the reactive interface. This work presents an efficient strategy for MOF generation and provides a promising pathway for the high-efficiency extraction of lithium from complex brine resources.