Efficient Lithium\nRecovery from Water Using Polyamide\nThin-Film Nanocomposite (TFN) Membrane Modified with Positively Charged\nSilica Nanoparticles

The separation of Li<sup>+</sup> from Mg<sup>2+</sup> in salt-lake\nbrines using nanofiltration (NF) has become the most popular solution\nto meet the rising demand for lithium, particularly driven by the\nextensive use of lithium-ion batteries. This study presents the fabrication\nof a uniquely designed polyamide (PA) thin-film nanocomposite (TFN)\nmembranes with ultrahigh Li<sup>+</sup>/Mg<sup>2+</sup> selectivity\nand enhanced water flux by covalently incorporating mixed ligands\nfunctionalized silica nanoparticles (F-SiO<sub>2</sub>NPs) into the\nselective PA layer and covalently bonding them to the membrane surface.\nIn this strategy, bare silica nanoparticles (SiO<sub>2</sub>NPs) were\nfunctionalized with mixed superhydrophilic ligands, including primary\namine and quaternary ammonium groups, resulting in a highly positive\nsurface charge primarily from the quaternary ammonium groups and enabling\ncovalent conjugation via amine groups. Among the F-SiO<sub>2</sub>NP-incorporated membranes, M500 containing 500 ppm of F-SiO<sub>2</sub>NPs exhibited the best performance. In a solution with 2000 ppm salt\nconcentration (Li<sup>+</sup>/Mg<sup>2+</sup> ratio of 1:20), the\nM500 membrane showed an improved Li<sup>+</sup>/Mg<sup>2+</sup> selectivity\nof 7.41 compared to the nonmodified TFC membrane, which had a selectivity\nof 5.05. Further surface conjugation of the M500 sample with 1500\nppm of F-SiO<sub>2</sub>NPs resulted in the C1500 membrane, demonstrating\nthe best performance among all of the surface-modified membranes.\nC1500 showed an outstanding Li<sup>+</sup>/Mg<sup>2+</sup> selectivity\nof 37.95, with a Mg<sup>2+</sup> rejection of 95.7% and a Li<sup>+</sup> rejection of −63.2%, and a water flux of 56.0 L m<sup>–2</sup> h<sup>–1</sup> at 70 psi. Notably, a 7.5-fold improvement\nin Li<sup>+</sup>/Mg<sup>2+</sup> selectivity over the TFC membrane\nwas achieved without compromising the water flux. This is evident\nfrom the nearly identical water flux values of the TFC, M500, and\nC1500 membranes, which were 57.1, 54.8, and 56.0 L m<sup>–2</sup> h<sup>–1</sup>, respectively. Considering key factors for\nlarge-scale applications, such as cost-effectiveness, environmental\nimpact, the abundance of synthetic precursors, and the maturity of\nsynthesis and tailoring technologies, SiO<sub>2</sub>NP-based modifications\noutperform all other reported approaches to date.