Efficient Lithium Recovery from Water Using Polyamide Thin-Film Nanocomposite (TFN) Membrane Modified with Positively...

The separation of Li⁺ from Mg²⁺ in salt-lake brines using nanofiltration (NF) has become the most popular solution to meet the rising demand for lithium, particularly driven by the extensive use of lithium-ion batteries. This study presents the fabrication of a uniquely designed polyamide (PA) thin-film nanocomposite (TFN) membranes with ultrahigh Li⁺/Mg²⁺ selectivity and enhanced water flux by covalently incorporating mixed ligands functionalized silica nanoparticles (F-SiO₂NPs) into the selective PA layer and covalently bonding them to the membrane surface. In this strategy, bare silica nanoparticles (SiO₂NPs) were functionalized with mixed superhydrophilic ligands, including primary amine and quaternary ammonium groups, resulting in a highly positive surface charge primarily from the quaternary ammonium groups and enabling covalent conjugation via amine groups. Among the F-SiO₂NP-incorporated membranes, M500 containing 500 ppm of F-SiO₂NPs exhibited the best performance. In a solution with 2000 ppm salt concentration (Li⁺/Mg²⁺ ratio of 1:20), the M500 membrane showed an improved Li⁺/Mg²⁺ selectivity of 7.41 compared to the nonmodified TFC membrane, which had a selectivity of 5.05. Further surface conjugation of the M500 sample with 1500 ppm of F-SiO₂NPs resulted in the C1500 membrane, demonstrating the best performance among all of the surface-modified membranes. C1500 showed an outstanding Li⁺/Mg²⁺ selectivity of 37.95, with a Mg²⁺ rejection of 95.7% and a Li⁺ rejection of -63.2%, and a water flux of 56.0 L m^-2 h^-1 at 70 psi. Notably, a 7.5-fold improvement in Li⁺/Mg²⁺ selectivity over the TFC membrane was achieved without compromising the water flux. This is evident from the nearly identical water flux values of the TFC, M500, and C1500 membranes, which were 57.1, 54.8, and 56.0 L m^-2 h^-1, respectively. Considering key factors for large-scale applications, such as cost-effectiveness, environmental impact, the abundance of synthetic precursors, and the maturity of synthesis and tailoring technologies, SiO₂NP-based modifications outperform all other reported approaches to date.