In our first study, we perform over 750 ion rejection experiments using representative synthetic brines that model the multicomponent compositions of the Salar de Atacama, Qaidam Lake, and the Salton Sea. By systematically varying the feed salinity (10–250 g L⁻¹) and pH (2 and 7), we demonstrate that acid pretreatment—employed industrially to prevent scaling—can enhance Li⁺/Mg²⁺ separation factors by up to 13× via the amplification of the Donnan potential. This enhancement is attributed to increased protonation of carboxyl and amino moieties in the polyamide matrix, generating a more positively charged surface that selectively impedes multivalent cation transport. Notably, we show that the predictive accuracy of separation performance increases by up to 80% when feed solutions replicate real-world anionic ratios (Cl⁻/SO₄²⁻), rather than simplified binary mixtures. To quantitatively elucidate these phenomena, we employ a Donnan-Steric Pore Model with Dielectric Exclusion (DSPM-DE), which successfully captures the effects of partitioning, ionic coupling, and transmembrane electric fields. Model calibration using experimental rejection data reveals clear evidence of charge-based transport coupling at neutral pH, which becomes attenuated under acidic conditions—further amplifying selective monovalent transport.
Building on this mechanistic foundation, we introduce a second innovation: a positively-coated NF membrane synthesized by covalently bonding a polyelectrolyte layer enriched in protonated ammonium (–NH2+) groups onto the polyamide surface. This coating imparts a stable, positive surface potential across a broad pH range (2–8), enabling robust Donnan exclusion of divalent ions under hypersaline conditions. Extensive membrane characterization—including zeta potential measurements, FTIR spectroscopy, SEM/TEM imaging, and quartz crystal microbalance analysis—confirms that the coating enhances electrostatic exclusion without compromising hydrophilicity or increasing fouling propensity. Molecular dynamics simulations further reveal that Coulombic energy barriers between –NH2+ sites and multivalent cations disproportionately impede Mg²⁺ and Co²⁺ transport while allowing Li⁺ to permeate. In over 8000 ion rejection measurements using both salt-lake brines and battery leachates, the coated membranes consistently achieve lithium purities exceeding 98% in a single-pass configuration, and reduce Mg²⁺ concentrations in permeates to as low as 0.14%. Long-term stability trials over 12 weeks in 0.5 M HCl confirm the membrane’s chemical resilience under harsh acidic conditions.
When integrated into a two-stage NF system, the coated membranes achieve final permeate Mg²⁺ concentrations below 0.031% from Chilean brines, while maintaining lithium purities above 99.5% from NMC battery leachates—demonstrating high separation efficiency with only a 14.7% increase in specific energy consumption. Robeson plot analysis confirms that the coated NF membrane occupies a Pareto-optimal region in the selectivity–permeability trade-off space. Collectively, these findings establish a robust design framework for tuning membrane electrostatics to optimize lithium selectivity under industrially relevant conditions. By marrying rigorous mechanistic insight with scalable materials innovation, this work advances both the fundamental understanding of ion transport in charged nanoporous media and the practical implementation of membrane-based direct lithium extraction (DLE) technologies for a decarbonized future.