Bioinspired Polypeptide Dendrimer-Modified Thin-FilmComposite Membranes for Selective Lithium-MagnesiumSeparation with DFT Insights

Selective ion transport in nanofiltration (NF) enables sustainable lithium (Li+) recovery. While many membranes rely on strong positive charge for Li⁺/Mg2⁺ separation, we show that negatively charged membranes can also excel using a biomimetic approach. Inspired by biological ion channels that achieve cation selectivity via specific binding sites despite their negative charge, we designed a nitrogen-rich polypeptide dendrimer (amino acid–based) bearing carboxylate coordination sites with higher affinity for Mg2⁺ than Li⁺, while moderating the membrane's net negative charge. This biomimetic design enhanced Li+ recovery by inhibiting Mg2+ transport through stronger interactions, thereby allowing for preferential Li+ permeation. This process occurred through a combination of electrostatic modulation and ligand-assisted coordination. Density functional theory (DFT) calculations indicated strong oxygen-donor coordination: lysine motifs bind hydrated Mg2+ (E ≈ −170 kcal.mol−1) far more strongly than Li+ (E ≈ −50.2 kcal.mol−1). The optimized membrane achieved Li+/Mg2+ selectivity of 15.6 at neutral pH with 23 LMH flux, and 136 at pH 4, highlighting strong performance in acidic feeds. Long-term tests showed ∼0.4% leaching over 10 days with stable rejection and enrichment of Li⁺ (feed Li⁺/Mg2⁺ increased from 0.05 to 0.20). Antifouling tests showed a twofold lower flux-decline ratio and higher flux-recovery than the unmodified TFC.