Charged Functional Groups Drive Nanofiltration Li⁺/Mg²⁺ Selectivity

Polyamide nanofiltration (NF) membranes offer a scalable and energy-efficient pathway for lithium concentration from lake brines and battery leachates, but practical implementation hinges on achieving high selectivity of Li⁺ over Mg²⁺. The active layer of these membranes can be positively or negatively charged, carrying both amine groups that can be protonated and carboxyl groups that can be deprotonated with an ionization state that is set by the feed pH. Here, molecular dynamics simulations are used to elucidate how pH-dependent charged functional groups within the polymeric nanostructure of NF membranes govern Li⁺/Mg²⁺ selectivity, arising from electrostatic charge interactions between ions and functional groups at the molecular scale as well as steric size exclusion within the membrane pore structure. Although single-salt Li⁺ or Mg²⁺ feed solutions exhibit similar ion penetration behavior, mixed Li⁺/Mg²⁺ feeds show markedly enhanced Li⁺/Mg²⁺ selectivity at low concentrations when the membrane is positively charged. This selectivity arises because Mg²⁺ interacts more strongly than Li⁺ with repulsive protonated amine (NH₂⁺) groups, suppressing divalent ion transport, an effect that emerges specifically when the two cations compete for the same Cl- ions in a mixed feed. In contrast, for negatively charged membranes, attractive interactions with deprotonated carboxylate (COO-) groups strongly hinder the transport of both ions, resulting in poor selectivity. Ion clustering within membrane pores further reduces transport through steric effects. These results provide molecular-level insight into how charged functional groups control mono/divalent ion selectivity when competing ions are present and highlight positively charged membranes as optimal platforms for lithium separation applications.