Understanding Ion Transport and Selectivity of Lanthanide Series in Zwitterionic Polymer Membranes

Lanthanide separation is vital for rare-earth recycling and clean-energy technologies, yet conventional solvent extraction is energy-intensive and environmentally harmful. Zwitterionic polymer membranes offer a sustainable alternative, though the microscopic basis of lanthanide selectivity remains unclear. We employ molecular dynamics simulations to probe lanthanide transport in P(AMA-MPC-TFEMA) polyzwitterionic membranes and their crosslinked analogs. Crosslinking slows ion motion, but lanthanide–zwitterion interactions remain weak. The membrane environment, however, accentuates hydration and ion-pairing differences across the series: light lanthanides exhibit stronger hydration and weaker ion pairing, leading to slower diffusion, while heavy lanthanides show enhanced cation–anion association and clustered motion. These results indicate that selectivity stems from the competition between hydration and ion pairing rather than polymer morphology alone, providing molecular-level insights for designing efficient zwitterionic membranes for sustainable lanthanide separations.