Probing the Rare-Earth Element Ion Binding Capabilities of Synthetic Polyanions and Polyzwitterions

Polyelectrolytes and polyzwitterions have shown great promise for a wide range of applications, including ion-separation technologies for heavy metal recovery. Despite significant advances in the characterization of charged polymers in different hydrated environments, the mechanism by which local ionic structure and polymer architecture affects selective ion capture has yet to be established. Our work investigates the interactions between charged polymers—a polyanion and a polyzwitterion—with Rare-Earth Element (REE) cations. Using small-angle X-ray scattering, turbidity measurements, and ion-coupled plasma mass spectrometry, we analyze the polymer phase separation behaviors and selective ion binding across varying pH conditions. Our findings reveal that in slightly basic environments, the polyzwitterion exhibits higher affinity for heavier REEs, while the polyanion predominantly captures lighter REEs. These trends are linked to energetic costs of ion dehydration and microstructural characteristics of the formed precipitates. Understanding the interplay between polymer structure, solution conditions and ion identity will allow for the design of more efficient and selective ion-separation systems for REE recovery.