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Abstract
Polyelectrolytes and polyzwitterions have shown great promise for a wide range of applications, including ion-separation technologies for the recovery of critical minerals, like the Rare-Earth Elements (REE). In this field, one potential strategy is the use of polymers that form reversible crosslinks in the presence of specific multivalent ions. 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. In this dissertation, we discuss the interactions between charged polymers—two polyanions and a polyzwitterion—with Rare-Earth Element (REE) cations. Specifically, this work examines the phase separation behavior of almost identical, fully ionized, carboxylate-bearing polymers upon the addition of trivalent REE ionic species. The effects of ion identity and monomer chemical structure on polymer phase behavior are systematically studied using optical microscopy, inductively coupled plasma mass spectrometry (ICP-MS), and small angle X-ray scattering (SAXS). We show that carboxylate-bearing polymers phase separate upon the addition of REE3+ ions, systematically taking up stoichiometric amounts of ion in the process. Phase separation proceeds in a similar manner regardless of the trivalent cation used and is unaffected by other non-trivalent ionic species. In the presence of multiple REE3+ ions, the studied polymers show preferential uptake of specific elements into the precipitate, primarily samarium and adjacent elements. In slightly basic environments, the polyzwitterion exhibits higher affinity for heavier REEs compared to the polyanions. These selectivity trends are linked to energetic costs of ion dehydration and microstructural characteristics of the formed precipitates. Phase separation can be reversed by the addition of acid, catalyzing the release of ions back into solution. These results provide a framework for the design and development of more efficient and sustainable selective ion-separation systems for REE recovery.