(391f) Selective Electrochemical Separation of Cations: Rare Earth Element Recovery and Water Softening

Rare earth elements (REEs) have become critical resources in various electronic devices and renewable energy technology; due to their unique magnetic, phosphorescent, and catalytic properties. There are currently several ways to separate REEs, including chemical precipitation, solvent extraction, ion exchange, and adsorption; however, these methods are highly inefficient, produce significant waste, and produce low-purity REEs. Ion removal using electrochemical separation methods is highly selective and green. However, these methods are typically used for anion adsorption.

Electrochemical techniques can be used to capture and release REEs reversibly using redox-copolymers. Combining the functionalities of ion-exchange groups, such as carboxylic acids, for REE adsorption, and redox groups, such as ferrocene moieties that can be controlled electrochemically for regeneration/desorption results in a redox switchable copolymer. We observed an yttrium (Y) adsorption capacity of 69.4 mg/g polymer, and electrochemical regeneration of these electrodes yielded a regeneration efficiency close to 100%, as well as similar uptake and recovery for other rare earth elements, including cerium (Ce), neodymium (Nd), europium (Eu), gadolinium (Gd), and dysprosium (Dy), establishing the concept of electrochemically regenerable ion-exchange copolymers for REE recovery. Thus, electrochemical controls can be used in REE recovery systems, providing highly effective methods that provide efficient recovery of REEs without the use of chemical reagents while also offering high modularity and sustainability.

Finally, redox-copolymers have also been utilized for electroseparating other cations, not just REEs, such as alkaline-earth cations for water softening. Various technologies can be used to remove hard ions from water, including ion exchange, chelation, distillation, and reverse osmosis. We demonstrate 100% selectivity towards binding divalent hard ions over monovalent competing ions by the redox-copolymer. By electrochemically mediated regeneration, hard ions can be removed from sorbent materials and regenerated without any regenerative chemicals needed for multiple cycles.