Ion Separations Using Electrodialysis Through Membranes Coated with Polyelectrolyte Multilayers

<p>Ion separations are essential in addressing global challenges ranging from clean water production and resource recovery to sustainable manufacturing practices. Conventional ion separation techniques such as precipitation or solvent extraction are effective, but they require high energy consumption or toxic solvents. Membrane-based ion separations are energy efficient, and they can operate continuously often without the need for additional reagents. </p> <p>This dissertation presents innovative electrodialysis (ED) strategies employing cation exchange membranes (CEMs) and multilayer polyelectrolyte films (MPFs) to enable selective separation among cations, extending the application of traditional ED.  </p> <p>First, MPF-coated CEMs were developed to effectively separate monovalent ions (e.g., Li+ and K+) from multivalent ions (e.g., Mg2+) with exceptionally high selectivity. By optimizing ED conditions, this method achieved monovalent ion recoveries up to 80% and purities of 99.5 mol%. Moreover, the membranes also show Li+/Mg2+ selectivities around 100 when Mg2+ is in a 20-fold excess in the source phase at an ionic strength of 0.6 M, demonstrating the potential of this ED method for Li+ extraction from brines. Adjusting current profiles during ED improved current efficiency, emphasizing the importance of electrical conditions in selective ion separation. </p> <p>Next, the work examined the potential of selectively extracting rare-earth elements (REEs) such as La3+ via Donnan partitioning into CEMs. Although significant La3+/Na+ partitioning selectivities (>100) occurred, ED transport selectivities were lower due to the low La3+ mobility within the membrane and concentration polarization in solution boundary layers. A computational model that integrates Donnan equilibria and coupled differential transport equations simulated multi-ion fluxes across solution boundary layers and membranes. The simulation underscored the critical importance of accounting for concentration polarization in ED processes. </p> <p>Finally, a novel "in-series" ED configuration combining bare and MPF-coated Nafion membranes was introduced to enhance the selective concentration and isolation of multivalent ions, such as REEs, from monovalent ions. This approach achieved substantial concentration enhancement (up to 25-fold) and high purity (93 mol%, 99 wt%) of REE cations by controlling MPF characteristics and ED operational parameters. Additionally, the study revealed crucial insights into managing accumulation and cathode solutions to maintain system electroneutrality and prevent REE precipitation. </p> <p>Together, this dissertation contributes significantly toward efficient, scalable, and economically viable ion separation technologies with ED. By leveraging ED configurations, membrane coatings, and optimized operating conditions, this dissertation provides a series of ED advancements for addressing critical separation challenges in water purification and resource recycling.</p>