Ion-exchange membranes are attractive for salt recovery and brackish water desalination because they provide high selectivities between cations and anions. Nevertheless, typical ion-exchange membranes show only modest selectivities among cations or anions, and such selectivities are important in applications including water softening, acid recovery, and salt purification. This dissertation explores coating of membranes with polyelectrolyte multilayers (PEMs) or conductive films to enhance selectivity in electrically driven ion transport. Adsorption of PEMs on Nafion membranes gives rise to high monovalent/divalent cation selectivities in electrodialysis (ED), but the high cost of Nafion may preclude its use in many ED applications. This work demonstrates that relatively inexpensive Fujifilm cation-exchange membranes modified with protonated poly(allylamine) (PAH)/poly (4-styrenesulfonate) (PSS) films have extremely high K+/Mg2+ cation selectivities >1000 in ED. The high exclusion of Mg2+ suggests a complete and dense PEM, presumably because the smooth Fujifilm surface allows formation of a continuous coating. The PEM formed on the anode side of the membrane is essential for the high selectivity, whereas the cathode-side coating contributes only a small amount to resistance of the membrane system and little selectivity. Current density-voltage curves and transference numbers suggest that water splitting occurs at overlimiting currents. Overall, these highly selective membranes may be attractive for salt purification and salt recovery using ED. Although PEM-modified cation-exchange membranes show high cation-selectivity during ED, the current efficiency is only ~0.5, which implies that unwanted ions carry 50% of current. Adsorption of (PDADMAC/PSS)n films on Nafion membranes leads to high monovalent /divalent cation selectivity in ED, and moreover, the monovalent cation current efficiency is as high as 0.8. (PDADMAC/PSS)3PDADMAC films give the highest current efficiency in both K+/Mg2+ and Li+/Co2+ separations. The high current efficiency presumably results from the high aqueous swelling of (PDADMAC/PSS)n films to increase the monovalent cation permeance. However, (PDADMAC/PSS)n films are not stable in solutions with high salt concentrations, so future work should aim to increase the film stability. The high selectivity of PEMs partly stems from the high surface charge of the thin film. Thus, instead of introducing membrane surface charge through adsorption of polyelectrolytes, this work also aimed to develop conductive membranes to create surface charge using an electrical potential applied between the conducting membrane and a reference electrode in the source phase. The potential-dependent surface charge should alter cation and anion partitioning into the membrane. Electroless deposition of gold followed by electrosynthesis of poly(3,4-ethylenedioxythiophene) gives highly conductive membrane with sheet resistances <100 Ω/. Unfortunately, the ion fluxes do not significantly change with applied potential. Finally, this dissertation investigates the possible mechanism of high salt rejection in nanofiltration through PEM-coated NF270. The surface charge may create regions of nonelectroneutrality in the membrane, and the low concentration of the excluded ion in this region should control the resistance to salt transport.
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