Layer-by-layer (LBL) deposition of polyelectrolyte multilayers (PEMs) on membranes greatly enhances ion-transport selectivities in dialysis and nanofiltration. This dissertation explores monovalent/divalent ion separations in dialysis through PEM-coated membranes and examines the effect of ionic strength and pH on the ion fluxes, selectivities and limiting currents. Coated anion-exchange membranes (AEMs) show Cl-/SO42- selectivities as high as 140, and a moderate increase in pH gives a five-fold increase in the limiting current through modified cation-exchange membranes (CEMs).
Development of AEMs with selectivities among anions may expand the applications of these membranes in salt separations. This work shows that LBL coating of aliphatic polyamide AEMs with poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films enhances Cl-/SO42- dialysis selectivities. Importantly, the extent of this enhancement depends on the source-
phase salt concentrations. With a source phase containing a 0.01 M NaCl, 0.01 M Na2SO4 mixture and a receiving phase of 0.01 M Na2CO3, a (PSS/PAH)5PSS coating (5 PSS/PAH “bilayers” capped with a layer of PSS) increases the Cl-/SO42- selectivity from 1.7 to 5.3 in diffusion dialysis and from 1.3 to 7.4 in electrodialysis. More important, the diffusion dialysis selectivity of a coated membrane increases to 140 when the salt concentrations in the source phase are 0.1 M. Even with bare membranes, selectivity increases to 13 with 0.1 M source-phase salt concentrations. Partitioning studies and modeling suggest that the increased Cl-/SO42- selectivities at high source-phase salt concentrations stem from enhanced Cl- partitioning and electromigration that disproportionately decreases SO42- flux in the AEM.LBL polyelectrolyte coatings on membranes are attractive for their high selectivities between monovalent and divalent ions. However, deposition of PSS/PAH films on ion-exchange membranes leads to low ion fluxes that restrict productivity or current efficiency. Remarkably, increasing the solution pH from 6.5 to 8.3 enhances cation fluxes and limiting currents through (PAH/PSS)5PAH-coated CEMs while still achieving order of magnitude selectivity increases compared to bare membranes. In Donnan dialysis through (PAH/PSS)5PAH-coated Nafion, at pH 8.3 the K+/Mg2+ selectivity is 100, and the K+ flux is 22 times that at pH 6.5. Moreover, at the higher pH the electrodialysis limiting current through these membranes increases ~5-fold. This enables electrodialysis separation of K+ and Mg2+ with a current efficiency around 0.75, a selectivity of 18, and 50% K+ recovery from the source phase. Transmembrane potential measurements and increases in Cl-/SO42- selectivities (from 9.5 at pH 6.5 to ~70 at pH 8.3) for porous alumina membranes coated with (PSS/PAH)5PSS films suggest that the coating becomes cation-permselective at higher pH. Increased cation transference numbers in polyelectrolyte coatings likely lead to the enhanced limiting currents for coated Nafion at higher pH. Finally, (PAH/PSS)5PAH-coated Nafion shows a Li+/Mg2+ Donnan dialysis selectivity >1000 at neutral pH and a selectivity of 10 at pH 8.3. However, the Li+ flux is 6 times greater at pH 8.3, which might make operation at the higher pH desirable.