Moderate pH changes alter the fluxes, selectivities and limiting currents in ion transport through polyelectrolyte multilayers deposited on membranes

Abstract

Layer-by-layer (LBL) polyelectrolyte coatings on membranes are attractive for their high selectivities between monovalent and divalent ions. However, deposition of poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on ion-exchange membranes leads to low ion fluxes that restrict productivity or current efficiency. This study shows that increasing the solution pH from 6.5 to 8.3 enhances cation fluxes and limiting currents through (PAH/PSS)5PAH-coated cation-exchange membranes 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 >700 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.