Separation of Ions Using Polyelectrolyte-Modified Nanoporous Track-Etched Membranes

Selective ion exclusion from charged nanopores in track-etched membranes allows separation of ions with different charges or mobilities. This study examines pressure-driven transport of dissolved ions through track-etched membranes modified by adsorption of poly(styrene sulfonate) (PSS)/protonated poly(allylamine) (PAR) films. For nominal M 30 nm pores modified with a single layer of PSS, Br-/SO42- selectivities are similar to 3.4 with SO42- rejections around 85% due to selective electrostatic exclusion of the divalent anion from the negatively charged pore. Corresponding membranes containing an adsorbed PSS/PAH bilayer are positively charged and exhibit average K+/Mg2+ selectivities >10 at 8 mM ionic strength, and Mg2+ rejections are >97.5% at ionic strengths <5 mM. The high rejection of Mg2+ compared to SO42- likely results from both a smaller pore size after deposition of the PAR layer and higher surface charge because of Mg2+ adsorption. Simultaneous modeling of K+ and Mg2+ rejections using the nonlinearized Poisson-Boltzmann equation gives an average modified pore diameter of 8.4 +/- 2.1 nm, which does not vary significantly with ionic strength. This diameter is smaller than that calculated from hydraulic permeabilities and estimated pore densities, suggesting that narrow regions near the pore entrance control ion transport. In addition to simple electrostatic exclusion, streaming potentials lead to differing rejections of Br- and acetate in PSS/PAH-modified pores, and of Li+ and Cs+ in PSS-modified pores. For these cases, electrical migration of ions toward the feed solution results in higher rejection of the more mobile ion.