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

Abstract

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) (PAH) films. For nominal 30 nm pores modified with a single layer of PSS, Br(-)/SO4(2-) selectivities are ∼3.4 with SO4(2-) 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(+)/Mg(2+) selectivities >10 at 8 mM ionic strength, and Mg(2+) rejections are >97.5% at ionic strengths <5 mM. The high rejection of Mg(2+) compared to SO4(2-) likely results from both a smaller pore size after deposition of the PAH layer and higher surface charge because of Mg(2+) adsorption. Simultaneous modeling of K(+) and Mg(2+) 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.

Preparation of Chemically-Tailored Copolymer Membranes with Tunable Ion Transport Properties

“…15,21,61 Water flux and solute rejection experiments were conducted over the same range of solution pH as the salt rejection experiments to explore the possibility of structural rearrangements causing the observed salt rejections. Only the amine-functionalized membrane was used in these experiments because the sulfonic acid membrane did not demonstrate a pH dependent salt rejection.…”

Design and Performance Evaluation of Hybrid Nanofiltration Membranes Based on Multiwalled Carbon Nanotubes and Polyelectrolyte Multilayers for Larger Ion Rejection and Separation

“…These characteristics such as nanoscale thickness and high functional group density, combined with their ease of fabrication, make PEMs very attractive for the fabrication of nanofiltration membranes. Several articles have explored the possibilities of PEMs for nanofiltration and reverse osmosis using different combinations of polyelectrolytes with very high retention for specific ions . For practical applications as nanofiltration and reverse osmosis membranes, PEMs must be assembled on top of membrane supports.…”

Sequential coating of nanopores with charged polymers: A general approach for controlling pore properties of self-assembled block copolymer membranes

“…used single and multiple polyelectrolyte multilayers of poly(styrene sulfonate) and poly(allylamine hydrochloride) to separate K + from Mg 2+ ions in aqueous solution. 16 The initial layer of poly(styrene sulfonate) was bound to the track-etched polycarbonate membrane through hydrophobic interactions. The charged environment created by the layer inside of the 50 or 30 nm track-etched pore ( Figure 4) led to a selectivity of K + /Mg 2+ >10 at 8 mM ionic strength.…”

“…16 A low molecular weight was chosen to avoid the risk of the polymer spanning the pore as the layer-by-layer assembly decreases the pore size. This was evidenced by the fast fouling of the membrane with an attempt to use poly(styrene sulfonate) with a molecular weight of 75kDa during initial tests ( Figure 14).…”