Thin-film composite bicontinuous cubic lyotropic liquid crystal polymer membranes: Effects of anion-exchange on water filtration performance

Abstract

The effects of anion-exchange on the filtration performance of thin-film composite (TFC) membranes with an active layer consisting of a nanostructured, lyotropic (i.e., surfactant) liquid crystal (LLC) polymer were investigated. These TFC LLC membranes are made by the in situ cross-linking of reactive amphiphiles (i.e., surfactants) that self-organize in the presence of glycerol into a type I bicontinuous cubic (QI) phase that contains a uniform, 3D-interconnected pore network lined with tethered cationic moieties and free mobile anions in the pores. In this study, a systematic series of experiments were performed to independently investigate how monovalent cations and anions affect transport in these TFC QI membranes. TFC QI membranes exposed to feed solutions that contain different cations (i.e., Li+(aq), Na+(aq), and K+(aq)) but have the same monovalent anion as the free mobile anion in the membrane (i.e., Br–(aq)) have a constant flux and a high rejection (>98%). When the cation is kept constant (i.e., Na+(aq)) and the anion in the feed is varied (i.e., Cl–(aq), Br–(aq), NO3(aq)–, and I–(aq)) and allowed to partially anion-exchange with the membrane, a high rejection is maintained (≥96%), but the flux significantly changes depending on the anion in the feed solution. The flux of the TFC QI membranes can also be repeatedly be cycled by contacting the membranes with different anions. Control experiments with completely anion-exchanged TFC QI membranes (i.e., with Cl–(aq), Br–(aq), NO3(aq)–, and I–(aq)) showed that the rejection of sodium salts and uncharged organic solutes was virtually the same for all of the completely anion-exchanged membranes. As a whole, these results demonstrate that the flux of these TFC QI membranes can be tuned by changing the anion with little to no change in the rejection performance. The unique performance characteristics of TFC QI membranes may offer advantages over conventional NF and RO membranes for water purification applications or other aqueous separations.