Green TEAM: Surface-Initiated Free Radical Polymerization of Tethered Electrolyte Active-Layer Membranes

Abstract

The recovery of valuable species like lithium and uranium is of growing importance as we turn toward sustainable energy sources. Nanofiltration (NF) membranes are of growing interest as potential candidates for tackling challenging separations between species of similar chemistry, size, and charge. Recently, a form of polyelectrolyte nanofiltration membrane with relatively large pores was developed: the tethered electrolyte active-layer membrane (TEAM). Utilizing surface-initiated atom transfer radical polymerization (SI-ATRP), neutral monomer precursors were grown from an ultrafiltration cellulose substrate and modified into ionizable groups. However, SI-ATRP requires harsh organic solvents, metal catalysts, and several synthesis steps, rendering the synthesis environmentally unfriendly and challenging at an industrial scale. In this work, we will develop a scalable, greener TEAM using surface-initiated free radical polymerization (SI-FRP). Monomers, methacroylcholine chloride (MACC), acrylic acid (AA), and sodium 4-vinylbenzenesulfonate (SVBS) were grafted on the surface of commercial ultrafiltration cellulose membranes as positive and negative TEAMs, respectively. The positive TEAMs showed rejection of 97% of CaCl2 and 83% of NaCl in 2 mM salt concentration with a permeability of around 4.3 L m-2 h-1 bar-1, whereas 95% Na2SO4 and 78% of NaCl rejection with a permeability of 8.2 L m-2 h-1 bar-1 was attained with the negative TEAMs. In addition, the highest monovalent selectivity was determined for all TEAMs in the mixture containing 75% divalent ions, with a cation selectivity of about 8 and an anion selectivity of about 9.