Greener TEAMs: Tethered electrolyte active-layer membranes produced by surface-initiated free radical polymerization

Abstract

As the world shifts toward sustainability, the recovery of valuable elements involved in energy storage and production, such as lithium and uranium, has gained increasing importance. Nanofiltration (NF) membranes are garnering attention as promising candidates for addressing complex separations involving species with similar chemical properties, sizes, and charges. Tethered electrolyte active-layer membranes (TEAMs) are a notable development within this realm. Production of TEAMs previously involved surface-initiated atom transfer radical polymerization (SI-ATRP) to grow neutral polymer precursors from an ultrafiltration cellulose substrate and subsequently modify repeat unit sidechains into ionizable groups. However, SI-ATRP uses harsh organic solvents, metal catalysts, and multistep synthesis. In this study, we demonstrate a more environmentally sustainable and simplified alternative by utilizing surface-initiated free radical polymerization (SI-FRP) to synthesize greener TEAMs. Monomers of methacroylcholine chloride, acrylic acid, and sodium 4-vinylbenzenesulfonate were grafted from the surface of commercial ultrafiltration cellulose membranes. Both positive and negative TEAMs rejected more than 95 % of divalent salts and 80 % of monovalent salts, with permeability ranging from approximately 4 to 8 Lm⁻²h⁻¹bar⁻¹. In addition, monovalent selectivity was probed using varied proportions of monovalent versus divalent co-ions. SI-FRP-TEAMs exhibited monovalent over divalent cation selectivity of ∼8 and anion selectivity of ∼9 with 75 % divalent co-ions. This study is an essential step in positioning TEAMs as a more accessible platform for fundamental membrane transport exploration and development of ion selective membranes for resource recovery.