(152x) Green TEAM: Surface-Initiated Free Radical Polymerization of Tethered Electrolyte Active-Layer Membranes.

Solute-solute separations are required for the recovery of ionic forms of these substances from natural waters, which is important for resource recovery. Nanofiltration (NF) membranes are gaining popularity as a possible solution to difficult separations between chemically and electrically similar species. Recently, the tethered electrolyte active-layer membrane (TEAM), an alternative to polyelectrolyte multilayer membranes, was introduced. 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 study, we used surface-initiated free radical polymerization (SI-FRP) to build a more sustainable, green, and industrially scalable production methodology for TEAMs. The process and reaction conditions in SI-FRP are greener as they allow the use of safe solvents (water and ethanol) and non-toxic initiators, for the direct synthesis of polyelectrolyte polymers from charged monomers. Synthesis time and number of steps are reduced with SI-FRP in comparison to SI-ATRP-produced TEAMs.

In this study, the surface of commercial ultrafiltration cellulose membranes were modified by grafting-from with methacroylcholine chloride (MACC), acrylic acid (AA), and sodium 4-vinylbenzenesulfonate (SVBS) to produce positive and negative TEAMs. Attenuated total reflection - Fourier transform infrared spectroscopy (ATR-FTIR) spectra and changes in water contact angles were consistent with signature peaks and expected hydrophobicity of grafted brushes, respectively. Compared to TEAMs produced using SI-ATRP, TEAMs synthesized using SI-FRP showed around a 20% greater salt rejection. The maximum salt rejection for positively charged PMACC TEAMs was around 83% NaCl, 97% CaCl₂, and 16% Na₂SO₄ at 2 mM single salt concentrations, with 4.3 Lm^-2h^-1bar^-1 pure water permeability. Under the same experimental conditions, negatively charged PSVBS TEAMs outperformed all previously synthesized negative TEAMs, rejecting nearly 78% NaCl, 95% Na₂SO₄, and 7% CaCl₂ with 8.2 Lm^-2h^-1bar^-1 pure water permeability compared to PAA TEAMs with about 57% NaCl, 86% Na₂SO₄, and 4% CaCl₂ rejection and having 3.6 Lm^-2h^-1bar^-1 pure water permeability. All TEAMs were shown to reduce the pore size to a tenth or the size of pores of unmodified cellulose membrane or even smaller. In addition, the highest monovalent selectivity was observed for all TEAMs in mixed salt solutions containing 75% divalent ions, with cation and anion monovalent selectivity of around ~8 and 9 for both positive and negative TEAMs, respectively.

The findings of this study pave the way for the environmentally friendly synthesis of a novel polyelectrolyte membrane with potential uses in water softening, resource recovery, desalination of brackish water, the fabrication of novel electrodialysis membranes, synthesis of antibacterial and antifouling membranes for wastewater treatment plants, and supramolecular channels for fast ion transport.