Simultaneously Modulating Nanofiltration Membrane Surface Charges and Pore Size in Pursuit of Effective Cation Separation via a Bi-Functional Monomers-Interfered IP Reaction

Reducing the membrane surface negative charge density and properly enlarging the pore size could benefit the selective and efficient separation of divalent cations from monovalent cations desired by many applications (e.g., Li+/Mg2+ separation) using nanofiltration (NF) by synergistically exploiting the Donnan and steric effects. This study provides a one-step tactic for the preparation of such membranes by simply introducing a small percentage of sulfuryl chloride (SO2Cl2) into the organic phase of trimesoyl chloride (TMC). This method is adaptable to the classical interfacial polymerization (IP) technique. The key conditions include a sufficient high concentration of piperazine (PIP) in the aqueous phase (e.g., 2.0 wt %) and a low-to-medium concentration ratio of sulfuryl chloride to TMC. An as-prepared membrane, which manifested a high Li+/Mg2+ selectivity of 22.2, exhibited a high rejection of MgCl2 at 92%, a low rejection of Na2SO4 at 34%, and a relatively high water permeance of 11.1 L m–2 h–1 bar–1. We propose that the sulfuryl chloride monomers mainly played an interferential role rather than largely being incorporated into the polyamide matrix. This was demonstrated by the very low sulfur element composition in the active layer and the substantially enlarged membrane pore size in comparison to that of the control membrane. Adding sulfuryl chloride into the organic phase could allow more PIP monomers to diffuse through the enlarged structure to the superficial surface of the membrane, leaving more unreacted amine groups endowed with positive charges. Moreover, the addition of sulfuryl chloride could greatly change the membrane surface and structural morphologies, and in turn the surface roughness, by increasing the number of large ring-like structures and/or rougher nodules that could cover almost the entire membrane surface. Therefore, this work demonstrated that the membranes fabricated by the novel approach could have practical applications for cation separation.