High rejection seawater reverse osmosis TFC membranes with a polyamide-polysulfonamide interpenetrated functional layer

Current polyamide thin-film composite (TFC) membranes for seawater reverse osmosis (SWRO) require enhanced rejection capabilities for high salinity application and the removal of neutral micropollutants. In this study, we developed high rejection SWRO membranes utilizing m-phenylenediamine (MPD) as the water phase monomer, trimethyl chloride (TMC) as the organic phase monomer, and naphthalene-1,3,6-trisulfonyl chloride (NTSC) as a molecular plug. Xylene was added to the heated organic phase to compensate for the flux loss due to the tightened functional layer. The membranes were characterized in detail with ATR-FTIR, XPS, SEM, AFM, WCA, surface zeta potential, Positron annihilation spectroscopy (PALS), and quartz crystal microbalance (QCM). Our results indicate that adding xylene into the organic phase enhances flux, while heating the organic phase to 90^oC benefits both flux and rejection. Furthermore, the addition of NTSC further improves the rejection. The variation in membrane flux correlates with the functional layer morphology under SEM and the membrane rejection properties correspond well with the functional layer free volume results by PALS. Due to the low reactivity of sulfonyl chloride, NTSC can only form oligomers embedding into the polyamide network, resulting in significantly higher rejection of sodium chloride (99.90 %) and boron (92.39 %) with a flux of 37.85 L m⁻² h⁻¹ under the conditions of 35000 ppm NaCl and 5 ppm boric acid at 800 psi. This work demonstrates that high rejection SWRO membranes can be successfully achieved by an in situ “swelling and filling” method.