Surface-engineered sulfonation of ion-selective nanofiltration membrane with robust scaling resistance for seawater desalination

Inorganic fouling is one of the most critical issues in the seawater desalination process; it significantly deteriorates the desalination performance of the current seawater reverse osmosis (SWRO) membranes. Conventional nanofiltration (NF) membranes, which act as the pretreatment medium, are unable to effectively fractionate the scaling ions, Ca2+ and SO42−, resulting in fouling transfer from SWRO to the NF membranes. To address this issue, a new class of nanofiltration membranes was proposed and designed by surface engineering a custom-made sulfonated polyamide thin-film composite. Specifically, a new functional monomer, 4-aminobenzenesulfonic acid (ABSA), was chemically grafted onto a membrane surface via carbodiimide crosslinking chemistry. Due to the intentional optimization of mean pore sizes and surface charges, the as-prepared NF membranes (named BM-ABSA) exhibited extremely low Ca2+ rejections (6.1 ± 0.4%) and moderate SO42−rejections (57.5 ± 0.1%), corresponding to a higher ion selectivity (9.5) toward seawater compared to that of the benchmark commercial NF270 membrane (1.5). More importantly, the replacement of carboxylate groups in the substrate membrane matrix with the sulfonate groups considerably lowered the probability of the Ca–COOH chelation effect, which, in turn, facilitated steady NF-pretreatment processes without significant fouling. The subsequent SWRO process, using the BM-ABSA membrane-pretreated permeate as the feed solution, also proceeded well over a short-term operation with limited gypsum fouling. Overall, this study highlighted the effective inhibition of scaling by sulfonated NF membranes in the pretreatment process, validating their potential application for robust and efficient seawater desalination.