pH-Fractionated lignin enables high-selectivity, chlorine-tolerant polyester thin-film composite nanofiltration membranes

Polyamide thin-film composite (TFC) membranes provide high permeability and strong ion rejection in nanofiltration and reverse osmosis, but oxidants like chlorine can impair their long-term performance. To overcome this, alternative chemistries are needed to improve durability while maintaining separation efficiency. Lignin-based polyester TFC membranes are promising, offering better antifouling and chlorine resistance. However, their lower salt rejection hampers wider adoption. Here, we used distinct molecular-weight fractions of lignin to fabricate polyester TFC membranes and assessed how fraction-specific physicochemical properties govern membrane structure and performance. Lignin was separated into five fractions (B1–B5) via pH fractionation. Among the resulting membranes, the B4-derived TFC (M-B4) achieved the best overall performance, delivering 98% rejection of Na 2 SO 4 and 41% rejection of NaCl with a water flux of 45 LMH. The B4 fraction isolated at pH 3.1, with a smaller particle size and a high phenolic hydroxyl content (2.1 mmol g −1 ), yielded a highly crosslinked polyester with increased hydrophilicity and surface roughness. M-B4 also exhibited excellent antifouling performance, with a flux recovery ratio (FRR) of 99% after three cycles, and strong operational stability, with only a 2% reduction in Na 2 SO 4 rejection and a 5.5% decrease in water flux after 12 h of extended filtration. Additionally, after four days of chlorine exposure, Na 2 SO 4 rejection decreased by just 2%, while water flux increased by 9%. Finally, we performed a cost analysis for large-scale manufacturing and benchmarked the estimated production cost against reported prices for conventional polyamide TFC membranes. This innovative yet simple strategy positions lignin-based polyester membranes as sustainable and efficient alternatives to polyamide membranes. • pH-fractionated lignin enables tunable polyester TFC NF selective layers. • M-B4 delivers ~45 LMH flux with ~98% Na 2 SO 4 rejection. • Higher phenolic-OH fractions yield denser, more selective films. • Excellent antifouling: up to ~99% flux recovery after 3 cycles. • High reliability under chlorine exposure and extended filtration run.