Thin-film composite forward osmosis membrane with superior alkaline stability

The polyamide selective controls water permeation and selectivity in thin-film composite (TFC) membranes. In this study, we fabricated TFC forward osmosis (FO) membranes supported on ultrafiltration (UF) polyethersulfone (PES) through interfacial polymerization (IP) of 2,6-diaminopyridine (DAP) and trimesoyl chloride (TMC). IR spectroscopic and XPS analysis confirmed the successful formation of the DAP-polyamide selective layer on the PES substrate. The DAP-TFC membrane’s surface exhibited higher hydrophilicity, less roughness, and higher crosslinking density than conventional TFC membrane (MPD-TFC) prepared via IP of meta-phenylene diamine (MPD) and TMC. The robust DAP-TFC membrane exhibited higher water flux (6.7 LMH) and lower specific solute flux (SSF) of K+ (2.2 g/L) in FO testing using 0.6 M NaCl feed solution and 3.0 M KOH draw solution. Additionally, lower forward solute flux for Na⁺ (5.8 gMH) and Cl^- (9.5 gMH) and SSFs Na⁺ (0.9 g/L) and Cl^- (1.4 g/L) were achieved. Most importantly, the DAP-TFC membrane demonstrated excellent stability under extreme pH conditions, maintaining integrity for 60 h under FO mode with 0.6 M NaCl feed solution and 3 M KOH draw solution, in contrast to conventional MPD-TFC, which disintegrated after 5 h. The enhanced DAP-TFC membrane stability is attributed to formation of iminol tautomer, stabilizing H-bonding, facilitating reversible cation capture, and preventing hydrolysis, thereby improving chemical resistance and ion rejection. The alteration of the PA selective layer using DAP offers efficient approach for fabricating FO membranes with superior alkaline stability, holding great potential for industrial applications requiring high pH stability, particularly in challenging environments and demanding applications.