Piperazine derivatives monomer-based TFC nanofiltration membranes with enhanced mineral/organic selectivity for efficient surface water purification

Thin-film composite nanofiltration (NF) membranes are attractive for drinking-water treatment; however, conventional polyamide (PA) selective layers rarely combine high water permeance, efficient organic-matter rejection and low mineral-salt rejection. Herein, 1,4-bis(3-aminopropyl) piperazine (BAPP) was used as an aqueous-phase amine monomer to fabricate PA NF membranes via interfacial polymerization, with the BAPP concentration systematically varied to regulate membrane formation. BAPP concentration controlled the balance between monomer diffusion and interfacial reaction, thereby tuning the crosslinking degree, pore size and free-volume fraction of the selective layer. Decreasing the BAPP concentration markedly enhanced water permeance while progressively lowering salt rejection. The optimized BAPP-0.15 membrane balanced network integrity with open transport channels, delivering a water permeance of 19.1 L m⁻² h⁻¹ bar⁻¹ (1.5 times that of commercial NF270), DOC removal of 75.5%, UV₂₅₄ removal of 74.2% and a Ca²⁺/DOC selectivity of 40.9. Fluorescence spectroscopy and UV₂₅₄-based molecular-weight distribution analysis further confirmed the effective removal of humic-like, protein-like and medium-molecular-weight aromatic organics from natural surface water. This work provides a facile monomer-concentration-regulation strategy for designing NF membranes with enhanced mineral/organic selectivity for healthy drinking-water production.