Surface functionalization of aromatic polyamide nanofiltration membranes for the selective removal of metal ions and pharmaceutical contaminants

Growing global water scarcity and increasing contamination of water resources by industrial effluents, heavy metals, and pharmaceutical residues have increased the demand for treatment technologies. Nanofiltration (NF) membranes are widely used due to their ability to selectively remove divalent ions and organic micropollutants while operating at lower pressures than reverse osmosis. However, conventional polyamide thin-film composite (TFC) membranes remain limited by the permeability–selectivity trade-off, fouling susceptibility, and pH-dependent instability of surface charge. In this work, aminomethylphosphonic acid (AMPA) was incorporated as a functional additive during interfacial polymerization. Its phosphonic acid groups provide stable negative charge across a wide pH range, while amine functionalities participate in polymerization, enhancing membrane hydrophilicity and interactions with divalent metal ions. TFC membranes were fabricated using piperazine (PIP) and trimesoyl chloride (TMC) with and without AMPA. The membranes were characterized by ATR-FTIR, AFM, contact angle measurements, and zeta potential analysis. Separation performance was evaluated through cross-flow filtration tests measuring flux and rejection of inorganic salts, divalent metal ions, neutral probes, and pharmaceutical compounds. AMPA-modified membranes exhibited higher water permeance while maintaining rejection of divalent salts. Zeta potential measurements confirmed a more negative and stable surface charge profile, while multi-cation experiments indicated enhanced retention of metal ions due to combined Donnan exclusion and coordination with phosphonate groups. The MWCO was approximately 290 g mol⁻¹, and fouling tests showed improved antifouling behavior and high flux recovery. Overall, AMPA incorporation during interfacial polymerization represents an effective strategy to enhance permeability, selectivity, and fouling resistance in nanofiltration membranes for water treatment.