Impact of in-situ silica nanoparticles-grafted polyamide nanocomposite membranes for simultaneous removal of pollutants from wastewater

A pristine polyamide (PA) nanofiltration membrane was benchmarked against two silica-based thin-film nanocomposite (TFN) membranes to clarify how silica–polymer interfaces govern multipollutant removal. TFN-1 was prepared by in-situ incorporation of pre-synthesized silica nanoparticles (Si-NPs) functionalized with a polydopamine–melamine (PDA–MM) composite, while TFN-2 incorporated Si-NPs modified with a 3-aminopropyltriethoxysilane–trimesic acid–melamine (APTES–TA–MM) composite. Surface analyses revealed that TFN-2 possessed greater hydrophilicity and a higher density of –OH/–COOH/–NH₂ groups; FESEM confirmed a more uniform nanoparticle dispersion, features that correlated with superior separations. In a Sterlitech™ test cell, TFN-1 achieved a water flux of 220 L m⁻² h⁻¹, and TFN-2 maintained a stable flux across all tested contaminants while preserving higher selectivity, with a stable flux of 90 L m⁻² h⁻¹. Mechanistically, separations arise from the synergy of size exclusion through the dense PA layer, electrostatic (Donnan) repulsion from the negatively charged surface (especially for TFN-2), and adsorption/chemisorption via amine and hydroxyl‑rich moieties that enable hydrogen bonding and π–π interactions. TFN-2 achieved immediate rejections of ≈ 98 % for hydrocarbons, ≈ 93 % for heavy-metal ions, and 73 % for SO₄²⁻.