Highly efficient As(III) removal via alkali-stable nanofiltration for sustainable water purification

Conventional polyamide nanofiltration (NF) membranes (PA-M₀) exhibit poor As(III) removal efficiency primarily due to the neutral molecular form (H₃AsO₃) of As(III) under typical conditions. Although alkaline environments can convert As(III) into an anionic species to enhance separation, most NF membranes lack chemical stability at high pH. Herein, we develop a series of alkali-resistant NF membranes (PA-M_T, T = 1-24 h, representing the reaction time) through H₂SO₄/EtOH-induced physical swelling and chemical reconstruction of PA-M₀, enabling efficient As(III) removal in alkaline media. The optimized membrane, PA-M_6 h, improves As(III) rejection substantially from 19.9 % to 90.2 % as feed pH increases from 7 to 10, while maintaining a high water flux of 17.7 LMH/bar, This performance considerably surpasses that of PA-M₀ and previously reported NF systems (typically 10 - 60 % rejection). Furthermore, PA-M_6 h exhibits exceptional antifouling properties, sustaining 91.5 % As(III) rejection and a stable flux of 13.7 LMH/bar over extended cycles in the presence of humic acid foulants - more than doubling the removal efficiency of PA-M₀ (5.9 LMH/bar). This study advances molecular-scale interfacial engineering in membrane design, promoting the development of environmentally sustainable separation technologies and providing a scalable solution for arsenic-safe water purification.