Architected Composite Reverse Osmosis Membrane with Multibarrier for Enhanced Ammonium Selectivity
Yi-Yu Ling, Kexin Yuan, SIMING XIE, Xin Li, Wei Wang, Haoran Feng, Xian Bao, Jun Ma
Abstract
To overcome the limited ammonium (NH 4 + –N) rejection (∼93%) of conventional reverse osmosis (RO) membranes in wastewater treatment, a novel thin-film composite RO (TFC-RO) membrane with ammonium-rejecting functional “fillings” throughout the polyamide (PA) matrix was fabricated by doping polyamidoamine (PAMAM) dendrimers into the PA layer. This unique filling-integrated architecture endowed the RO membrane with an exceptional NH 4 + –N rejection of 98.39%. Mechanistically, PAMAM dendrimers optimize monomer diffusion kinetics and reaction thermodynamics during interfacial polymerization (IP), regulating the self-limiting effect to form a defect-reduced PA layer with homogenized free-volume characteristics, thereby improving size-sieving capabilities while maintaining favorable water permeability (3.74 L/m 2 ·h·bar). Concurrently, the embedded abundant amine groups (−NH 2 and R 3 N) undergo protonation to form highly positive charges (−NH 3 + and R 3 NH + ) within the PA layer, synergizing with the negatively charged membrane surface (−COO – ) to establish a robust multiscale electrostatic barrier. This system directly repels NH 4 + and indirectly enhances rejection by impeding Cl – transport via the Donnan effect. Furthermore, the structural similarity between the protonated −NH 2 (−NH 3 + ) and NH 4 + ions generates a “concentration trap” within the PA layer via molecular mimicry, inducing site-specific simulation that elevates intramembrane NH 4 + concentration to establish a chemical potential barrier to oppose NH 4 + diffusion. The synergistic “structure-charge-concentration” mechanism intrinsic to the filling-integrated design thus effectively inhibits NH 4 + migration while concurrently enabling integrated fouling resistance through optimized surface architecture and charge-regulated interfacial interactions. This study offers an advanced membrane separation technology, contributing significantly to low-carbon–water recycling and sustainable development goals.