Architected Composite Reverse Osmosis Membrane with Multibarrier for Enhanced Ammonium Selectivity

To overcome the limited ammonium (NH₄⁺-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 NH₄⁺-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/m²·h·bar). Concurrently, the embedded abundant amine groups (-NH₂ and R₃N) undergo protonation to form highly positive charges (-NH₃⁺ and R₃NH⁺) within the PA layer, synergizing with the negatively charged membrane surface (-COO^-) to establish a robust multiscale electrostatic barrier. This system directly repels NH₄⁺ and indirectly enhances rejection by impeding Cl^- transport via the Donnan effect. Furthermore, the structural similarity between the protonated -NH₂ (-NH₃⁺) and NH₄⁺ ions generates a "concentration trap" within the PA layer via molecular mimicry, inducing site-specific simulation that elevates intramembrane NH₄⁺ concentration to establish a chemical potential barrier to oppose NH₄⁺ diffusion. The synergistic "structure-charge-concentration" mechanism intrinsic to the filling-integrated design thus effectively inhibits NH₄⁺ 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.