Coagulation-assisted membrane immobilization of Prussian Blue analog nanoparticles for ultra-selective ammonium separation

Abstract

Selective recovery of ammonium from complex aqueous environments is critical for circular nutrient economy and mitigating eutrophication. Highly selective nanoscale sorbents like Prussian Blue analogs offer an alternative to zeolites, but in continuous-flow systems, maximizing active-material loading often compromises structural stability and sorption kinetics. Here, we present a novel coagulation-assisted stabilization strategy to engineer a mixed-matrix membrane featuring a 50 wt% loading of zinc hexacyanoferrate nanoparticles, a record high loading for water filtration membranes. By doping the polyethersulfone casting solution with Fe³⁺, we induced a reduction in nanoparticle surface charge, triggering controlled intramembrane aggregation. These aggregates fill the evolving membrane voids during phase inversion, prevent nanoparticle leaching, and enable exceptional ammonium removal capacity of 2.3 ± 0.3 g·m⁻² in short contact times (10-26 sec). Crucially, high permeate fluxes may improve convective/dispersive access of NH₄⁺ to exchange sites within the ZnHCF membrane structure, overcoming the diffusion limitations of conventional packed beds. Pilot-scale evaluations (0.613 m²) under dynamic, low-concentration conditions successfully validated the system’s robust reusability and practical viability. This coagulation-based stabilization establishes a highly versatile platform for mixed-matrix membrane engineering, readily adaptable to immobilize diverse functional nanoparticles for a broad spectrum of separation applications, thus bridging the gap between ultra-selective nanomaterials and scalable membrane engineering.