Selective Separation of Ammonium from Wastewater Using Ion Conducting Channels of a Prussian Blue Analogue

Wastewater treatment plants represent a centralized collection point of various resources, including ammonium, that could be recovered for the production of nitrogen fertilizer. Here, we show the use of copper hexacyanoferrates (CuHCFs) as ion-conducting channels for the selective recovery of ammonium from wastewater. This model Prussian blue analogue was deposited at the surface of a cation exchange membrane (CEM) by a simple layer-by-layer precipitation method. The resulting CuHCF-CEM showed a selectivity of 5.0 ± 0.4 for NH4+ relative to Na+ using a binary mixture of synthetic wastewater (5 mM NH4Cl, 20 mM NaCl, 0.17 mA cm–2), which was nearly 5 times greater than the bare CEM. The improved selectivity was attributed to the CuHCF layer wherein NH4+ could be migrated faster as well as more favorably occupied relative to Na+. The selectivity of CuHCF-CEM for NH4+ was maintained relatively high compared with that of CEM in the presence of competing cations in a real domestic wastewater sample. This proof-of-concept study shows that selective ion-conducting channels of CuHCFs can provide a means to ammonia recovery from wastewater in electro-driven membrane processes.