Ammonium salt production in NH₃-CO₂-H₂O system using a highly selective adsorbent, copper hexacyanoferrate

Ammonia is a beneficial material that is widely used in agriculture, but its emission into the atmosphere causes air pollution. Recently, Prussian blue (PB) and its analogs (PBA) were found to be ammonia adsorbents with high selectivity and capacity. In this study, we utilized a highly potent PBA adsorbent, copper hexacyanoferrate (CuHCF), to desorb ammonia and turn it into a reusable form. Because the reported NH₃-CO₂-H₂O system phase diagram suggests the possibility of the recovery of solid NH₄HCO₃, we examined whether adsorbed ammonia desorbs into the saturated ammonium hydrogencarbonate solution (sat. NH₄HCO₃aq). We demonstrated that 40% of adsorbed ammonia desorbed into sat. NH₄HCO₃aq. After the desorption, CO₂ was blown into the washing liquid, and NH₄HCO₃ precipitated, which was confirmed by Fourier transform infrared spectroscopy. The molar amount of solid NH₄HCO₃ was almost equal to that of desorbed ammonia. Our findings pave the way for recovery of ammonia as a valuable product from waste gas.