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.