Selective removal of ammonium ions with transition metal hexacyanoferrate (MHCF) electrodes

This study investigated the potential of using transition metal hexacyanoferrates (MHCF) to selectively capture NH4+ over Na+ in brine for nutrient recovery from wastewater. The electrochemical characteristics of five different MHCF (M is Ni, Cu, Fe, Co or Zn), known as Prussian blue analogues (PBA), was investigated using a hybrid capacitive deionization device, and their desalting capacity, stability and energy consumption were evaluated. By adjusting the mass ratio of electrodes, applied voltage, and ion concentration, the relationship between NH4+ removal and selectivity was explored. Results showed that all five MHCF electrodes exhibited high selectivity (> 4) for NH4+ over Na+, but the selectivity is not stable due to comprehensive factors (such as the structural stability of electrode, solution composition, and current density, etc.). Among them, the NiHCF electrode showing the best performance in terms of specific desalination capacity (similar to 1.23 mmol/g), charge efficiency (>80%), and stability (82.5% capacity retention after 50 cycles). This study demonstrates that PBA has promising potential as NH4+ selective electrodes for nutrient removal and recovery in wastewater.