Selective capture of ammonium ions from municipal wastewater treatment plant effluent with a nickel hexacyanoferrate electrode

Currently, intercalation materials such as Prussian blue analogs have attracted considerable attention in water treatment applications due to their excellent size-based selectivity toward cations. This study aimed to explore the feasibility of using a nickel hexacyanoferrate (NiHCF) electrode for selective NH4+ capture from effluent from a municipal wastewater treatment plant. To assess the competitive intercalation between NH4+ and other common cations (Na+, Ca2+), a NiHCF//activated carbon (AC) hybrid capacitive deionization (CDI) cell was established to treat mixed-salt solutions. The results of cyclic voltammetry (CV) analysis showed a higher current response of the NiHCF electrode toward NH4+ ions than toward Na+ and Ca2+ ions. In a single-salt solution with NH4+, the optimized operating voltage of the hybrid CDI cell was 0.8 V, with a higher salt adsorption capacity (51.2 mg/g) than those obtained at other voltages (0.1, 0.4, 1.2 V). In a multisalt solution containing NH4+, Na+, and Ca2+ ions, the selectivity coefficients of NH4+/Ca2+ and NH4+/Na+ were 9.5 and 4.9, respectively. The feasibility of selective NH4+ capture using the NiHCF electrode in a hybrid CDI cell was demonstrated by treating the effluent from a municipal wastewater treatment plant (WWTP). The intercalation preference of the NiHCF electrode with the WWTP effluent was NH4+>K+>Na+>Ca2+>Mg2+, and NH4+ showed the highest salt adsorption capacity among the cations during consecutive cycles. Our results revealed that cations with smaller hydrated radii and lower (de)hydration energies were more favorably intercalated by the NiHCF electrode. The results provide important knowledge regarding the use of intercalation-type electrodes for selective nutrient removal and recovery from wastewater.