High-capacity and selective ammonium removal from water using sodium cobalt hexacyanoferrate

A new NH₄ ⁺ adsorbent with high capacity and selectivity, sodium cobalt(ii) hexacyanoferrate(ii) (NaCoHCF, Na Co(ii) [Fe²⁺(CN)₆] ·zH₂O), was prepared. The adsorption performance was investigated by varying the mixing ratio of [Fe(CN)₆]^4- to Co²⁺ during synthesis, R _mix. The ammonia capacity was found to be proportional to R _mix, indicating that the NH₄ ⁺ capacity can be increased by increasing the Na⁺-ion content in NaCoHCF. To conduct a detailed study, we prepared homogeneous nanoparticles by flow synthesis using a micromixer with R _mix = 1.00. Even on the addition of a saline solution (NaCl) with an Na⁺-ion concentration of 9350 mg L^-1, the capacity was maintained: q _max = 4.28 mol kg^-1. Using Markham-Benton analysis, the selectivity factor, defined by the ratio of equilibrium constants for NH₄ ⁺ to that for Na⁺, was calculated to be α = 96.2, and 4.36 mol kg^-1 was found to be the maximum capacity. The high selectivity of NaCoHCF results in good NH₄ ⁺-adsorption performance, even from seawater. In comparison with other adsorbents under the same conditions and even for a NH₄Cl solution, NaCoHCF showed the highest capacity. Moreover, the coexisting Na⁺ caused no interference with the adsorption of ammonium by NaCoHCF, whereas the other adsorbents adsorbed ammonia only slightly from the saline solution. We also found that the pores for NH₄ ⁺ adsorption changed their sizes and shapes after adsorption.