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, NayCo(II) [Fe2+(CN)6]x·zH2O), was prepared. The adsorption performance was investigated by varying the mixing ratio of [Fe(CN)6]4− to Co2+ during synthesis, Rmix. The ammonia capacity was found to be proportional to Rmix, 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 Rmix = 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: qmax = 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.