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

A new NH adsorbent with high capacity and selectivity, sodium cobalt(ii) hexacyanoferrate(ii) (NaCoHCF, Na Co(ii) [Fe(CN)] ·HO), was prepared. The adsorption performance was investigated by varying the mixing ratio of [Fe(CN)] to Co during synthesis, . The ammonia capacity was found to be proportional to , indicating that the NH 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 = 1.00. Even on the addition of a saline solution (NaCl) with an Na-ion concentration of 9350 mg L, the capacity was maintained: = 4.28 mol kg. Using Markham-Benton analysis, the selectivity factor, defined by the ratio of equilibrium constants for NH to that for Na, was calculated to be = 96.2, and 4.36 mol kg was found to be the maximum capacity. The high selectivity of NaCoHCF results in good NH -adsorption performance, even from seawater. In comparison with other adsorbents under the same conditions and even for a NHCl 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 NH adsorption changed their sizes and shapes after adsorption.