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

A new NH 4 + adsorbent with high capacity and selectivity, sodium cobalt(ii) hexacyanoferrate(ii) (NaCoHCF, Na y Co(ii) [Fe 2+ (CN) 6 ] x · z H 2 O), was prepared. The adsorption performance was investigated by varying the mixing ratio of [Fe(CN) 6 ] 4- to Co 2+ during synthesis, R mix . The ammonia capacity was found to be proportional to R mix , indicating that the NH 4 + 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 NH 4 + 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 NH 4 + -adsorption performance, even from seawater. In comparison with other adsorbents under the same conditions and even for a NH 4 Cl 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 4 + adsorption changed their sizes and shapes after adsorption.