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)·zH(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.