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

Abstract

A new NH₄⁺ adsorbent with high capacity and selectivity, sodium cobalt(II) hexacyanoferrate(II) (NaCoHCF, Na_yCo(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, 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.