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

Jiang, Yong, Minami, Kimitaka, Sakurai, Koji, Takahashi, Akira, Parajuli, Durga, Lei, Zhongfang, Zhang, Zhenya, Kawamoto, Tohru
RSC advances 2018 v.8 no.60 pp. 34573-34581
adsorbents, adsorption, ammonia, ammonium, ammonium chloride, cobalt, ferrocyanides, iron, mixing ratio, nanoparticles, seawater, sodium, sodium chloride
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.