Principle of Ferrocyanide Coprecipitation for Cs Removal
The reaction of soluble Fer salts (K, Na, or H compounds) with metal (Fe, Cu, Zn, Ni, Cd, Mn, etc.) ions in solution produces insoluble metal-Fer complexes. Fer ion and metal can precipitate as such compounds as A2M3[Fe(CN)6]2・nH2O, A2MFe (CN)6・nH2O, M2Fe(CN)6・nH2O, or as mixtures of these, depending on concentrations of alkali metal ions (designated as A+) and divalent transition metal ions (M2+) in the solution. The elemental composition and crystal structure of precipitates also vary with the combination of soluble Fer salt (e.g., lithium Fer, sodium Fer, or potassium Fer) and transition metal salts (e.g., chloride, nitrate, or sulfate salts) used . Some trivalent metals (e.g., Fe3+) also precipitate with Fer.
The insoluble Fer compounds preferentially incorporate Cs into their structure by multiple mechanisms such as ion exchange, isomorphic substitution, and adsorption. Distribution of Cs to metal-Fer precipitates is known to vary depending on solution pH and chemical characteristics of Fer solids. The distribution coefficient values (ml/g) are in the range of 104 to 106 for K-Co-Fer, 105 to 106 for Na-Ni-Fer, 105 for Na-Cu-Fer, 104 for K-Cu-Co-Fer, and 103 for K-Zn-Fer and Zn-Fer . The coefficient value for Cs with Zn-Fer is small compared to those with Fe-Fer, Cu-Fer, and Ni-Fer complexes, but the aforementioned distribution coefficient values could have been underestimated owing to insufficient removal of colloidal Fer solids from solution. Alkali metal (i.e., principal constituents such as Na and K in solution) substitution in metal-Fer complexes also results in changes in Cs distribution . The pH ranges for Cs distribution to preformed Fer complexes of Ni(II), Zn(II), Cu(II), and Fe(III) are reported to be 0 to 10, 1 to 8, 0 to 8, and 0 to 6, respectively . Variations in Cs distribution to solid Fer within these pH ranges are also reported . It is known as well that Fer should not be used in highly caustic and acidic solutions, because it is chemically decomposed in these reaction conditions.
The size of Fer precipitates is important as it determines settling velocity, which is a crucial factor for separation of Cs-containing solids in solution. Iron-Fer precipitates are often found as colloidal particles, and their separation by gravity settling method is difficult whereas Ni-Fer precipitate settles more easily. The physical properties of precipitates depend on preparation procedures. For example, in strongly oversaturated solutions, very fine crystalline particles with a disordered lattice and higher solubility are formed incipiently, whereas an inactive solid phase is formed in slightly oversaturated solutions .
Solubility of metal-Fer precipitate is also a governing factor for Cs removal. The reported solubility product values for pure Fe4[Fe(CN)6]3, Zn2[Fe(CN)6], Cu2[Fe (CN)6], and Ni2[Fe(CN)6] are 3.3 x 10-41,4 x 10-16, 1.3 x 10-16, and 1.3 x 10-15, respectively . In the case of Ni2[Fe(CN)6], Fer concentration in solution should be higher than 10-5 M for insoluble Fer precipitate to be formed. Because the solubility of fresh metal-Fer compounds can be higher than that of pure compounds, the minimum Fer concentration used in this study was 10-4 M.
There are three different ways to use Fer to remove Cs: (1) addition of soluble Fer salts and metal elements to waste solution (in situ formation of Fer solid), (2) addition of freshly prepared insoluble Fer-metal complex slurry to waste solution, and (3) use of Fer-metal adsorbents in solution. The distribution of Cs to insoluble Fer compounds is highest when the in situ Fer formation method is applied. If appropriately used, the in situ method is the best for both decontamination and waste volume reduction.