from the F1 accident), applied Fer precipitation techniques to the uncontaminated
MSW extracts, and then proceeded to rad-Cs-contaminated waste treatment.
29.2 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 A 2 M 3 [Fe(CN) 6 ] 2 ・nH 2 O, A 2 MFe
(CN) 6 ・nH 2 O, M 2 Fe(CN) 6 ・nH 2 O, or as mixtures of these, depending on concentrations of alkali metal ions (designated as A
+
) and divalent transition metal ions
(M
2+ ) 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 [13]. Some trivalent metals (e.g., Fe
3+ ) 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 10
4 to 10
6 for K-Co-Fer, 10
5 to 10
6 for Na-Ni-Fer,
10
5 for Na-Cu-Fer, 10
4 for K-Cu-Co-Fer, and 10
3 for K-Zn-Fer and Zn-Fer [5]. 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 [14]. 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 [14]. Variations in Cs distribution to solid Fer within these pH
ranges are also reported [15]. 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 [16].
Solubility of metal-Fer precipitate is also a governing factor for Cs removal. The
reported solubility product values for pure Fe 4 [Fe(CN) 6 ] 3 , Zn 2 [Fe(CN) 6 ], Cu 2 [Fe
29 Volume Reduction of Municipal Solid Wastes Contaminated with Radioactive. . .
333
MSW extracts, and then proceeded to rad-Cs-contaminated waste treatment.
29.2 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 A 2 M 3 [Fe(CN) 6 ] 2 ・nH 2 O, A 2 MFe
(CN) 6 ・nH 2 O, M 2 Fe(CN) 6 ・nH 2 O, or as mixtures of these, depending on concentrations of alkali metal ions (designated as A
+
) and divalent transition metal ions
(M
2+ ) 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 [13]. Some trivalent metals (e.g., Fe
3+ ) 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 10
4 to 10
6 for K-Co-Fer, 10
5 to 10
6 for Na-Ni-Fer,
10
5 for Na-Cu-Fer, 10
4 for K-Cu-Co-Fer, and 10
3 for K-Zn-Fer and Zn-Fer [5]. 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 [14]. 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 [14]. Variations in Cs distribution to solid Fer within these pH
ranges are also reported [15]. 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 [16].
Solubility of metal-Fer precipitate is also a governing factor for Cs removal. The
reported solubility product values for pure Fe 4 [Fe(CN) 6 ] 3 , Zn 2 [Fe(CN) 6 ], Cu 2 [Fe
29 Volume Reduction of Municipal Solid Wastes Contaminated with Radioactive. . .
333