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Results and Discussion

Metal concentrations in waste samples extracted with oxalic acid (samples 3–6, Table 29.1) were in general high. The scrubber wastewater from an industrial waste incinerator (sample 7, Table 29.1) showed very high Zn concentration. Not well metals in samples 8–12 (Table 29.1) could be measured because of the constraint on elemental analyses of rad-Cs-contaminated samples. Nevertheless, Table 29.3 Removal of cesium (Cs) from the 1 M oxalic acid extract with different dilution factor applied before ferrocyanide (Fer) coprecipitation (the solution pH was 5 and Fer was 0.1 mM for all samples)

Sample code

100 times dilution (~0.01 M oxalic acid)

20 times dilution (~0.05 M oxalic acid)

10 times dilution (~0.1 M oxalic acid)

3

97

99

93

4

97

95

93

5

95

97

96

6

98

95

92

the data suggest that metal concentrations in rad-Cs-contaminated sewage sludge (samples 10–12, Table 29.1) were similar to those in the extracts of uncontaminated sewage sludge (samples 1, 2, 4, 5, and 6, Table 29.1). Samples 8 and 9 (Table 29.1) showed low metal concentrations as they were stabilized waste materials that were treated to reduce heavy metal leaching.

The removals of Cs in different samples and test conditions are summarized in Tables 29.2, 29.3, and 29.4. Table 29.2 shows Cs removal efficiencies (%) for samples 1–7 under different pH and K-Fer concentrations without an addition of metals. The tests were conducted for pH 3–10 and soluble Fer salt concentrations 0.1–1.0 mM. The results indicated that insoluble Fer complexes were formed with metals present in the waste extracts (Table 29.1) upon addition of soluble K-Fer salts, resulting in high Cs removal efficiencies. Fer complexes could be formed with any of the metals such as Fe, Mn, Cu, and Zn present in sufficient concentrations (Table 29.1) to precipitate 0.1 mM Fer ions. Cs removal in sample 7 was lower (e.g., 74 % at pH 5) than those in other samples, although transition metals (particularly Zn) in the sample were abundant for the formation of insoluble Fer complexes. In a control experiment discussed in our previous work [4], we investigated on the effect of Zn concentration on Cs removal, and found that Cs removal by Fer solids tends to be low when Zn is present at pH 5. Formation of the Zn-Fer complex, which is known to have a comparatively low Cs distribution factor, probably reduced Cs removal in the sample. The Cs removal in sample 7 at pH 3 increased to 92 %, possibly because of the formation of iron-Fer complex, which has a high Cs distribution factor [4]. Moreover, Cs removal increased with increasing K-Fer concentration. However, this leads to increased amount of precipitate in the solution, which is not preferable from the aspect of waste volume reduction.

Table 29.3 shows Cs removal from oxalic acid extracts. In a separate experiment, we examined the effect of oxalic acid concentration on Fer coprecipitation method and concluded that for 0.1 mM Fer concentration, oxalic acid concentration should be 0.01 M or less for precipitation of insoluble Fer compounds. In actual waste extracts, oxalic acid is consumed by calcium present in the wastes, and hence actual oxalic acid concentrations are lower than those in the original reagents. The data in Table 29.3 show that Cs removal is possible with 20 times (nominal concentration ¼ 0.05 M) as well as 10 times (nominal concentration ¼ 0.1 M) dilutions of 1.0 M oxalic acid extract.

Table 29.4 Removal of radioactive cesium (rad-Cs) from samples 8–12

Sample code

Fer concentration (mM)

pH

Metal added

Concentration of added metal (mM)

Cs-137

removal (%)

Nominal oxalic acid concentration (M)a

Fly ash K

8

0.1

3

Fe(III)

1.8

100

0

3

Fe(III)

0.4

100

3

Fe(II)

1.8

93

5

Ni

0.2

100

3,

5, 7,

9

Zn

0.1–1

0

Fly ash N

9

0.1

5

None

0

23

0

3

Fe(III)

1.8

52

3

Fe(II)b

1.8

100

5

Fe(II)

1.8

91

5

Ni

0.1

58

5

Ni

0.2

62

5

Zn

0.1

0

5

Zn

0.4

26

5

Zn

1

3

Fly ash CM (water extract)

10

0.1

0.1

2.2

None

92

0

5

Nickel ferrocyanide

96

Fly ash CM (0.1 M oxalic acid extract)

11

0.1

3

None

0

36

0.05

0.3

3

None

0

96

0.1

5

Cupper ferrocyanide

82

0.1

5

Nickel ferrocyanide

93

0.2

5

Nickel ferrocyanide

100

Fly ash CI (0.1 M or 0.5 M oxalic acid extract)

12

0.3

3

Ni

0.6

69

0.05

0.3

5

Ni

0.6

80

0.05

0.1

3

None

0

69

0.25

0.1

5

None

0

62

0.25

aIn actual waste extracts, oxalic acid is consumed by calcium present in the wastes, and hence actual oxalic acid concentrations are lower. The concentration listed in this table is nominal concentration, not considering the consumption of oxalic acid

bRemoval of metals: Fe 99 %, Cu 60 %, Zn 0 %

Table 29.4 shows results of coprecipitation tests conducted with fly ash extracts contaminated with rad-Cs as the result of the F1 accident. On-site analysis of samples 8 and 9 using a portable voltammetry instrument revealed that metal concentrations in the samples were relatively low. Addition of K-Fer alone in sample 8 resulted in 23 % rad-Cs removal. In fact, metal salts had to be added before the addition of soluble Fer salts for the formation of insoluble of Fer complexes in the samples. We, therefore, compared removals of rad-Cs with different metals (e.g., Ni(II), Fe(II), Fe(III) or Zn(II)). In sample 9, light greencolored Ni-Fer precipitate was formed when Ni and soluble Fer salt were added, but rad-Cs removals were only 58–62 %. The removal increased to almost 100 % only when 1.8 mM Fe(II) (in excess to 0.1 mM Fer) was used. Apparently, the removal of rad-Cs changed significantly for sample no. 9 depending on the type of iron salt (ferric or ferrous iron) used with K-Fer, but the reason remains unknown at this point. In contrast, the extract of molten fly ash sample 8 showed almost complete removal of rad-Cs with Fer-Fe(II), Fer-Fe(III), and Fer-Ni coprecipitation. The results may be explained by the existence of colloidal, nonionic rad-Cs (e.g., sorbed on suspended particles, but passed through 0.45-μm-pore-size filter) in the incinerator fly ash extract sample 9, because Cs in such form does not precipitate with Fer-Ni, but it precipitates with Fe through coagulation-precipitation mechanisms.

For rad-Cs-contaminated sewage wastes (samples 10–12, Table 29.4), on-site analysis for metal contents showed rather high Zn concentrations for samples 11 and 12 whereas Fe was prevalent in sample 13. We, therefore, conducted co-precipitation tests at pH 3 to produce Fe-Fer rather than Zn-Fer, that has a low Cs distribution factor, or added Ni-Fer in place of K-Fer to prevent formation of Zn-Fer in the samples.

Overall, very high rad-Cs removals (>95 %) were observed for contaminated waste extracts (samples 8–11), although we did not have enough time to optimize coprecipitation conditions for sample 12.

 
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