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.
29.5 Conclusion
Selective removal of Cs using Fer precipitation was conducted with extracts of
sludge and fly ashes generated from municipal water treatment plants and waste
incineration plants in the areas affected by the F1 accident. More than 95 % rad-Cs
removals were achieved for an optimized combination of pH, Fer concentration,
and type of added metal salts. The chemical form (ionic or particulate) of Cs in
waste extracts, heavy metal leaching from the wastes (i.e., whether the waste had
undergone stabilization treatment), and Zn concentration influenced Cs removal.
The results undoubtedly suggest that knowledge of principal metal content is very
important for successful application of the Fer coprecipitation technique to remove
rad-Cs from contaminated wastes.
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Y. Fujikawa et al.
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.
29.5 Conclusion
Selective removal of Cs using Fer precipitation was conducted with extracts of
sludge and fly ashes generated from municipal water treatment plants and waste
incineration plants in the areas affected by the F1 accident. More than 95 % rad-Cs
removals were achieved for an optimized combination of pH, Fer concentration,
and type of added metal salts. The chemical form (ionic or particulate) of Cs in
waste extracts, heavy metal leaching from the wastes (i.e., whether the waste had
undergone stabilization treatment), and Zn concentration influenced Cs removal.
The results undoubtedly suggest that knowledge of principal metal content is very
important for successful application of the Fer coprecipitation technique to remove
rad-Cs from contaminated wastes.
340
Y. Fujikawa et al.