Leaching Characteristics of Cs from the Decomposed Cu Ferrocyanide Adsorbent Solidified by Portland Cement and Geopolymer

Abstract

The use of cement technology is under investigation for the disposal of ¹³⁷Cs, which is a type of radioactive waste and which was dispersed as a result of the accident. Here, we present the results of a study on the leaching characteristics of Cs from solidified bodies. Combustible materials contaminated with radioactive Cs released by the accident at the TEPCO's Fukushima Daiichi NPS are incinerated to reduce their volume, and the incineration residue is further reduced by melting treatment. Cs exists as water-soluble salts in the molten fly ash. The Cs can be transferred to the liquid phase by washing, and the Cs can be further concentrated by ion-exchange chromatography using Cs adsorbents such as ferrocyanide transition metal salts. The adsorbent used must be stabilized for final disposal. We present an example of using copper ferrocyanide as an adsorbent in this process. Copper ferrocyanide was calcined and decomposed, then solidified with Portland cement or metakaolin geopolymer. Since cement hydrate has no cation exchange property, all Cs leached into pure water, but copper ferrocyanide, after pyrolysis, became cation exchangeable in the alkaline environment of Portland cement, resulting in a Cs leaching rate of only 20%. The Cs leaching rate from the geopolymer solidified body was 2% because the geopolymer is an ion exchanger and has a large Cs ion selectivity for Na ions. However, from the viewpoint of workability during mixing, there was an upper limit to the amount of material to be treated, and cement solidification was superior in terms of volume reduction.

Appendix: Cs+ Ion Selectivity of Geopolymer Against Na+ and Cs Leaching in Saline Water

Cs adsorption by geopolymer (GP-) is caused by ion-exchanging reaction as shown in Eq. (A-1).

$$\mathrm{GP}-\mathrm{Na}+{\mathrm{Cs}}^{+} \iff \mathrm{ GP}-\mathrm{Cs}+{\mathrm{Na}}^{+}$$

(A-1)

Geopolymers are cation exchangers, but their cation exchange capacity is determined by the amount of four-coordinated Al. The alkali metal ions fixed in the geopolymer are equal to the number of moles of Al.

Cs⁺ ion selectivity of geopolymer against Na⁺ at the equilibrium is defined as Eq. (A-2).

$${K}_{\mathrm{Cs}/\mathrm{Na}}=\frac{\left[\mathrm{GP}-\mathrm{Cs}\right][{\mathrm{Na}}^{+}]}{\left[\mathrm{GP}-\mathrm{Na}\right][{\mathrm{Cs}}^{+}]}$$

(A-2)

[] means ion concentration in mol/kg for solid or in mol/L for solution. GP-X means X ion containing geopolymer.

Fig. A-1.

Cs⁺ selectivity coefficient against Na⁺ of geopolymer with various Si/Al ratios and at various Cs adsorption degree (Cs/Al) [13].

Three different geopolymers with different Si/Al molar ratios were synthesized from metakaolin and water glass. 27Al-NMR showed that all Al was four-coordinated. The three geopolymers were immersed in CsCl/NaCl mixtures of different concentrations and the KCs/Na was determined from the Cs + ion depletion from the liquid phase according to Eq. (A-2), as shown in Fig. A-1 KCs/Na was higher with a higher Si/Al molar ratio, i.e. a lower cation exchange capacity. However, it does not increase so much when Si/Al is increased from 1.69 to 2.04. KCs/Na also decreases with increasing Cs adsorption rate (Cs/Al). Both 23Na-NMR and 133Cs-NMR show a chemical shift change with increasing Cs adsorption, indicating that the ion exchange sites in the geopolymer have different Cs selectivities and are therefore filled by Cs in the order of sites with large Cs selectivities. The synthesized geopolymer is X-ray amorphous, but as type A zeolites with an Al/Si ratio of 1 and low Cs selectivity and phillipsite with an Al/Si ratio of 1.67 and high Cs selectivity are known, it is possible that local structures similar to both are mixed in the geopolymer.