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Study for the Formation of Sponge-like Ion Exchanger Containing Sodium-form Synthetic Mica and Its Cesium Adsorption Characteristics

As an adsorbing material for recovering cesium from radioactive contaminated water, a sponge-like compound was synthesized using sodium-form synthetic mica (Na-TSM) as an ion exchanger which does not contain lithium and is inexpensive and readily available. The cesium adsorption ratio from aqueous solutions was 80% or higher in low cesium concentration region and the desorption ratio was less than 5%. It suggests that synthesized sponge-like material is much adapted as cesium adsorbent.


Introduction
The unprecedented earthquake disaster that hit eastern Japan on March 11, 2011 has raised many problems even seven years thereafter. Among them, the environmental pollution caused by accidents at Fukushima Daiichi Nuclear Power Plant is a grave situation, and there are many inhabitants who have not yet returned due to the seriousness of the pollution. The problem of contamination by radioactive substances can be roughly divided into two. One is the problem of the nuclear power plant itself. The other is the local pollution by spread radioactive materials. Focusing on contaminated water, the authors consider two factors. Firstly, seawater injected for cooling the nuclear reactor just after the accident became a large amount of contaminated water at the nuclear power plant. Secondly, the groundwater flowing down to the sea from mountainside became also contaminated water as it passed through the polluted area. These contaminated waters are stored in tanks, and beautiful forests were transformed to forests of tanks with an increasing amount of contaminated water. Tokyo Electric Power Co. is working on the contaminated water countermeasures following the three mottoes; "Remove", "Do not approach" and "Prevent leak" 1) .
As for "Remove" among them, a multi-nuclide removal equipment, ALPS, has been introduced and tests are underway for full-scale operation. Before introducing contaminated water into ALPS, it is necessary to remove cesium and strontium.
For such a purpose, cesium adsorption apparatus such as KURION and SARRY are used. In these devices, the key collecting material is mainly zeolite and a lot of research has been conducted regarding recovery of cesium [2][3][4][5][6] . Meanwhile, in areas where radioactive substances were spread, decontamination was carried out at many public facilities and individual housing in order to reduce air dose below the standard, leading to the generation of contaminated water. It is mandatory to recover contaminated water, to remove cesium before discharge to sewers and to serve methods for passing through sandbags using zeolite and filtering after coagulation sedimentation, etc.
The authors' research regarding buffer materials in the geological disposal of spent nuclear fuel reported that sodium taeniolite had high total cesium-ion adsorption capacity and sodium-form synthetic mica, Na-TSM, had high ion-exchange selectivity for cesium ion 7) . Na-TSM belongs to the same tetrafluoride mica group as sodium taeniolite but does not contain lithium. In layered clay minerals, interlayer interchangeable cations are generally involved in ion-exchange phenomenon. In Na-TSM, the exchange occurs between interlayer sodium and external cesium ions.
However, ion-exchange sites are not only interlayer zones but other sites are also involved 8) . When these substances are utilized as buffer materials, their powder form is not a matter but rather a merit because it is meant to fill the gaps. The sponge-like Na-TSM obtained after ion exchange described above was dried in the air without rinsing.
Ultrapure water of 20 mL was taken in a plastic container and the dried sample was added. Reaction and measurement were carried out under the same conditions as the ion-exchange experiment. Table 1 shows the amounts of materials used for each sponge. The sponge prepared with the same formulation as sodium taeniolite 9) was described as "Former Sponge".

Results and Discussion
Photographs of sponge-like Na-TSMs synthesized with the amounts listed in Table 1 are shown in Fig. 1 into the interlayer of smectite is up to two layers for sodium taeniolite and ten layers for Na-TSM 10,11) . When ultrapure water is added to Na-TSM/PVF/DMA mixture, Na-TSM is apter to swell than sodium taeniolite. During the dry process, the shrinkage of interlayer spacing is larger than sodium taeniolite because dehydration is more. At same time, PVF also diminishes following the contraction between layers. As a result, the sponge became smaller such as Former Sponge as seen in Fig. 1. This phenomenon could be partly explained by the fact that the contraction of the sponge was improved by decreasing the ratio of Na-TSM. Sponge A and Sponge B differed in thickness due to the difference in the amount of Nonetheless, in this study, the purpose is to recover the clay mineral itself after absorbing target ions from contaminated water and allow the treated water below environmental standards to be discharged to ordinary sewage, etc. If the clay mineral was powder, it should be filtered. Therefore, molding in some way is indispensable to easy operation. The authors started molding of sodium taeniolite because it has high total cesium adsorption capacity rather than of its selectivity. In the attempts to granular and to form film, the authors succeeded in molding by taking advantage of the hydrophobicity of poly(vinylidene fluoride), gave the name "sponge-like form" to the product and reported its ion-exchange properties 9) . In this method, recovery ratio is 95.4% when the initial  Comparing Sponge B and powder Na-TSM, it can be seen that the adsorption ratio decreased as a result of molding. In Na-TSM, interlayer sodium ion underwent ion exchange for cesium ion in the reaction solution with excluding the interlayer water and the interlayer distance shrinked 7) . Figure   2 shows  and sponge-like sodium taeniolite 9) . Adsorption ratios were calculated as follows. Water on the surface was wiped off before measurement.  The results of the desorption experiment are shown in Table 2. Desorption ratio is calculated as follows. With reaction solutions of 5 mmol/L and 10 mmol/L, the desorption ratios on Sponge A are higher than that of other samples. This result suggests the higher adsorption ratios contribute to the lower desorption ratios. In such sponge-like Na-TSMs, the desorption ratios are less than 5%, therefore a sponge-like Na-TSM after use can safely be stored even if in contact with moisture of the level of environmental water.

Conclusion
Sponge-like Na-TSM was successfully molded using poly(vinylidene fluoride) as a binder. It exhibited a somewhat higher recovery ratio compared with the sponge-like sodium taeniolite. Furthermore, the desorption experiment demonstrated that the desorption ratio of cesium is less than 5% in the used sponge-like Na-TSM with high adsorption ratio, suggesting that a sponge-like Na-TSM is useful as cesium recovery material.