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Separation and recovery of radioactive cesium and strontium by metal-organic-frameworks

配位高分子による放射性セシウムとストロンチウムの分離回収

南川 卓也; 小越 友里恵; 桑原 彬; 山田 鉄兵*; Zanella, M.*; Jansat, S.*; Manning, T.*; Rosseinsky, M.*

Nankawa, Takuya; Ogoshi, Yurie; Kuwahara, Akira; Yamada, Teppei*; Zanella, M.*; Jansat, S.*; Manning, T.*; Rosseinsky, M.*

液体廃棄物中から、放射性セシウム(Cs)とストロンチウム(Sr)を取り除く材料としては、ゼオライトなど細孔性の材料が知られているが、これらの細孔は非常に安定であるため、細孔中からこれらの元素を取り出し、回収することや、細孔のサイズを自由に変化させることは困難である。一方、最近研究が進んできた配位高分子(MOF)は、配位子と金属を選択することで細孔をデザインできるため、細孔の大きさを自由に変化させることができる。また、この材料の細孔は、溶液の変化に敏感なため、一度取り込んだゲスト分子も溶液条件を変化させれば容易に取り出すことができる。本研究では、新規なMOFである(NH$$_{4}$$)[Ln(C$$_{2}$$O$$_{4}$$)$$_{2}$$(H$$_{2}$$O)] (Ln=Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu)を合成し、MOFの細孔サイズをこれまでにない方法で制御し、調整することで、溶液中からCsやSrを選択的に分離した。また、一度取り込んだCsやSrをMOFから回収する実験を行い、放射性廃棄物から有用な放射性金属元素を回収できる可能性を示した。

Radioactive Cesium(Cs) and Strontium(Sr) in liquid waste can be separated by porous materials such as zeolite. The pore sizes of these materials seems to identify metal ion radius or hydrated ion radius of Cs or Sr. But since it is very difficult to change the pore sizes of these materials, we cannot investigate the relationship between the pore sizes and Cs or Sr uptake behavior of these materials. And since the frameworks of these materials are rigid, it is very hard to recover Cs or Sr absorbed in the materials and we have to dispose Cs or Sr containing material as nuclear waste. But if Cs and Sr can be recovered from the materials, we can make use of these radioactive nuclides as radiation or heat source. On the other hand, metal-organic-frameworks (MOF) have attracted much interest in this two decades because of its fascinating characters such as gas absorption, catalytic or optical properties. The advantages of MOF over zeolites or other porous materials are a highly-ordered structure composed of metal ions and organic ligands, and highly controllable pore sizes that can be controlled by the length of ligand and ionic radii of metal ions. Moreover, structures of MOF can easily be influenced by solution phase. So, it becomes easy to recover guest ions from MOF. In this study, we synthesized a novel series of MOF, (NH$$_{4}$$)[Ln(C$$_{2}$$O$$_{4}$$)$$_{2}$$(H$$_{2}$$O)]. The structure is shown in Fig.1. All the structures the MOF are isostructural. The cell lengths and pore sizes tend to be decreased with the increase of atomic number in response to the decrease of ionic radius known as lanthanide contraction. And we investigated the behavior of Cs or Sr uptake by changing lanthanide ions in the MOF. Moreover, we found that Cs can be recovered from the MOF and we can reuse the MOF again. According to these results, we found a new material to recover Cs from radioactive liquid waste to decrease the amount of waste and make use of Cs as radiation source.

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