Cesium-137 is one of the main radionuclides produced by nuclear fission of Uranium-235 or Plutonium-239. According to several disposal plans of the high�level radioactive waste, it is isolated from an environment by engineered and/or natural barriers for hundred thousand years. Then chemical properties of barrier materials are crucial for the isolation, and they have been intensely studied experimentally and theoretically.It is well known that weathered illite, which is one of the good barrier materials, has special properties: selective and irreversible adsorption of cesium. From successive experiments for a long time, it is expected that it has a special sorption site called �frayed edge site (FES)�. However, the mechanisms of the special adsorptions have not been known.To study adsorption of cesium to FES of micaceous clay minerals, we performed first-principles calculations. We made a minimum model of FES, and evaluated the standard Gibbs free energy of formation of the ion-exchange reaction between potassium ion preoccupied FES and hydrated cesium ion by density functional theory. For convenient, experimental values of the standard Gibbs free energy of hydration for hydrated ions are used. To investigate the role of the wedge-shape structure of FES, we vary the interlayer distance of FES.Our result shows that FES adsorbs cesium when the interlayer-distance is enough large. This implies that not only dehydration tendency of cesium but also weathering plays crucial role for the adsorption. From additional calculation, we showed that the mechanism of the adsorption is competition between the ion radius of cation and the interlayer distance.ReferenceM. Okumura, H. Nakamura, and M. Machida, J. Phys. Soc. Jpn. 82, 033802 (2013).

First-princinciples studies of cesium adsorption to frayed edge sites of micaceous clay minerals

248th National Meeting (2014)

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