10:45 〜 11:00
[MAG34-06] 化学種解析と第一原理計算による粘土鉱物への金属イオンの吸着構造の系統的理解
キーワード：粘土鉱物、ラジウム、セシウム、EXAFS、第一原理計算
Clay minerals are ubiquitous at the Earth’s surface and absorb various cations. Therefore, the adsorption reaction on the clay minerals has an important role in the environmental behavior of various cations [1]. For example, radioactive cesium (Cs) emitted from the Fukushima Daiichi Nuclear Power Plant accidents was fixed within 5 cm of the surface soil because of its strong adsorption on clay minerals. On the other hand, radioactive strontium (Sr) was not fixed by the soil. This difference was caused by the difference in the adsorption structures of Cs+ and Sr2+ on clay minerals. Cs+ adsorbed on the clay minerals is dehydrated, forming inner-sphere (IS) complexes, while Sr2+ is hydrated, outer-sphere (OS) complexes [2]. Then, what is the factor controlling the IS/OS complexes? Candidates for the factors are hydration energy, ionic radius, covalency of an adsorbed ion, etc. However, it has been unclear. Therefore, this study systematically conducted extended X-ray absorption fine structure (EXAFS) measurements and ab initio calculations to investigate the factors.
EXAFS is a powerful tool to investigate a local structure of a target element because of its high sensitivity and selectivity. This study measured EXAFS spectra of various cations {Cs, rubidium (Rb), potassium (K), Sr, barium (Ba), lanthanum (La), yttrium (Y), and lutecium (Lu)} adsorbed on clay minerals, vermiculite and montmorillonite, to clarify which elements form IS complexes and which elements OS complexes. Furthermore, EXAFS spectra of radium (Ra) adsorbed on clay minerals were also obtained. Ra2+ is the largest alkaline earth element and has no stable isotopes, causing difficulty in treating Ra. Therefore, this study established the method to obtain Ra LIII-edge EXAFS spectrum safety, following the Japanese act [3]. Ab initio calculation is also a powerful tool to investigate the adsorption structures at the atomic level. This study calculated the potential energies of various cations in the interlayer sites of collapsed vermiculite without water molecules. These calculations evaluate the stabilities of vermiculite.
EXAFS results showed that Cs+, Rb+, and K+ form IS complexes while Ba2+, Sr2+, La3+, Y3+, and Lu3+ form OS complexes when adsorbed on vermiculite or montmorillonite. Ra2+ adsorbed on vermiculite forms IS complexes, while Ra2+ on montmorillonite form OS complexes. Some previous studies suggested that the hydration energy regulates the IS/OS complexes [4]. However, the IS complex of Ra2+ cannot be explained by the hydration energy because of the high valence of Ra. The potential energies calculated by ab initio calculations showed that a larger cation has a stronger affinity for clay minerals, suggesting the importance of a structural matching effect between the adsorbed cation and siloxane rings in the tetrahedral layers in the clay minerals. However, the structural matching effect alone cannot explain all EXAFS results. Therefore, this study concluded that both the hydration energy and the structural matching effect are the important factors regulating the IS/OS complexes on the clay minerals. Furthermore, the hydration energy and structural matching effect can be evaluated by hydration enthalpy and ionic radius, respectively. These systematic understandings help us to predict and explain the environmental behavior of various elements.
References:
[1] D. Langmuir et al., Aqueous environmental geochemistry, Prentice Hall, Upper saddle river, 1997.
[2] A. Yamaguchi et al., J. Radioanal. Nucl. Chem. 317, 545 (2018).
[3] A. Yamaguchi et al., IScience 25, 104763 (2022).
[4] B.J. Teppen et al., Soil Sci. Soc. Am. J. 70, 31 (2006).
EXAFS is a powerful tool to investigate a local structure of a target element because of its high sensitivity and selectivity. This study measured EXAFS spectra of various cations {Cs, rubidium (Rb), potassium (K), Sr, barium (Ba), lanthanum (La), yttrium (Y), and lutecium (Lu)} adsorbed on clay minerals, vermiculite and montmorillonite, to clarify which elements form IS complexes and which elements OS complexes. Furthermore, EXAFS spectra of radium (Ra) adsorbed on clay minerals were also obtained. Ra2+ is the largest alkaline earth element and has no stable isotopes, causing difficulty in treating Ra. Therefore, this study established the method to obtain Ra LIII-edge EXAFS spectrum safety, following the Japanese act [3]. Ab initio calculation is also a powerful tool to investigate the adsorption structures at the atomic level. This study calculated the potential energies of various cations in the interlayer sites of collapsed vermiculite without water molecules. These calculations evaluate the stabilities of vermiculite.
EXAFS results showed that Cs+, Rb+, and K+ form IS complexes while Ba2+, Sr2+, La3+, Y3+, and Lu3+ form OS complexes when adsorbed on vermiculite or montmorillonite. Ra2+ adsorbed on vermiculite forms IS complexes, while Ra2+ on montmorillonite form OS complexes. Some previous studies suggested that the hydration energy regulates the IS/OS complexes [4]. However, the IS complex of Ra2+ cannot be explained by the hydration energy because of the high valence of Ra. The potential energies calculated by ab initio calculations showed that a larger cation has a stronger affinity for clay minerals, suggesting the importance of a structural matching effect between the adsorbed cation and siloxane rings in the tetrahedral layers in the clay minerals. However, the structural matching effect alone cannot explain all EXAFS results. Therefore, this study concluded that both the hydration energy and the structural matching effect are the important factors regulating the IS/OS complexes on the clay minerals. Furthermore, the hydration energy and structural matching effect can be evaluated by hydration enthalpy and ionic radius, respectively. These systematic understandings help us to predict and explain the environmental behavior of various elements.
References:
[1] D. Langmuir et al., Aqueous environmental geochemistry, Prentice Hall, Upper saddle river, 1997.
[2] A. Yamaguchi et al., J. Radioanal. Nucl. Chem. 317, 545 (2018).
[3] A. Yamaguchi et al., IScience 25, 104763 (2022).
[4] B.J. Teppen et al., Soil Sci. Soc. Am. J. 70, 31 (2006).