2:45 PM - 3:00 PM
[HCG20-05] Microscopic structural changes in cesium adsorbed on clay minerals due to concentration variations
Keywords:Clay mineral, cesium, XAFS, DFT calculation
The radioactive cesium (Cs) released by the Fukushima Daiichi Nuclear Power Plant accident strongly has been adsorbed on clay minerals and fixed in the surface layer of the soil. Similarly, adsorption reactions of clay minerals significantly influence the environmental behavior of various elements. However, these adsorption reactions are complex and remain partially unexplored. One complexity lies in the fact that clay minerals possess multiple adsorption sites with different affinities and adsorption capacities, and the dominant adsorption sites change depending on the concentration of adsorbed ions. Typically, molecular-level information about cesium adsorbed onto clay minerals is obtained using samples with high concentrations of adsorption (ppm order or higher). However, in natural environments, cesium is often adsorbed at much lower concentrations (ppt order). Consequently, there is a concern that molecular-level information obtained from laboratory experiments may not be directly applicable to understanding environmental behavior.
To address this issue, this study systematically elucidated changes in adsorption structures across a wide range of concentrations by combining molecular-level experiments and density functional theory (DFT) calculations. This approach aimed to clarify how adsorption sites of clay minerals change. Additionally, the study evaluated the bonding characteristics.
To create Cs adsorption samples with different dominant adsorption sites, adsorption experiments were conducted by varying the concentration of Cs in the solution from 10-9 M to 10-1 M. The resulting solid-phase samples were analyzed using X-ray diffraction (XRD) to measure interlayer spacing. To obtain molecular-level information, extended X-ray absorption fine structure (EXAFS) and high-energy resolution fluorescence detection X-ray absorption near-edge structure (HERFD-XANES) measurements were performed at SPring-8. Furthermore, simulations of stable structures and bonding characteristics were conducted using the Vienna Ab initio Simulation Package (VASP) and the Local-Orbital Basis Suite Towards Electronic-Structure Reconstruction (LOBSTER).
The adsorption isotherm results suggested the presence of three adsorption sites, and XRD results confirmed systematic changes in interlayer spacing. Analysis of EXAFS revealed that the distance between Cs+ and neighboring oxygen atoms systematically changed depending on the adsorption site. Combining these results with DFT calculations indicated that Cs+ initially adsorbs on the frayed edge sites (FES) with relatively narrow interlayers. As the adsorption concentration increases, Cs+ adsorbs on the FES with relatively wider interlayers, and at higher concentrations, it induces interlayer contraction.
Additionally, HERFD-XANES results showed that the interaction between adsorbed Cs+ and clay minerals was predominantly ionic, regardless of the adsorption site. This finding was consistent with bonding evaluations conducted through DFT calculations.
To address this issue, this study systematically elucidated changes in adsorption structures across a wide range of concentrations by combining molecular-level experiments and density functional theory (DFT) calculations. This approach aimed to clarify how adsorption sites of clay minerals change. Additionally, the study evaluated the bonding characteristics.
To create Cs adsorption samples with different dominant adsorption sites, adsorption experiments were conducted by varying the concentration of Cs in the solution from 10-9 M to 10-1 M. The resulting solid-phase samples were analyzed using X-ray diffraction (XRD) to measure interlayer spacing. To obtain molecular-level information, extended X-ray absorption fine structure (EXAFS) and high-energy resolution fluorescence detection X-ray absorption near-edge structure (HERFD-XANES) measurements were performed at SPring-8. Furthermore, simulations of stable structures and bonding characteristics were conducted using the Vienna Ab initio Simulation Package (VASP) and the Local-Orbital Basis Suite Towards Electronic-Structure Reconstruction (LOBSTER).
The adsorption isotherm results suggested the presence of three adsorption sites, and XRD results confirmed systematic changes in interlayer spacing. Analysis of EXAFS revealed that the distance between Cs+ and neighboring oxygen atoms systematically changed depending on the adsorption site. Combining these results with DFT calculations indicated that Cs+ initially adsorbs on the frayed edge sites (FES) with relatively narrow interlayers. As the adsorption concentration increases, Cs+ adsorbs on the FES with relatively wider interlayers, and at higher concentrations, it induces interlayer contraction.
Additionally, HERFD-XANES results showed that the interaction between adsorbed Cs+ and clay minerals was predominantly ionic, regardless of the adsorption site. This finding was consistent with bonding evaluations conducted through DFT calculations.