3:00 PM - 3:15 PM
[HCG20-06] Molecular dynamics simulations of free energy profiles of cesium ions on adsorption to interlayers of Na-montmorillonite with different composition.
Keywords:Molecular dynamics simulation, Clay minerals, Radioactive cesium
The buffer material that inhibits the migration of radionuclides plays a critical role in the long-term safety of geological disposal of high-level radioactive waste. Bentonite is the primary candidate for the buffer material to confine radionuclides. In particular, montmorillonite, which is the major component of bentonite, is a promising adsorbent of radionuclides. The adsorption of radionuclides has been confirmed by many experiments. However, this adsorption is essentially an atomic scale phenomenon, which is difficult to be addressed by experiments. Thus, using classical molecular dynamics (MD), we investigated the diffusion/adsorption behavior of cesium and sodium ions in Na-montmorillonite and water systems. We primarily evaluated the free energy of the cesium and sodium adsorption reaction to Na-montmorillonite. In the previous paper [1], the authors evaluated the free energy profiles of adsorption of cesium and sodium ions to the interlayer of Na-montmorillonite from bulk water in micropore. However, they found that there was not a large deviation between these free energy profiles, i.e., the affinities of the cesium and sodium ions to Na-montmorillonite are almost the same. We extended this previous study to larger system sizes and a more accurate evaluation of the free energy.
The details of our simulation are as follows. Molecular dynamics simulation was done using the GROMACS 2024.3 software package. The system we considered consists of two phases: the interlayer of Na-montmorillonite and bulk water. The border between the two phases is described by the edge of Na-montmorillonite. The interaction of the atoms in the clay and water molecules is described by the CLAYFF force field. We evaluated the free energy profiles of cesium and sodium ion adsorption to the interlayer of Na-montmorillonite from bulk water. We used the umbrella sampling and weighted histogram analysis methods to calculate their free energy profiles. We performed NVT simulation. Moreover, we put the pairs of Na+ and Cl- into the bulk water to control the salinity of the system.
As a result, we found that the minimum of free energy profiles of these cations locates in the interlayer of Na-montmorillonite, and the minimum of free energy profile of cesium is lower than that of sodium. This result shows that the affinity of cesium ion to Na-montmorillonite is stronger than that of sodium ion. Moreover, there is a difference in the free energy profiles of these cations in the interlayer. The free energy profiles of cesium ions have spiky structures in the interlayer. On the other hand, the free energy profiles of the sodium ion are almost uniform in the interlayer. We found that this difference corresponds to the adsorption mechanism of these cations to Na-montmorillonite. The results suggest that cesium ion forms inner-sphere complex at these local minimums and sodium ion forms outer-sphere complex. Furthermore, we confirmed that the affinities of both cations to Na-montmorillonite decrease with salinity increase, and these cations maintain their adsorption mechanism. The gaps between the free energies of both cations in the interlayer do not depend on salinity. Furthermore, we found that sodium ion is adsorbed to the edge of Na-montmorillonite, which was reflected in the free energy profile, i.e., the free energy profile for the sodium ion has small spiky forms near the edge.
In the presentation, we discuss layer charge and layer size dependence on the free energy profiles of these cations.
Acknowledgement
The Ministry of Economy, Trade and Industry of Japan has funded a part of the work as “Project on Research and Development for Validating Safety Assessment of Geological Disposal: Development of the Technology for Integrating Radionuclide Migration Assessments” (Grant Number: JPJ007597, Fiscal Year 2024).
References
[1] Rotenberg, B., Marry, V., Vuilleumier, R., Malikova, N., Simon, C., and Turq, P. (2007). Geochimica et Cosmochimica Acta, 71, 50895101.
The details of our simulation are as follows. Molecular dynamics simulation was done using the GROMACS 2024.3 software package. The system we considered consists of two phases: the interlayer of Na-montmorillonite and bulk water. The border between the two phases is described by the edge of Na-montmorillonite. The interaction of the atoms in the clay and water molecules is described by the CLAYFF force field. We evaluated the free energy profiles of cesium and sodium ion adsorption to the interlayer of Na-montmorillonite from bulk water. We used the umbrella sampling and weighted histogram analysis methods to calculate their free energy profiles. We performed NVT simulation. Moreover, we put the pairs of Na+ and Cl- into the bulk water to control the salinity of the system.
As a result, we found that the minimum of free energy profiles of these cations locates in the interlayer of Na-montmorillonite, and the minimum of free energy profile of cesium is lower than that of sodium. This result shows that the affinity of cesium ion to Na-montmorillonite is stronger than that of sodium ion. Moreover, there is a difference in the free energy profiles of these cations in the interlayer. The free energy profiles of cesium ions have spiky structures in the interlayer. On the other hand, the free energy profiles of the sodium ion are almost uniform in the interlayer. We found that this difference corresponds to the adsorption mechanism of these cations to Na-montmorillonite. The results suggest that cesium ion forms inner-sphere complex at these local minimums and sodium ion forms outer-sphere complex. Furthermore, we confirmed that the affinities of both cations to Na-montmorillonite decrease with salinity increase, and these cations maintain their adsorption mechanism. The gaps between the free energies of both cations in the interlayer do not depend on salinity. Furthermore, we found that sodium ion is adsorbed to the edge of Na-montmorillonite, which was reflected in the free energy profile, i.e., the free energy profile for the sodium ion has small spiky forms near the edge.
In the presentation, we discuss layer charge and layer size dependence on the free energy profiles of these cations.
Acknowledgement
The Ministry of Economy, Trade and Industry of Japan has funded a part of the work as “Project on Research and Development for Validating Safety Assessment of Geological Disposal: Development of the Technology for Integrating Radionuclide Migration Assessments” (Grant Number: JPJ007597, Fiscal Year 2024).
References
[1] Rotenberg, B., Marry, V., Vuilleumier, R., Malikova, N., Simon, C., and Turq, P. (2007). Geochimica et Cosmochimica Acta, 71, 50895101.