Assessing the long-term safety of radioactive waste buried underground requires an evaluation of the migration of metal ions leached into groundwater through pore spaces and rock fractures. Metal cations in groundwater adsorb to the pore surfaces, inhibiting migration. Therefore, it is essential to understand the adsorption of cations to the pore surfaces of rocks for the safety assessment of the disposal repository. The degree of adsorption of metal ions on a rock surface is often evaluated using the distribution coefficient, but the structure and size of pores in the rock are not considered when evaluating adsorption of metal ions [1].

Previous studies reported that the pore size in rocks, especially those in the deep subsurface, reaches the nanometer scale, which may affect the state of water inside [2, 3]. It remains unclear, however, how the adsorption of ions on the surfaces of small pores differs from those in larger spaces. NRA has conducted a joint research program with the University of Tokyo to clarify how the pore sizes affect the adsorption reactions of different ions [4–6]. Because natural rocks have the pore geometry with broad size distributions, complex shapes, and network structures, mesoporous silicas are chosen as their simplified analogues, considering their apparent similarities with rock-forming minerals, as well as their relatively uniform pores.

Surface charge density inside pores of mesoporous silica
(Reproduced from [4] with permission from the PCCP Owner Societies.)

We focused on the deprotonation reaction of hydroxyl groups, a fundamental surface reaction, and investigated the dependence of the surface charge density on pore size by determining the surface charge densities of six types of mesoporous silicas with micropores and mesopores at different ionic strengths and pH levels from batch titration tests. The surface complexation model assuming a potential distribution based on the Poisson-Boltzmann equation in cylindrical coordinates (Figure 1) was fitted to the obtained surface charge densities to relate the electrostatics near the surface to the surface reaction. The results showed that the absolute values of the surface charge densities decreased with decreasing pore diameter due to the overlap of the electrical double layers. Furthermore, the capacitance of the Stern layer optimized by fitting decreased with decreasing pore diameter, especially in pores smaller than 4 nm in diameter, which suggested that the dielectric constants of water decreased near the surfaces of small pores.

Adsorption of cesium and strontium on mesoporous silica
(Reproduced from [5] with permission from the PCCP Owner Societies.)

We investigated the effect of the pore size on the adsorption of two cations with different valence, Cs⁺ and Sr²⁺, on mesoporous silicas with different pore size distributions. The amount of Sr²⁺ adsorbed per unit surface area did not differ significantly among the silicas, whereas that of Cs⁺ was particularly high for silicas with a larger fraction of micropores. The results of X-ray absorption fine structure analysis showed that both ions form outer-sphere complexes with the mesoporous silicas. The results of adsorption experiments were analyzed by fitting using the surface complexation model described above with the optimized capacitance of the Stern layer for different pore sizes, and we found that the intrinsic equilibrium constant for the adsorption of Sr²⁺ is constant regardless of the pore size, whereas that of Cs⁺ increases as the pore size decreases. The decrease in the relative permittivity of water inside pores with a decrease of the pore size can be interpreted to cause a change in the hydration energy of Cs⁺ in the second coordination sphere upon adsorption (Figure 2). The reasons for the different confinement effects on the adsorption reactions of Cs⁺ and Sr²⁺ were discussed based on the distance of the adsorbed ions from the surface and the chaotropic and kosmotropic nature of Cs⁺ and Sr²⁺, respectively.

Adsorption of europium on mesoporous silica [6]

We analyzed the adsorption states of Eu³⁺ on mesoporous silicas with different pore distributions in comparison with nonporous silica using time-resolved laser-induced fluorescence spectroscopy. The amount of Eu³⁺ adsorbed per unit surface area did not differ significantly depending on the presence or absence of small pores, but the hydration state of the adsorbed ions differed. In addition, at high pH, Eu³⁺ multinuclear complexes formed only on the mesoporous silica surface.

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References

[1] Oscarson, D.W., Hume, H.B., 1998. Effect of the Solid: Liquid Ratio on the Sorption of Sr²⁺ and Cs⁺ on Bentonite, Adsorption of Metals by Geomedia. Academic Press. https://doi.org/10.1016/B978-012384245-9/50013-X

[2] Wang, Y., 2014. Nanogeochemistry: Nanostructures, emergent properties and their control on geochemical reactions and mass transfers. Chem. Geol. 378–379, 1–23. https://doi.org/10.1016/j.chemgeo.2014.04.007

[3] Zachara, J., Brantley, S., Chorover, J., Ewing, R., Kerisit, S., Liu, C., Perfect, E., Rother, G., Stack, A.G., 2016. Internal Domains of Natural Porous Media Revealed: Critical Locations for Transport, Storage, and Chemical Reaction. Environ. Sci. Technol. 50, 2811–2829. https://doi.org/10.1021/acs.est.5b05015

[4] Murota, K., Saito, T., 2022. Pore size effects on surface charges and interfacial electrostatics of mesoporous silicas. Phys. Chem. Chem. Phys. 24, 18073–18082. https://doi.org/10.1039/D2CP02520E

[5] Murota, K., Takahashi, Y., Saito, T., 2023. Adsorption of cesium and strontium on mesoporous silicas. Phys. Chem. Chem. Phys. 25, 16135–16147. https://doi.org/10.1039/D3CP01442H

[6] Murota, K., Aoyagi, N., Mei, H., Saito, T., 2023. Hydration states of europium(III) adsorbed on sicilas with nano-sized pores. Appl. Geochem. 152, 105620. https://doi.org/10.1016/j.apgeochem.2023.105620