Effect of different interlayer counter-ions on montmorillonite swelling; Key controlling factors evaluated by molecular dynamic simulations

Yotsuji, Kenji*; Tachi, Yukio; Sakuma, Hiroshi*; Kawamura, Katsuyuki*

Genshiryoku Bakkuendo Kenkyu (CD-ROM), 29(2), p.63 - 81, 2022/12

https://doi.org/10.3327/jnuce.29.2_63

The understanding of the swelling phenomenon of montmorillonite is essential to predict the physical and chemical behavior of clay-based barriers in radioactive waste disposal systems. This study investigated the key factors controlling crystalline swelling behavior of montmorillonite with different interlayer counter-ions by molecular dynamics (MD) simulations. On the basis of the comparisons between MD simulated and experimental results, the water content in the interlayer in five homoionic (Na, K, Cs, Ca and Sr) montmorillonite was strongly correlated to the hydration number and the preference of an outer- or inner-sphere complex of each counter-ion. The detailed analysis for these results offer insights that the hydration number is controlled by the hydration free energy, the volume and the distribution of each interlayer counter-ion. The systematic MD simulations with virtually variable parameters clarified that the hydration free energy and the charge of interlayer counter- ions compete as influencing factors, and the control the formation rate of an outer-sphere complex of each counter-ion. The empirical relationships between these key factors will allow essential insights into predicting the swelling behavior of montmorillonite with different interlayer counter-ions.

Effect of interlayer cations on montmorillonite swelling; Comparison between molecular dynamic simulations and experiments

Yotsuji, Kenji*; Tachi, Yukio; Sakuma, Hiroshi*; Kawamura, Katsuyuki*

Applied Clay Science, 204, p.106034_1 - 106034_13, 2021/04

https://doi.org/10.1016/j.clay.2021.106034

Stability of montmorillonite edge faces studied using first-principles calculations

Sakuma, Hiroshi*; Tachi, Yukio; Yotsuji, Kenji; Suehara, Shigeru*; Arima, Tatsumi*; Fujii, Naoki*; Kawamura, Katsuyuki*; Honda, Akira

Clays and Clay Minerals, 65(4), p.252 - 272, 2017/08

https://doi.org/10.1346/CCMN.2017.064062

Structure and stability of montmorillonite edge faces (110), (010), (100), and (130) of the layer charges y = 0.5 and 0.33 are investigated by the first-principles electronic calculations based on the density functional theory. Stacking and single layer models are tested for understanding the effect of stacking on the stability of montmorillonite edge faces. Most stacking layers stabilize the edge faces by making hydrogen bonds between the layers; therefore, the surface energy of stacking layers is reduced rather than the single layer model. This indicates that the surface energy of edge faces should be estimated depending on the swelling conditions. Lowest surface energies of (010) and (130) edge faces were realized by the presence of Mg ions on the edge faces. These edge faces have a strong adsorption site for cations due to local negative charge of the edges.

Diffusion model considering multiple pore structures in compacted bentonite

Yotsuji, Kenji; Tachi, Yukio; Okubo, Takahiro*

CMS Workshop Lectures, Vol.21, p.251 - 257, 2016/06

https://www.clays.org/other-cms-publications-wrksh21/

We have developed integrated sorption and diffusion model (ISD model) for assessment of diffusion parameters consistent with sorption processes in compacted bentonite. The ISD model gives consistent consideration to porewater chemistry, sorption and diffusion processes in compacted bentonite. The diffusion component based on the electric double layer theory and the homogeneous pore model in the ISD model accounts consistently for cation De overestimation and anion exclusion in narrow pores. The current ISD model can quantitatively account for diffusion of monovalent cations and anions, however, the model predictions disagree with diffusion data for multivalent cation and complex species. To improve the applicability of the model, it is necessary to consider the atomic level interactions between solute, solvent or clay mineral, and try that we apply the current ISD model to heterogeneous pore structure. In this study we try the application of the current ISD model to multiple pore structure. As results of numerical analysis of these models, the salinity dependence of effective diffusivity for the multi-pore model is comparatively smaller than that for the homogeneous pore model and the current diffusion model is improved.

Integrated sorption and diffusion model for bentonite, 2; Porewater chemistry, sorption and diffusion modeling in compacted systems

Tachi, Yukio; Yotsuji, Kenji; Suyama, Tadahiro; Ochs, M.*

Journal of Nuclear Science and Technology, 51(10), p.1191 - 1204, 2014/10

https://doi.org/10.1080/00223131.2014.914453

The integrated sorption and diffusion (ISD) model for compacted bentonites was developed based on the consistent combination of (1) the porewater chemistry model, (2) the thermodynamic sorption model (TSM), and (3) the diffusion model based on the electrical double layer (EDL) theory. This ISD model was successfully tested for various actinides with a complex chemistry (Np(V), Am(III), U(VI) under conditions where carbonate complexes are formed), by using published diffusion and sorption data (, , ) as a function of partial montmorillonite density. Quantitative agreements were observed as same as monovalent cations (Cs, Na), anions (Cl, I, TcO) previously reported. The ISD model could be therefore seen to be able to predict sorption and diffusion behavior of various complex radionuclides in compacted bentonites.

Diffusion and sorption of Cs, Na, I and HTO in compacted sodium montmorillonite as a function of porewater salinity; Integrated sorption and diffusion model

Tachi, Yukio; Yotsuji, Kenji

Geochimica et Cosmochimica Acta, 132, p.75 - 93, 2014/05

https://doi.org/10.1016/j.gca.2014.02.004

Sorption and diffusion of radionuclides in compacted bentonite are key processes in the safe geological disposal. The effects of porewater salinity on diffusion and sorption of Cs, Na, I and HTO in compacted montmorillonite were investigated by experimental and modeling approaches. Effective diffusion coefficients (De) and distribution coefficients (Kd) in montmorillonite (dry density of 800 kg/m) were measured under different salinities (0.01-0.5 M). The De showed cation excess and anion exclusion effects, which were influenced by salinity. The Kd values for Cs and Na were sensitive to salinity. The Da trends of Cs and Na are mostly independent on salinity. These results were interpreted by the integrated sorption and diffusion (ISD) model coupling the ion exchange model and the EDL diffusion model in narrow pores. This ISD model was further applied to diffusion data of monovalent cations and anions in compacted bentonite, performed well for a wide range of compaction.

Advanced diffusion model in compacted bentonite based on modified Poisson-Boltzmann equations

Yotsuji, Kenji; Tachi, Yukio; Nishimaki, Yuichiro*

Proceedings of 5th International Meeting on Clays in Natural and Engineered Barriers for Radioactive Waste Confinement (USB Flash Drive), p.427 - 428, 2012/10

We have developed integrated sorption and diffusion model (ISD model) for assessment of diffusion parameters consistent with sorption processes in compacted bentonite. Conventional ISD model is unsatisfactory because for multivalent cation/anion and complex ion model predictions disagree with experimental data, and because the model contains additional non-physical fitting parameter. Accordingly we extract the factors influencing the assessment from fundamental assumptions of the conventional ISD model. In this study we incorporated the excluded volume effect and the dielectric saturation effect into ISD model. As results of numerical analysis of these models the considering factors influence hardly the effective diffusivity. Therefore it does not mean that the disagreement with experimental data are caused by considering factors in this report.

Diffusion and sorption of Cs, I and HTO in samples of the argillaceous Wakkanai Formation from the Horonobe URL, Japan; Clay-based modeling approach

Tachi, Yukio; Yotsuji, Kenji; Seida, Yoshimi*; Yui, Mikazu

Geochimica et Cosmochimica Acta, 75(22), p.6742 - 6759, 2011/11

https://doi.org/10.1016/j.gca.2011.08.039

Diffusion and sorption behaviors of Cs, I and HTO in samples of the Wakkanai Formation from Horonobe URL were investigated as a function of ionic strength (IS) of synthetic groundwater by through-diffusion and batch sorption experiments. The values showed cation excess and anion exclusion effects, which were strongly dependent on IS; for Cs decreased as IS increased, for I showed the opposite dependency and for HTO showed no dependence. The Kd values for Cs determined by through-diffusion and batch experiments were in good agreement and decreased with IS as a result of competitive ion exchange. Diffusion and sorption behaviors were interpreted based on the clay-based modeling approach in assuming the clay components of illite and smectite control diffusion and sorption mechanisms. The model predicted the and Kd trends obtained by the series of experiments reasonably well, implying the key contribution of the clay particle and nano-size pore for ionic migration.

Development of the mechanistic sorption and diffusion model/database for safety assessment of geological disposal; Prototype model/database for bentonite systems

Tachi, Yukio; Yotsuji, Kenji; Suyama, Tadahiro; Ochs, M.*; Yui, Mikazu

JAEA-Research 2009-069, 83 Pages, 2010/03

JAEA-Research-2009-069.pdf:14.56MB

This report presents the prototype model/database for integrated sorption and diffusion (ISD2009) in bentonite systems as basis for performance assessment (PA) of geological disposal. The sorption model is based on a relatively simple 1-site surface complexation/diffuse layer model in combination with 1-site ion exchange model. The model parameters for surface chemistry/sorption are evaluated based on existing datasets for Na-montmorillonite, which covers key geochemical conditions, in consistent way. The diffusion model based on homogeneous pore structure and electrical double layer theory was developed and coupled with batch-based sorption model, then was validated by using data in compacted montmorillonite. The sorption and diffusion models were also tested by applying data in bentonite systems. The model and related parameters developed for key radionuclides such as Cs, Np(V), Ni, Am, etc. are integrated to ISD2009 database, which could be useful in future PA exercise.

A Study on edge surface structures and reactivity for clay minerals by molecular orbital method

Yotsuji, Kenji*; Takeda, Seiji; Kimura, Hideo

Nihon Genshiryoku Gakkai Wabun Rombunshi, 7(2), p.166 - 176, 2008/06

https://doi.org/10.3327/taesj.J07.008

To clarify the dissolution mechanism of clay minerals, the basal and edge surface structures of dioctahedral 2:1 phyllosilicate was optimized using semi-empirical molecular orbital method. The stability of the surface structure was estimated by the bond length, bond strength and the bond energy on bonds of interest along the basal and edge surfaces. The optimized basal surface structure of montmorillonite resides at a local minimum on the potential energy surface, because each normal mode for the optimized structure has real vibrational frequency. The ideal (010)-type edge surface of pyrophyllite was initialized using crystal chemical methods. The edge surface structure that was protonated allowing for pH-dependency was optimized. On optimized edge surfaces structural relaxation occurred in all pH-conditions, then it was found that Si-O bonds in the silanol groups were stronger, and Al-O bonds in the aluminol groups were weaker, than corresponding bonds in the bulk structures, respectively. The rate-determining step is thought to be governed by hydrolysis of outer Si-O bonds on edge surfaces in alkaline dissolution of pyrophyllite and by hydrolysis of Al-O bonds in bridging Si-O-Al bonds on edge surfaces in acid dissolution.