講演情報
[MIS19-02]陽イオン交換を伴う粘土凝集挙動の熱力学的考察
*佐藤 久夫1、長嶋 剣2 (1.日本原燃株式会社埋設事業部、2.北海道大学低温科学研究所)
キーワード：
凝集、陽イオン交換、界面自由エネルギー、古典的核形成理論
1. Context
On the necessity to evaluate hydraulic property of bentonite barrier for radioactive waste repository, we often estimate its permeability based on the power law equation as a function of pore void/clay volume ratio. On the other hand, geochemical simulation to predict the volume change of clay with time is another important aspect to verify the integrity of bentonite barrier (smectite-bearing soil material). When we discuss about such a simulation involving smectite with cationic exchange, actual pore is often ignored because of equation for above permeability based on clay volume as dry state. Actual pore space in hydrated clay is always smaller than that conventionally considered for the permeability estimation. Also, effective pore resulted from aggregation of hydrated cation exchanged clay should be different from porosity based on density. Here, we conducted thermodynamic consideration to predict clay pore space resulted from swelling and aggregation behavior after hydration and cationic exchange of some montmorillonites.
2. Experimental and theoretical
We prepared some montmorillonite clay as KF, KBP, KRP purified from Japanese bentonites, KunigelV1, Kunibond and Tsugaru2, respectively. These were once processes as totally cation-free to be H-type by NH4-exchanging and heating at 200 degC. Then, mica substrates were coated with these clays and served for contact angles measurement [1] along with hanging solution droplet measurement (Fig. 1a). Results were calculated to be interfacial free energy at following solution, H2O and NaCl, KCl, CaCl2, and MgCl2 at ionic strength 0.5. These obtained data were used to clarify the mechanism of aggregation and its size.
3. Results and Discussion
Interfacial free energies, gS/L of montmorillonite with cation-exchange showed a range of 40 to 60 mJ/m2 (Fig. 1b). H2O and K runs showed quite different gS/L variation among these montmorillonites. Roughly, the gS/L increases in order from KF, KBP to KRP which have grain sizes of 100, 150, and 300 in diameter.
Using obtained gS/L, we estimate minimum volume of stacks with the following insight of formation of swelled clay aggregation based on the total energy balance among hydration, size, stack number, DLVO potential, interfacial free energy and ambient pressure for suspended and aggregated states, with referring classical nucleation theory.
If we calculate the energy of the stacks, critical number of stacks can be estimated for each cation-type of montmorillonite. Especially Ca-montmorillonite showed larger stack than the other cationic type, so that Ca-solution can stabilize montmorillonite layering. This may increase pore space and permeability.
Fig. 1 Wetting angles measured for H2O and CaCl2 solutions on KRP montmorillonite by photomicrography (a), and the compared interfacial free energy of between cation-exchanged montmorillonites and solutions (b).
Reference:
[1] Giese et al. (1990) Surface energies of some smectite clay minerals. Proc, the 9th Int. Clay Conf. Strasbourg, 1989, Vol II: Surface chemistry. Structure and mixed layering of clays.
On the necessity to evaluate hydraulic property of bentonite barrier for radioactive waste repository, we often estimate its permeability based on the power law equation as a function of pore void/clay volume ratio. On the other hand, geochemical simulation to predict the volume change of clay with time is another important aspect to verify the integrity of bentonite barrier (smectite-bearing soil material). When we discuss about such a simulation involving smectite with cationic exchange, actual pore is often ignored because of equation for above permeability based on clay volume as dry state. Actual pore space in hydrated clay is always smaller than that conventionally considered for the permeability estimation. Also, effective pore resulted from aggregation of hydrated cation exchanged clay should be different from porosity based on density. Here, we conducted thermodynamic consideration to predict clay pore space resulted from swelling and aggregation behavior after hydration and cationic exchange of some montmorillonites.
2. Experimental and theoretical
We prepared some montmorillonite clay as KF, KBP, KRP purified from Japanese bentonites, KunigelV1, Kunibond and Tsugaru2, respectively. These were once processes as totally cation-free to be H-type by NH4-exchanging and heating at 200 degC. Then, mica substrates were coated with these clays and served for contact angles measurement [1] along with hanging solution droplet measurement (Fig. 1a). Results were calculated to be interfacial free energy at following solution, H2O and NaCl, KCl, CaCl2, and MgCl2 at ionic strength 0.5. These obtained data were used to clarify the mechanism of aggregation and its size.
3. Results and Discussion
Interfacial free energies, gS/L of montmorillonite with cation-exchange showed a range of 40 to 60 mJ/m2 (Fig. 1b). H2O and K runs showed quite different gS/L variation among these montmorillonites. Roughly, the gS/L increases in order from KF, KBP to KRP which have grain sizes of 100, 150, and 300 in diameter.
Using obtained gS/L, we estimate minimum volume of stacks with the following insight of formation of swelled clay aggregation based on the total energy balance among hydration, size, stack number, DLVO potential, interfacial free energy and ambient pressure for suspended and aggregated states, with referring classical nucleation theory.
If we calculate the energy of the stacks, critical number of stacks can be estimated for each cation-type of montmorillonite. Especially Ca-montmorillonite showed larger stack than the other cationic type, so that Ca-solution can stabilize montmorillonite layering. This may increase pore space and permeability.
Fig. 1 Wetting angles measured for H2O and CaCl2 solutions on KRP montmorillonite by photomicrography (a), and the compared interfacial free energy of between cation-exchanged montmorillonites and solutions (b).
Reference:
[1] Giese et al. (1990) Surface energies of some smectite clay minerals. Proc, the 9th Int. Clay Conf. Strasbourg, 1989, Vol II: Surface chemistry. Structure and mixed layering of clays.