Effects of OH^– activity and temperature on the dissolution rate of compacted montmorillonite under highly alkaline conditions Open Access

Highly alkaline environments induced by cementitious materials in a deep geological disposal system for high-level radioactive waste are likely to alter montmorillonite, the main constituent of bentonite buffer materials, affecting adversely the physical and/or chemical barrier functions of the buffer materials. In order to evaluate the long-term alteration behaviour, the dissolution rate of montmorillonite should be determined.

Various studies on the dissolution rates of clays in alkaline solutions used different pulverized smectites (Savage et al., 1992; Bauer & Berger, 1998; Kubo et al., 1998; Sato et al., 2004; Rozalén et al., 2008, 2009), and determined the rates under much lower solid/solution ratios than those of the disposal sites (Table 1). These data need some modification in order to evaluate the dissolution of buffer materials under the confined disposal conditions. Savage & Liu (2015) reported that the most relevant water/clay ratio (‘porosity’) should be used in modelling and experimental systems based on thermodynamic calculations illustrating that the water/clay ratio affected the magnitude and nature of alteration. The dissolution rates in compacted sand-bentonite mixtures were determined experimentally and formulated under hydroxide ion activities, a_OH–, of 0.04–0.57 mol dm^–3 and temperatures of 50–170°C (Nakayama et al., 2004; Yamaguchi et al., 2007). The dissolution rates in the mixtures, however, were smaller than that in pulverized montmorillonite and depended more heavily on a_OH–. Yamaguchi et al. (2007) suggested that different a_OH– in pore water from that in the external solution might be responsible for this dependence, although this was not verified. Sawaguchi et al. (2013) studied the dissolution rates of compacted pure montmorillonite at 130°C and showed greater dissolution rates than those in the mixtures. The explanation given for the difference was that the dissolution of montmorillonite in the mixtures was inhibited by decreasing the a_OH– stemming from the dissolution of accessory minerals such as SiO₂ polymorphs (quartz, chalcedony, etc.) in the mixtures.

In the present study, the dependence of the dissolution rate of compacted montmorillonite (Kunipia-F) on OH^– activity and temperature were investigated experimentally to clarify the effect of accessory minerals on the dissolution rate under compaction.

The dissolution rate for montmorillonite under compaction was obtained through a time-dependent variation in the amount of remaining montmorillonite in the compacted montmorillonite altered by highly alkaline solution. The experimental method was the same as in previous studies (Nakayama et al., 2004; Yamaguchi et al., 2007; Sawaguchi et al., 2013) and the experimental conditions were the same as in a previous study by the present authors (Sawaguchi et al., 2013), except for the temperature conditions.

The montmorillonite (Kunipia-F, a commercial product of Kunimine Industries Co. Ltd., Japan) was compacted in specimen holders to obtain a dry density of 1060 kg m^–3 or porosity of 0.6. The porosity of 0.6 was selected by considering porosity variations of 0.4–0.8 through the experiment for the mixtures (Nakayama et al., 2004). The Si-, Al- and Ca-adjusted NaCl-NaOH mixed solutions used in the previous experiment (Nakayama et al., 2004) were also used in the present study (Table 2). The alkaline solutions were prepared by adding NaCl, NaOH, sodium metasilicate nonahydrate (Na₂SiO₃·9H₂O) and small amounts of aluminium standard solution (Al concentration of 1000 mg dm^–3 and HNO₃ of 0.2 mol dm^–3) and calcium standard solution (Ca concentration of 1000 mg dm^–3 and HNO₃ of 0.1 mol dm^–3) to deionized water to obtain chemical compositions shown in Table 2. Potassium, although usually present in cementitious water, was not added to the experimental solutions because the dissolution rate of montmorillonite in KOH-NaOH mixed solutions where K-feldspar was formed was identical to that in an NaOH solution (Yamaguchi et al., 2007). The specimen holder was soaked in the alkaline solution in a Teflon^® container assembly at room temperature under vacuum for 1 week to saturate the specimen (Fig. 1). Then, the assembly was contacted with 0.065 dm³ of the alkaline solution in a sealed stainless steel container and kept at 50, 70 and 90°C in an oven to observe montmorillonite dissolution over a reasonable time period. The alkaline solution was replaced by fresh solution when necessary to minimize changes in the concentration of OH^–. The necessity was decided by monitoring the pH of mock-up assemblies each week. When the measured value of pH decreased by >0.1 pH units from the initial value, the alkaline solution in all assemblies of the same condition was exchanged for the initial solution. Therefore the variations of pH in the assemblies were kept within ±0.2 pH units, during the alteration period. The pH was measured at ∼25°C using a ROSS^® combination-glass pH electrode (Orion 8172BNWP, Thermo Scientific, Inc., USA) calibrated with 7.00 and 10.01 pH buffers and 1 mol dm^–3 NaOH solutions. Each of the assemblies was taken from the oven at the fixed sampling time. The compacted montmorillonite specimen was pushed out from the holder by 2 mm at a time using a screw presser and was sliced with a steel blade. Each sliced piece was weighed, freeze-dried at –110°C, ground using a mortar and pestle, redried at 90°C and then analyzed for the amount of remaining montmorillonite by the conventional methylene-blue adsorption titration method (Hang & Brindley, 1970; Brindley & Thompson, 1970; Japan Bentonite Manufacturers' Association, 1991). In the present study, 2.5 × 10⁻⁴ kg of crushed sample and of pure montmorillonite (Kunipia-F) for calibration were used. Furthermore, the remaining reacted solid was analyzed by powder X-ray diffraction (XRD, Geigerflex, Rigaku Corp., Japan). The concentrations of Al and Ca in the contacted aqueous solutions were determined by inductively-coupled plasma atomic emission spectroscopy (ICP-AES, SII SPS 7800, SII Nanotechnology Inc., Japan) after filtering the solutions through a 0.45 μm PTFE (polytetrafluoroethylene) filter and acidifying into 0.5 N HNO₃. The concentration of Si was determined as SiO₂ by means of the molybdosilicate method (Rand et al., 1976; Japanese Standard Association, 2004; The Japan Society for Analytical Chemistry Hokkaido, 1994) and by UV spectrometry (U-3210, HITACHI, Ltd., Japan) after the filtration.

The montmorillonite content of each of the slices decreased with alteration time (Fig. 2). The XRD patterns of each slice also showed that the decrease in montmorillonite and the formation of secondary analcime (NaAlSi₂O₆), proceeded over time (Fig. 3). The alteration of primary minerals proceeded both at the centre and the edges of the specimens facing the alkaline solution so that uniform dissolution of montmorillonite in the compacted specimens was considered hereafter. The diffusive mass transfer of OH^– would be much greater than the rate of consumption in the compacted montmorillonite. The above-mentioned dissolution and formation of minerals is qualitatively consistent with the concentrations of Si and Al in the contacting solutions (Fig. 4). The concentrations of Si tend to be much greater than the initial values in all alkaline solutions, due to the dissolution of montmorillonite. The formation of analcime in the solutions rich in Si and Na concentrations probably consumed Al, which decreased the Al concentration. Although determination of the concentration of Ca was difficult due to interference with the high-concentration of Na, an increase in the concentrations of Ca was observed, probably due to the dissolution of montmorillonite, which contains ∼0.36 wt.% of exchangeable Ca (Ito et al., 1993).

The concentrations of OH^– ([OH^–], (mol dm^–3)) in alkaline solutions were kept within ±0.2 logarithmic units by changing the water with the new one during the reaction. The measured values of pH, pHe, through experimental time were averaged and converted into [OH^–] by the correlation equation between pHe and theoretically evaluated [OH^–] of the initial solutions shown in Table 2: [OH^–] = 10^{0.982 × pHe–13.6}. Furthermore, representative values of activities of hydroxide ion, a_OH–, for each experimental condition were determined by multiplying [OH^–] by the activity coefficients evaluated using the SIT (specific ion interaction) model (Lemire et al., 2001).

Assuming that the shift would be linear by 0.05, 0.1 or 0.2 mol dm^–3 or logarithmic by 0.3, 0.4 or 0.5 units, the dissolution rates of montmorillonite in the mixtures were estimated (Fig. 7). The curves given by shifting equation 3 by 0.05 mol dm^–3 and by 0.4 logarithmic units reproduce the dissolution rate data in the mixtures well. Furthermore, trial calculations by considering the shift of a_OH– were performed to reproduce the montmorillonite dissolution experiment of Yamaguchi et al. (2013), who compacted sand-bentonite mixture specimens immersed in 1 mol dm^–3 NaOH solution at 90°C for 14, 27, 51 and 67 days. Figure 8 shows the montmorillonite densities after each alteration period and the calculated values obtained using the dissolution rates from equation 3 and those after shifting. The calculated values obtained by shifting equation 3 by 0.3 logarithmic units reproduced the experimental data better than by 0.05 mol dm^–3. These results indicate that the dissolution rate in the mixtures can be explained by equation 3 considering a shift of the a_OH– in the mixtures, and that the shift may be logarithmic. The shift in logarithmic scale is more consistent than the shift in linear scale with the dependence on compacted montmorillonite, 1.3 ± 0.4, being closer to that for the mixtures, 1.4 ± 0.3. The shift in linear scale is not favoured because it gives an extremely low dissolution rate when applied to lower pH ranges.

The activation energy obtained (55 ± 16 kJ/mol) is consistent with that reported for the mixtures (51 kJ/mol, Yamaguchi et al., 2007) and with that determined for smectite dissolution in KOH solution (52 ± 4 kJ/mol, Bauer & Berger, 1998).

Yamaguchi et al. (2007) compared the formula of dissolution rate of montmorillonite in the mixtures with that of pure pulverized montmorillonite determined by flow-through experiments (Sato et al., 2004) and pointed out that the difference in the dependence on a_OH– made a large difference in the dissolution rate when the dependence was extrapolated to repository conditions. The formula obtained in the present study also gives a much lower dissolution rate than Sato et al. (2004) for a_OH–, < 0.01 mol dm^–3. However it gives greater dissolution rate than Yamaguchi et al. (2007) by ∼0.3 orders of magnitude.

In the present study, the dissolution rate of montmorillonite free of accessory minerals, under compacted conditions was formulated. In order to provide reliable values for the long-term prediction of bentonite buffer performance under disposal conditions by using the dissolution rate equation, further study is necessary to quantify the decrease in a_OH– accompanied by dissolution of accessory minerals such as SiO₂ polymorphs.

The dissolution rate of the compacted montmorillonite (Kunipia-F) was formulated as a function of a_OH– and temperature. The existence of accessory minerals decreased the dissolution rate of montmorillonite. The dissolution rate model developed can be used not only for pure montmorillonite but also for bentonite mixtures by taking into account the negative shift of a_OH– in the mixtures accompanied by dissolution of accessory SiO₂ polymorphs.

Part of this research was funded by the Secretariat of Nuclear Regulation Authority, Nuclear Regulation Authority, Japan (formerly the Nuclear and Industrial Safety Agency, Ministry of Economy, Trade and Industry, Japan). The authors acknowledge Drs Hisayoshi Mitamura and Tetsushi Nagano for performing the long-term experiments at the Waste Safety Testing Facility (WASTEF) of the Japan Atomic Energy Agency. Dr Logan Barr of the Japan Atomic Energy Agency is acknowledged for improving the English. Anonymous referees are acknowledged for their work on the manuscript and their helpful comments.