Taiji Chida

Dynamic Behavior of Colloidal Silica in the Presence of Solid Phase

Since silica undergoes polymerization, precipitation, and dissolution depending on the change in pH or temperature, the chemical behavior of silica would be much complicated when cement for the construction of geological disposal system greatly changes the pH (8 to 13) of groundwater. To clarify the dynamic behavior of silica in such an alkaline solution, the concentrations of silica in both soluble and colloidal form in the supersaturated solution in the presence of solid phase have been traced over a 40-day period. In the experiment, the concentration of silica in a soluble form was determined by the silicomolybdenum-yellow method, and the concentration of silica in soluble plus colloidal forms was determined by adjusting the pH of the solution to 13, where all the silica changes into a soluble form (mainly monomeric). In order to examine the dynamic behavior of colloidal silica with solid phase of silica, this study has used natural quartz and pure commercial amorphous silica, both in a size fraction of 74–149 μm, whose specific surface-area (BET, N 2 gas) were respectively 1.0 m 2 /g and 400 m 2 /g. The Na 2 SiO 3 solution (250 ml, pH>10, 298 K) was poured into a polyethylene vessel containing quartz or amorphous silica (0.1 g or 0.5 g), HNO 3 and a buffer solution. The pH of the solution was set to 8. The silica initially in a soluble form at pH>10 (6.8×10 -3 M or 1.2×10 -2 M) became supersaturated and either deposited on the solid surface or changed into the colloidal form. The ratio of silica in those form depended both on the initial concentration of soluble-silica and the surface area of the solid. The concentration of colloidal-silica gradually decreased, where the logarithm of its concentration decreased linearly against time after the concentration of soluble-silica decreased to a metastable concentration slightly higher than the solubility of soluble-silica.

Dissolution Rate of Colloidal Silica in Highly Alkaline Solution

For the performance assessment of the radioactive waste repository, it is important to clarify the dynamic behavior of silica (silicic acid and hydrous or unhydrous silicon dioxides). The behavior of silica would be complicated when cement is used for the construction of the repository, since silica takes various forms due to polymerization, precipitation and dissolution with change of pH or temperature. In order to know the fundamental kinetic property of silica, this study has examined the dissolution rate of colloidal-silica. In the experiment, the concentration of silica in a soluble form was determined by the silicomolybdenum-yellow method. In this study, soluble-silica was defined as silica reacting with molybdate reagent and coloring yellow, and colloidal-silica was defined as silica in liquid phase except for soluble-silica. Colloidal-silica was obtained through the polymerization process, where the pH value of silica solution was brought down from over 10 by HNO 3 solution. This study examined dissolution rate of colloidal-silica again by setting to 10 or 13 in pH-value and 288 K, 298 K or 313 K in temperature. In the experimental results, the dissolution reaction of colloidal-silica proceeded linearly with time, when the dissolution of colloidal silica was not restricted by the solubility of silica. To estimate the dissolution rate, we assumed df /dt = k * , where f is the soluble-silica fraction defined as the amount of soluble-silica divided by the silica amount introduced into the sample solution, t the time (s) and k * the rate constant (s -1 ). The activation energy for the dissolution of the colloidal-silica at pH 13 was estimated to be approximately 80 kJ·mol -1 which was similar to that for amorphous silica (solid phase) at pH 13. This suggests the same reaction mechanism for the dissolution of colloidal-silica and amorphous silica in highly alkaline solution.

Effects of Temperature on the Deposition Rate of Supersaturated Silicic Acid on Ca-type Bentonite

Na-type bentonite is commonly used as a tunnel backfilling material to prevent groundwater and radionuclide migration during the construction of a geological disposal system for high-level radioactive waste in Japan. However, host rock fractures with strong water flow can develop groundwater paths in the backfilling material. Especially, the alteration to Ca-type bentonite causes degradation of the barrier performance and accelerates the development of groundwater paths. Additionally, using cementitious materials gradually changes pH between 13 and 8. High alkaline groundwater results in high solubility of silicic acid; therefore, silicic acid is eluted from the host rock. Downstream, in the low alkaline area, the groundwater becomes supersaturated in silicic acid. This acid is deposited on Ca-type bentonite, thus leading to the clogging of the groundwater paths. In the present study, we investigate the silicic acid deposition rate on Ca-type bentonite under 288-323 K for depths greater or equal to 500 m. The results indicate that temperature does not affect the silicic acid deposition rate up to 323 K. However, in this temperature range, the deposition of silicic acid on Ca-type bentonite in backfilled tunnels results in clogging of the flow paths.

A Double Porosity Model to Describe Both Permeability Change and Dissolution Processes

Cement is a practical material for constructing the geological disposal system of radioactive wastes. However, such materials alter groundwater up to 13 in pH around the repository, changing the permeability of natural barrier. So far, the authors have examined the relation of permeability change with dissolution process by flowing a high pH solution (NaOH, 0.1 mM) into a bed packed with amorphous silica particles. Here, the particle diameters were adjusted to a size fraction of 74 to 149 μm by sieving. Its specific surface area was estimated as 350 m2/g by the BET method using nitrogen gas. The experimental results showed that the permeability did not immediately change although the soluble silicic acid continuously flowed out of the packed bed.This study proposes a new mathematical model considering the diffusion and dissolution processes in the inner pore of the particle. This model assumed that each packed particle (74 to 149μm in diameter) consists of the sphere-shaped aggregation of smaller particles (20 nm in diameter). OH− ions diffuse into the pore between such small particles, and simultaneously consumed by the reaction with small particles. The radius of the each packed particle (sphere-shaped aggregation of small particles) was defined by the length from the center of the aggregation to the region where the small particles still remains. Since the outer small particles more easily dissolve than inner small particles because of diffusion process of OH− ions, each packed particle gradually shrinks. The fundamental equations consist of a simple diffusion equation of spherical coordinates of OH− ions considering the reaction term, which is linked by the equation to describe the size change of small particles with time. Here, this model also considered a change (time and space) of the diffusion oefficient caused by the change of the porosity between small particles. Besides, the change of over-all permeability of the packed bed was evaluated by Kozeny-Carman equation and the calculated radii of packed particles. The dissolution rate constant already reported was used.The calculated result was able to well describe the experimental result, though there was no fitting parameter in the comparison with the experiment results. While the flow paths of underground cannot be simply simulated by a packed bed, this approach suggested that the dynamic behavior of permeability in a natural barrier depends also on non-uniformity of dissolution processes in inner pores (secondary pores) of minerals.Copyright

Fluorescence Emission Behavior of Eu(III) Sorbed on Calcium Silicate Hydrates Formed With No Dried Process

Calcium silicate hydrate (CSH) is a main component of cement-based material required for constructing the geological repository. As in many countries, since the repository in Japan is constructed below water table, we must consider the interaction of radionuclide with cement materials altered around the repository after the backfill. Using fluorescence emission spectra, so far, the authors have investigated the interaction of Eu(III) (as a chemical analog of Am(III)) with CSH gels formed with no dried process, considering a condition saturated with groundwater. However, in such fluorescence emission behaviors, a deexcitation process of OH vibrators of light water and a quenching effect caused by Eu-Eu energy transfer between Eu atoms incorporated in the CSH gel must be considered.This study examined the fluorescence emission behavior of Eu(III) sorbed on CSH gels formed with no dried process, by using La(III) (non-fluorescent ions) as a diluent of Eu(III). Furthermore, the CSH samples were synthesized with CaO, SiO2, and heavy water (D2O) as a solvent in order to avoid the obvious deexcitation process of OH vibrators of light water. This study prepared CSH samples with the Ca/Si ratio set to 1.6, 1.0, and 0.5. A 1 mM solution of a given combination of Eu(III) and La(III) (Eu(III) content: 100%, 67%, 50% or 33%) was added into CSH gel sample. The contact time-period of the CSH gel with the Eu(III)/La(III) solution was set to 60 days.In the results, the peak around 618 nm was split into two peaks of 613 nm and 622 nm in the cases of Ca/Si = 1.0 and 1.6. Then, the peak of 613 nm decreased with increment of Eu(III)/La(III) ratio. This means that the relative intensity of 613 nm is useful to quantify the amount of Eu(III) incorporated in CSH gel. Besides, the intensity peak of 584 nm decayed with increment of Eu/La ratio, suggesting a quenching effect due to Eu-Eu energy transfer. However, the decay behavior of the fluorescence emission did not depend on the Eu/La concentration ratio. That is, such a quenching effect is neglectable. Additionally, the low Ca/Si ratio samples underwent slow attenuation of fluorescence and showed profiles similar to those of high Ca/Si ratio samples. Therefore, low Ca/Si ratio samples also include the reaction forming a complex on the surface of CSH gel with Eu(III). In other words, even if Ca/Si ratio is lower than 1.0, CSH gels would retard the migration of radionuclides released from the repository.Copyright