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CC8.39.1

Dissolution Rate of Colloidal Silica in Highly Alkaline Solution Taiji Chida, Yuichi Niibori, Osamu Tochiyama, Hitoshi Mimura, Koichi Tanaka Department of Quantum Science and Energy Engineering, Graduate School of Engineering, Tohoku University, Sendai 980-8579, JAPAN ABSTRACT 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 HNO3 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−1which 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. INTRODUCTION Cement used for the construction of the radioactive waste repository may greatly alter the pH (8 to 13) of groundwater [1]. Among the various materials which may react with alkaline fluids, silica is considered to be the most important, since silica undergoes polymerization, precipitation and dissolution depending on the change in pH or temperature. Such behavior of

CC8.39.2

silica causes the alternation in the chemical and physical properties of fractured layer or of the host rock surrounding the repository, as shown in Figure 1. Further, colloidal-silica may affect the migration of the radionuclides released from the repository. Therefore, we need various kinetic data of silica under high pH condition, in order to predict such complicated behavior of silica and estimate the migration rates of the radionuclides in the disposal system also considering