Deposition Behavior of Supersaturated Silicic Acid in the Condition of Relatively High Ca or Na Concentration
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Deposition Behavior of Supersaturated Silicic Acid in the Condition of Relatively High Ca or Na Concentration Taiji Chida1, Yuichi Niibori1, Hayata Shinmura1 and Hitoshi Mimura1 Dept. of Quantum Science & Engineering, Graduate School of Engineering, Tohoku University, 6-6-01-2 Aramaki-aza-Aoba, Aoba-ku, Sendai 980-8579 JAPAN 1
ABSTRACT Around the radioactive waste repository, the pH of the groundwater greatly changes from 8 to 13 and the groundwater contains a relatively large quantity of calcium (Ca) and sodium (Na) ions due to cementitious materials used for the construction of the geological disposal system. Under such conditions, the deposition behavior of silicic acid is one of the key factors for the migration assessment of radionuclides. The deposition and precipitation of silicic acid with the change of pH and coexisting ions may contribute to the clogging in flow paths, which is expected as the retardation effect of radionuclides. Thus, this study focused on the deposition behavior of silicic acid under the condition of relatively high Ca or Na concentration. In the experiments, Na2SiO3 solution (250 ml, 14 mM, pH>10, 298 K) was prepared in a polyethylene vessel containing amorphous silica powder (0.5 g) as the solid phase. Then, a buffer solution (to adjust to 8 in pH), HNO3, and Ca(NO3)2 as Ca ions or NaCl as Na ions were sequentially added. Such a silicic acid solution becomes supersaturated, gradually forming colloidal silicic-acid and/or the deposit on the solid surface. In this study, the both concentrations of soluble and colloidal silicic-acid were monitored over a 40-day period. As a result, the deposition rate of silicic acid decreased with up to 5 mM in Ca ions. Besides, Na ions with up to 0.1 M slightly increased the deposition rate. Under the conditions of [Na+]>0.1 M or [Ca2+]>5 mM, the supersaturated silicic acid immediately deposited. These suggest that Na or Ca ions strongly affect the deposition behavior of supersaturated silicic-acid, depending on the surface alteration of solid phase, the change of zeta potential and the decrease of water-activity due to the addition of electrolytes (coexisting ions). INTRODUCTION Silicic acid around the geological disposal system for radioactive wastes undergoes dissolution, deposition and polymerization depending on the change of pH in the range from 8 to 13 which is caused by the cementitious materials used for the construction of the repository [1]. Some dynamic behaviors of silicic acid, such as polymerization and alteration of solid surface, may facilitate the migration rate of radionuclides from the repository, e.g., because of the chemical alteration of the flow path surfaces and the complex formation between the polymeric silicic acid and radionuclides [2, 3]. On the other hand, it is also suggested that the deposition and precipitation of silicic acid may clog main flow paths and the micro-cracks in rocks, and retard the migration of radionuclides [4]. In general, the chemical behaviors of silicic acid are greatly affected by not only the chan
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