Experimental Studies on the Interaction of Groundwater with Bentonite
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EXPERIMENTAL STUDIES ON THE INTERACTION OF GROUNDWATER WITH BENTONITE Y. Sasaki, M. Shibata, M. Yui and H. Ishikawa Power Reactor and Nuclear Fuel Development Corporation, 4-33 Muramatsu, Tokai-mura, Ibaraki-ken, 319-11, Japan ABSTRACT Interactions of sodium bentonite with distilled water and two types of synthetic groundwater were studied by batch experiments. In the experiments, clay and pure minerals were reacted at room temperature under aerobic and anaerobic condition as a function of time and liquid/solid ratio. The clay and pure minerals used in the experiments were Kunigel-VI (crude Na-bentonite), Kunipia F(purified Na-bentonite), purified Na-smectite(purified from Kunipia F), calcite and pyrite as accessory minerals. The chemical composition in the liquid phase was analyzed through centrifugation and ultra filtration. Alteration of the distribution of exchangeable cation in the bentonite was analyzed by NH4Ac and XRD. The results indicated that the interaction between bentonite(Kunigel-V 1) and groundwater under aerobic condition was described by ion exchange reaction of smectite, dissolution of calcite and oxidation of pyrite. From these experimental studies, the model of the interaction of groundwater with bentonite proposed by Wanner was modified. The comparison between calculation and experimental results showed good agreement and indicated that this model could be adopted to predict porewater chemistry of bentonite for performance assessment of geological isolation system of high level waste. INTRODUCTION In a geological isolation system of high level radioactive waste, the chemical interaction of engineered barrier materials with groundwater and radionuclides transport depends on the porewater chemistry of bentonite buffer material. Therefore, it's needed to interpret the interaction process and to predict the porewater chemistry in the bentonite. Some laboratory studies [1][2][3] have been performed on bentonite-groundwater interaction using Wyoming bentonite and synthetic groundwater. In these studies, they observed ion exchange of smectite, calcite dissolution and pyrite oxidation. As the
process of dominate aqueous concentration, the model of bentonite-groundwater interaction was proposed by Wanner[4], refering the result of these experiment. His model assumed ion exchange of smectite, calcite and quartz dissolution equilibrium. However, experimental studies were little to identfy dominant reaction that affected the solution composition and its variation with time. In this study, experiments on bentonite-water interaction were carried out using Japanese bentonite. In order to identfy dominant reactions, pure smectite and mixture of minerals were in use for these experiments. The model calculation was also carried out to interpret the variation of solution composition with time. EXPERIMENTAL Materials Two kinds of commercial bentonite from same deposit (Tsukinuno mine Yamagata Pref., Japan) were used. One is Kunigel-Vl(Kunimine Industries Co.) which is a crude Na-bentonite. The other is Kunipia-F
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