Experimental Indications of Effects of Surface Deprotonation on Na-Bentonite Porewater Chemistry in a Geological Reposit
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ABSTRACT Bentonite-water interaction was studied using a simple equilibrium model based on experimental measurements in order to describe bentonite porewater chemistry. Direct pH measurements for highly compacted bentonite and batch-type bentonite-water interaction experiments were performed under anaerobic conditions. In the direct pH measurements, resin particles doped with a pH indicator were sandwiched between a pair of bentonite columns immersed in a test solution. The experimental results showed that the solution compositions in equilibrium with bentonite depended on the bentonite to liquid ratio (B/L) and the initial solution composition. An equilibrium model assuming only fast equilibration processes between the bentonite minerals and the solution could be used to calculate the trends of pH and other ion concentrations with B/L. This study indicates that the surface deprotonation of smectite is a very important factor influencing the porewater chemistry in highly compacted bentonite. INTRODUCTION The porewater chemistry in compacted bentonite will affect the performance of engineered barrier systems in a high-level radioactive waste repository. Many studies have been carried out on the bentonite-groundwater interaction, especially with respect to modeling dissolution equilibria and cation exchange between bentonite and groundwater. Wieland et al. [2] proposed a bentonite surface chemical model, which included surface acid-base properties of smectite and predicted near-field chemistry under repository conditions. Surface complexation modeling has also been applied to smectite in sorption studies (e.g. Bradbury and Baeyens [3]). However it was uncertain whether those models could be applied to a highly compacted bentonite system and this has never been validated due a lack of information on porewater chemistry. Furthermore, some modeling studies on porewater chemistry have encountered experimental difficulties in controlling the partial pressure of CO2 during the analyses. This study demonstrates the reliability of a porewater chemistry model through experiments for a range of bentonite-water interaction systems, in order to identify which reactions would be likely to constrain the porewater chemistry under repository conditions.
EXPERIMENTAL Batch-type experiments on the interaction between bentonite and water Batch-type experiments on the interaction between bentonite and water were carried out in an Ar-gas glove box. Tsukinuno bentonite Kunigel-V1 (Kunimine Industry Co.) was used. The experimental conditions were: reaction periods between 14 ~ 200 days, bentonite/liquid ratios of 0.1 ~ 1,000 g/l, initial solutions of degassed-deionized water (DDW, pH 7), 0.5M NaCl solution (pH 7) and low-alkalinity-cement [4] equilibrated solution (pH 11). The aqueous phases were ultrafiltered (MWCO 10,000) following centrifugation, and chemical compositions were analyzed by inductively coupled plasma emission spectrochemical analysis (ICP), ion chromatography (IC) and atomic adsorption spectrochemical analysis (AAS). Th
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