Smectite Dissolution Kinetics in High-Alkaline Conditions
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Hiroshi Ohmoto1, Kathryn R. Spangler1, Yumiko Watanabe1, and Gento Kamei2 1 Astrobiology Research Center and Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, U.S.A. 2 Japan Nuclear Cycle Development Institute, Tokai, Ibaraki, 319-1194, Japan $%675$&7 To determine the initial rates and effects of silica in solution on the dissolution kinetics of smectite, short- and long-term batch experiments (0.5 hour to 30 days) were completed at three temperatures (T = 25, 50, and 75°C) using stock solutions pH adjusted by NaOH (pH = 12, 13, and 13.5) with varying initial silica concentrations (0, 30, 60, and 100 ppm). The following important characteristics were observed at pH = 12: (1) The concentrations of Al, Si, Mg, Fe, and Ti in solutions increase rapidly during the first ~2 hours and reach steady state (equilibrium) within ~5 days. (2) The concentration ratios of Al, Si, Fe, Mg, and Ti in solutions during the early (~9. ,1752'8&7,21 A variety of buildings and tunnels have been built in smectite-rich environments in many countries, including Japan. The current plan for underground nuclear waste repositories in Japan and other countries is to construct concrete facilities to house the waste materials, which will be placed in stainless-steel containers and surrounded by buffer materials (composed primarily of smectite). Upon contact with concrete, groundwater may become highly alkaline (pH ≈ 12-14). Therefore, long-term stability assessments of the underground repository sites and concrete facilities require accurate information on the dissolution and precipitation kinetics of a variety of minerals and rocks subjected to high-alkaline conditions. Understanding the formation and dissolution kinetics of smectite and other minerals in highly alkaline solutions is also important for studies on soil formation, water chemistry, and biological processes in highly evaporating lakes and ponds in tropical regions. Previous experimental studies suggest smectite dissolution rates generally increase with increasing pH (from ~8 to ~13) and with increasing temperature (from ~25° to ~80°C) [1-3]. Cama et al. [1] have also recognized that smectite dissolution rates decrease with increasing silica content of solution at pH 8.8 and 80°C. An important question for our investigation is
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whether silica plays the same role as an inhibitor of smectite dissolution in higher pH solutions. Here we report the experimental results on the dissolution kinetics of smectite as a function of T (25, 50, and 75°C) and initial silica content of solution (0, 30, 60, and 100 ppm) at pH = 12. (;3(5,0(17$/0(7+2'6 The smectite powder used in the experiments is composed of fine-grained crystals (between 0.05 and 0.5µm) with a specific surface area of 36.04 m2/g, determined by the BET (BrunauerEmmett-Teller) method. The smectite is a Ca-poor, Mg-Na-rich type (CaO = 0.01 wt%; MgO = 3.40 wt%; Na2O = 3.21 wt%) with the following structural formula (Equation 1): (N
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