Buffering of pH conditions in sodium bentonite
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Buffering of pH conditions in sodium bentonite Veli-Matti S. Pulkkanen, Aku P. Itälä and Arto K. Muurinen VTT Technical Research Centre of Finland, Otakaari 3 K, Espoo, P.O. Box 1000, FI-02044 VTT, Finland ABSTRACT A bentonite buffer is a part of the engineering barrier system of the geological disposal concept for spent nuclear fuel in Finland. The chemical conditions in the bentonite porewater determine the solubility, speciation and diffusion/sorption behaviour of the radionuclides in the bentonite. The OH-groups on the edge sites of the montmorillonite, the main component of bentonite, can buffer the pH conditions especially in the case that the buffering capacity of the accessory minerals is small. In this work, the pH conditions created by the interaction of sodium montmorillonite and an acetate buffer solution were studied experimentally in batch experiments and in compacted sodium montmorillonite. In the experiment with the compacted sample, the ends of an 18.4 mm long cylinder were exposed to an external solution of 0.3 M NaCl and 0.1 M acetate adjusted to pH 5 with NaOH. The effects were monitored by measuring the pH in the montmorillonite sample at 5 mm and 9.2 mm from the solution-montmorillonite interface as a function of time. The batch experiment was modelled considering the surface reactions of montmorillonite and the dissolution reactions of calcite and the added constituents to explain the observed phenomena. INTRODUCTION An important parameter which should be evaluated in the chemical evolution of a geological repository for radioactive wastes is the pH of the bentonite porewater. Until now, pH has been studied, for example, in batch experiments, by squeezing porewater from the bentonite or by geochemical modelling [1-4]. The direct measurement of pH in bentonite has been reported in our earlier papers [5,6] where solid IrOx pH electrodes prepared in accordance with the method by Yao et al. [7] were used. In an unpurified bentonite, pH is buffered by calcite and the hydroxyl groups on the edge sites of montmorillonite on the timescale of the experiments [8]. In the compacted clay, the progress of the pH change is determined by the diffusion and simultaneous chemical reactions which buffer the pH. In this work, the progress of the pH change caused by an acetate buffer in purified bentonite was studied experimentally in a batch experiment and in a cylinder of compacted clay, where the pH was measured directly. The batch experiment was also modelled using PHREEQC. EXPERIMENTAL The method described in [9] was followed to purify the MX-80 bentonite that was used in both the experiments. The purification comprised the removal of large particles, dissolution of solid carbonates by acid treatment, dissolution of iron oxides, removal of organic material, changing to the sodium form, and removal of the excess salt by washing and finally by dialysis.
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Batch experiment An acetate buffer of 0.3 M NaCl and 0.0125 M acetic acid adjusted to pH 5 with NaOH was used as a solution in the batch interaction expe
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