Comparison of Titration Methods Used in Characterization of Bentonite
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Comparison of Titration Methods Used in Characterization of Bentonite Rostislav Adam, Karel Štamberg and Barbora Drtinová Department of Nuclear Chemistry, Faculty of Nuclear Sciences and Physical Engineering, Czech Technical University in Prague, Břehová 7, 115 19 Prague 1, Czech Republic
ABSTRACT Titration of bentonite, or clay materials in general, can be carried out in two ways, namely by semi-continuous method and batch method. This study compares these two methods, and, in addition, presents the results of two different evaluation and regression procedures used for fitting of experimental titration curves and for calculation of corresponding parameters. Bentonites B75 and S65, and montmorillonite SAz-1, were chosen for this study. Regarding the semi-continuous method, the technique developed by Wanner et al. was used. As for the batch method, two-week contact time for selected solid/liquid ratio 1 g per 250 ml was used on the base of preliminary experiments. The evaluation of result obtained using the batch method with back titration correction gave more relevant data, on the other hand, with the use of this method is connected a risk of solid phase dissolution. The chemical equilibrium model (i.e. nonelectrostatic), incorporated into codes (I) PHREEQC + UCODE_2005, and (II) PDNLRG.fm (code package STAMB) + software FAMULUS, was used to the evaluation of the titration curves. The codes mentioned give only slightly different resulting values in the case of batch method. INTRODUCTION In underground radioactive waste repository construction, its safety is the main task. Therefore, not only geological formation as a natural barrier, but also the so-called engineering barrier material surrounding the containers with the radioactive waste has to be selected. These engineered barriers should prevent the penetration of water to the containers with radioactive waste in the repository and retard migration of radionuclides to the ground water for enough long time. Bentonites are good for both of these tasks due to their swelling and sorption properties, so they can fill the gaps between natural rock barrier and container wall in the repository. There are some methods which can be used to the characterization of bentonite sorption properties. In particular, this article regards the titration curve and cation exchange capacity (CEC) determination. Based on the titration curve evaluation, it is possible to obtain the values of protonation constants, characterizing the so-called edge-sites, and the ion-exchange constant characterizing the so-called layer-sites. In addition, the total concentrations of both sites mentioned above are determined, as well. In principal, on the edge-sites proceed in principal the surface complexation and on the layer-sites the ion exchange of cationic species. There are two ways how to obtain the values of titration curve parameters. The first one is the semi-continuous titration developed by Wanner et al. [1], the second one is the batch titration developed by
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