Determination of pore water composition during long term interaction of bentonite substrates with water media: Comparative study

Knowledge of pore water composition is crucial information in research of long-term stability of bentonite and is also crucial for any stability predictions based on geochemical modeling. This paper reports validation results of two frequently used extraction methods — centrifugation and leaching. Three types of bentonite materials were used in our study. The first one was the Czech raw bentonite from the Rokle deposit. The second type of bentonite was “Mock-Up-Cz”, which is a mixture of bentonite material from the Rokle deposit with finely ground quartz and graphite powder. The third type of material was the FEBEX bentonite obtained from the Cortijo de Archidona deposit. All the bentonite samples were placed into pressure vessels and interacted with natural water during a one-year experiment. Free water, obtained during these interactions, was used as a comparative pore water composition for both of the extraction methods. The high water content in saturated bentonite, reaching about 50 m%, allowed the use of both methods. Concentrations of Mg^2 +, Ca^2 +, K⁺ and Na⁺ in water were determined. Centrifugation could be generally considered as a more suitable method because of the smaller differences between free water composition and concentrations obtained from extracted pore water compared to the leaching method. However, actual differences will depend on the type of bentonite and on the composition of the water media. In some cases, the leaching method yields better results. Leaching usually leads to higher concentrations of alkali metals and lower concentrations of alkaline earth metals. This indicates that bentonite substrates are sensitive to the change of external conditions and quickly interact with the water media.