Diffusion of soluble salts under a temperature gradient after the hydration of compacted bentonite

Abstract

The transport of the soluble salts contained in a compacted bentonite has been studied in a hydration and heating experiment. The objective is to follow the processes of transport produced during and after the hydration of compacted bentonite in order to detect the generation of localized saline environments under the presence of a temperature gradient. Permanent saline environments may affect the surfacial and swelling properties of a bentonite when used as a clay liner for the isolation of high-level nuclear waste (HLNW). Several hermetic cells have been designed to constrain an unsaturated compacted bentonite cylinder (dry density of 1.65 g/cm 3). The bentonite is moisturized by injecting an aqueous solution in an opposite direction to a heat source. The tests are conducted during pre-saturation to post-saturation stages. The hydration is carried out with two different solutions, synthetic granitic water of low salinity and saline water. Two gradients with different average temperatures are generated: 100–60°C (16°C/cm) and 60–35°C (10°C/cm). Convection and diffusion processes govern the transport of salts in the compacted bentonite. The temperature is the variable that mostly influences the processes of transport, while the concentration of the injected solution had smaller importance. The convective transport of salts, driven by the hydration, occurs in the presence of the thermal gradient. By means of this mechanism, the dissolved ions concentrate on the heated area of the cell. The process permits the measurement of apparent diffusion coefficients ( D a) for ions and salts during the homogenization of concentration profiles in the post-saturation stages. D a for Cl −, SO 4 2−, Br −, Ca 2+, Mg 2+, Na +, K +, Sr 2+ and Zn 2+ have been calculated. The values vary in the range of 1.5×10 −10–4×10 −12 m 2/s; the ions Cl −, Na + and Ca 2+ being of higher mobility. Codiffusion of Ca 2+ and Na + with Cl − is the main mechanism found for salt transport in the compacted bentonite.