Determining Sorption Coefficients in Intact Rock Using an Electrical Potential Gradient as a Driving Force for Migration
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Determining Sorption Coefficients in Intact Rock Using an Electrical Potential Gradient as a Driving Force for Migration Magnus André, Maria E. Malmström, Ivars Neretnieks Chemical Engineering and Technology Royal Institute of Technology, KTH Teknikringen 26, 100 44 Stockholm, Sweden
ABSTRACT The transport of radionuclides in indigenous rock is greatly affected by the sorption of cations in the porous rock matrix. For the determination of sorption coefficients, batch experiments have traditionally been used. For these experiments, the rock sample is crushed into fine particles to reduce the experimental time. However, this procedure increases the specific surface area of the sample and the new surfaces created could have different sorption qualities than the naturally occurring surfaces, which may impair the results of sorption coefficient determinations. A new method for determining sorption coefficients in intact rock is being developed, using electromigration as a means to speed up the transport process, thereby allowing for faster equilibration between the rock sample and the tracer solution. Here, we report results from preliminary experiments, using cesium as a sorbing tracer, showing a consistent difference between sorption coefficients obtained using electromigration methods on intact rock samples and traditional batch experiments on crushed samples.
INTRODUCTION Geological repositories are considered for permanent disposal of nuclear waste in Sweden. The crystalline rock enclosing the repository acts as a natural barrier to transport of radionuclides in the case of a repository failure. The nuclides can diffuse in the micropores of the rock and interact with the external and internal surfaces of the porous rock matrix. Sorption reactions in the porous rock matrix may strongly retard the nuclide transport and may cause the rock to act as a sink for the radionuclides. Sorption coefficients are traditionally determined using batch experiments with crushed rock. The diffusivity of ions is low in the rock matrix and decreasing the diffusional lengths by crushing the sample reduces the time needed to reach equilibrium. The crushing process increases the specific surface area of the crystalline rock sample and might create rock surfaces with different sorption properties than the naturally occurring internal surfaces of the bedrock [12]. It is thus probable that the sample preparation in batch experiments increases the per mass sorption capacity of the sample and leads to an overestimation of the radionuclide sorption and retardation in the rock. Sorption experiments using intact drill cores from site investigations could provide sorption data free from artefacts of the crushing process. However, the internal surfaces of the intact samples are not directly exposed to the tracer solution and the low diffusivity in the rock matrix would cause prohibitively long experimental times. Faced with this problem, Löfgren and Neretnieks [3] proposed the use of electromigration as a way of reducing the experimental time
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