A modification of the electromigration device and modelling methods for diffusion and sorption studies of radionuclides in intact crystalline rocks

To determine the diffusion and sorption properties of radionuclides in intact crystalline rocks, a new electromigration device was built and tested by running with I− and Se(IV) ions. By introducing a potentiostat to impose a constant voltage over the studied rock sample, the electromigration device can give more stable and accurate experimental results than those from the traditional electromigration devices. In addition, the variation in the pH of the background electrolytes was minimised by adding a small amount of NaHCO3 as buffers. To interpret the experimental results with more confidence, an advection-dispersion model was also developed in this study, which accounts for the most important mechanisms governing ionic transport in the electromigration experiments. Data analysis of the breakthrough curves by the advection-dispersion model, instead of the traditional ideal plug-flow model, suggest that the effective diffusivities of I− and Se(IV) are (1.15 ± 0.06) × 10−13 m2/s and (3.50 ± 0.86) × 10−14 m2/s, respectively. The results also show that I− is more mobile than Se(IV) ions when migrating through the same intact rock sample and that their sorption properties are almost identical.