Diffusion and retention of sodium and strontium in Opalinus clay: Comparison of sorption data from diffusion and batch sorption measurements, and geochemical calculations

The retention of 22Na and 85Sr on Opalinus clay from Benken and the Mont Terri Rock Laboratory (Switzerland) was measured by through- and out-diffusion measurements on intact rock samples, and by batch sorption on crushed material. The sorption values obtained from these measurements were compared with one another and with ones calculated from a selectivity coefficient/geochemical modelling approach. One of the main aims of the work was to test to what extent sorption values deduced from diffusion experiments on intact rock were compatible with those measured in batch tests on crushed material and with sorption values calculated from geochemical modelling. This is an important consideration in repository performance assessment studies because apparent diffusion coefficients are often calculated from batch Rd values and effective diffusion coefficients measured using tritiated water.In general, there was excellent agreement between the distribution ratios for Na obtained by all 3 approaches for both sources of Opalinus clay. For Sr the agreement was also good, but the calculations based on selectivity coefficients in the case of Benken, and the batch sorption data in the case of Mont Terri material were a factor of ∼2 different from the values found in the other two cases. Nevertheless, a factor of ∼2 difference in the worst case is not considered to be significant when the errors associated with the different methods are considered realistically.The uptake mechanism for both elements in the Opalinus clay system is cation exchange and the results strongly indicate that there is no significant difference between the exchange capacity available in the dispersed and in the intact rock systems i.e. reduced site accessibility in intact Opalinus clay is not an issue where sorption by cation exchange is occurring.