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.