Batch sorption experiments with iodine, bromine, strontium, sodium and cesium on Grimsel mylonite

Currently PSI and Nagra are carrying out a series of single fracture migration experiments in the underground laboratory at the Grimsel Test Site. A laboratory support programme has been running in advance of the migration experiments. The current aims of this programme are to identify suitable sorbing radionuclides for future tracer tests and to determine their sorption properties. In this report, the results of a series of sorption experiments carried out with 131I, 82Br, 85Sr, 22Na and 137Cs on fracture infill material are reported. These static batch sorption experiments are complemented by dynamic core infiltration experiments. Although the provision of sorption isotherm data as input to the transport codes and as an aid to assessing the feasibility of migration experiments with sorbing tracers was one of the main aims of this work, an equally important task was to obtain a quantitative understanding of the mechanism of sorption on mylonite for the chosen radionuclides. A previous study in which the geochemistry of the migration fracture, the chemistry of the groundwater and physico-chemical interaction of the infill material with groundwater were investigated, is summarized below and forms the background and basis for these sorption studies. Previous work showed that the fracture infill material ('mylonite') behaved as a weak cation exchange medium. On the basis of these results, 85Sr and 22Na were put forward as two tracers likely to exhibit 'ideal' sorption characteristics. It was essential to test the validity of the assumptions and predictions by performing laboratory sorption experiments with 22Na and 85Sr in the groundwater-mylonite system. In the previous report it was suggested that tracer migration tests be carried out with a radioisotope of Cs as an example of a radionuclide exhibiting a non-linear sorption isotherm. The present sorption experiments were designed with the intention of testing whether the concepts and models developed for homoionic clays might also be applied to the infill material being studied here. (author) 7 figs., 12 tabs., 42 refs