Sorption of caesium and strontium onto calcium silicate hydrate in saline groundwater. Experimental and modelling studies on the competitive sorption between cations

In the concept for TRU waste disposal in Japan, cement is a potential waste packaging and backfilling material and is expected to provide chemical containment. The sorption of radionuclides onto cement materials, which retards the migration of aqueous radionuclides in the porewater, is a very important parameter when considering the release of radionuclides from the near field of a cementitious radioactive waste repository. Many safety assessment calculations currently assume radionuclide retardation as linear sorption equilibrium and describe it with a distribution ratio (Rd value). Further data to support and justify this assumption would be very useful. In this study, the competitive sorption between caesium or strontium, and sodium is studied to simulate the effect of the ionic strength of saline groundwater on the sorption of the cations by measuring the sorption of caesium and strontium in solution with a range of sodium chloride concentration (10-4 - 10-2 mol dm-3) using the batch sorption technique. A competition in sorption between caesium or strontium and sodium was observed. On the bases of the experimental results, we discussed the competitive sorption between caesium or strontium and sodium using a modelling approach, in which the ion-exchange model is employed and the presence of some calcium sites in C-S-H gel with different selectivity coefficients of ion-exchange is assumed, by considering the composition and the structure of C-S-H gel. The modeling calculation results predicted the measured Rd values well and also described the competition of sorption between caesium or strontium and sodium. (author)