| Summary: | Recrystallization and solid-solution formation with barite is considered as relevant retention mechanism for <sup>226</sup>Ra in long-term scenarios of nuclear waste management. Here, we studied the effect of ionic strength and the presence of Sr in solution upon the Ra-uptake kinetics and final Ra concentrations in solution by recrystallizing barite in solution with varying Sr and NaCl concentration and temperature for up to 1000 days. Final Ra-concentrations were interpreted based on thermodynamic modelling. Our results indicate a slight decrease of the retention potential of barite for Ra but little effect on the uptake kinetics due to the increase of ionic strength from. 0.1 mol/kg to 1.0 mol/kg of NaCl. The final concentrations at solid/liquid ratio of 0.5 g/kg are well described based on available thermodynamic models whereas at 5 g/kg additional Ra uptake probably due to kinetic effects was observed. On the contrary, the presence of Sr in solution can have a significant inhibiting kinetic effect on the uptake kinetics and lower the final Ra-uptake. In some cases, with low solid/liquid ratio or at ambient conditions, Sr completely inhibits barite recrystallization. In all other cases, Ra, Ba and Sr were taken up as thermodynamically predicted at the end of the experiments.
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