The Uptake of Strontium by Calcium Silicate Hydrates under High pH Conditions: An Experimental Approach to Distinguish A
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7KH8SWDNHRI6WURQWLXPE\&DOFLXP6LOLFDWH+\GUDWHVXQGHU+LJKS+&RQGLWLRQV $Q ([SHULPHQWDO$SSURDFKWR'LVWLQJXLVK$GVRUSWLRQIURP&RSUHFLSLWDWLRQ3URFHVVHV J. Tits, E. Wieland, J.-P. Dobler, and D. Kunz Paul Scherrer Institute, Laboratory for Waste Management, CH 5232 Villigen PSI, Switzerland $%675$&7 The interaction of Sr(II) with CSH phases has been investigated to distinguish adsorption from co-precipitation processes and to assess the relevance of these processes for performance assessments. Batch-type sorption experiments were carried out to study the adsorption of Sr(II) on synthetic CSH phases at different C:S ratios. Co-precipitation experiments were conducted by precipitating CSH phases in the presence of Sr(II) at two different precipitation rates. Distribution ratios (Rd) of Sr(II) obtained from the sorption and co-precipitation studies were found to be similar in value. The results indicate that co-precipitation processes do not enhance Sr(II) uptake by CSH compared to adsorption. In both the co-precipitation and adsorption experiments the same sorption sites are accessible to Sr(II). The precipitation rate of the CSH phases has no significant influence on the Rd values. ,1752'8&7,21 For disposal options involving the isolation of low- and intermediate-level radioactive waste in a cementitious repository, uptake by calcium silicate hydrate (CSH) phases is expected to play an important role in retarding the migration of radionuclides in both the near field and the far field [1]. In the near field, CSH phases are major components of hardened cement paste, and the interaction of hyperalkaline fluids from the repository with the mineral components of sedimentary rocks produces a range of CSH-type secondary minerals in the far field. Studies concerning the retention of trace concentrations of radionuclides by CSH phases have mainly focused on adsorption as the dominant uptake mechanism (e.g., [2, 3, 4]). However, for some radionuclides other uptake mechanisms such as co-precipitation (solid-solution formation) may play an additionally important role. Especially when adsorption is weak, other uptake mechanisms might become important for the immobilisation of radionuclides. The present study aims at elucidating the role of adsorption and co-precipitation as possible retention mechanisms for Sr on CSH phases. Sr(II) is an important radioelement present in different types of radioactive waste and serves as a chemical analogue for Ra(II). Moreover, Sr adsorption on CSH phases is relatively weak compared to the stronger sorbing actinides, and therefore, coprecipitation could be an additional important process enhancing the Sr uptake by these solids. The partitioning of a radionuclide during co-precipitation processes may depend on the precipitation rate, as indicated from previous studies on calcite [5, 6]. Since precipitation rates of CSH phases under repository conditions can vary widely, e.g., very fast precipitation during the hydration of cement or very slow precipitation when secondary phases in th
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