Study of Radium Behavior in Contact With Calcium-Silicate-Hydrates

Radium-226 is an important radionuclide with respect to the long-term safety of repositories for low and intermediate-level radioactive wastes, as well as deep geological repositories for high-level radioactive waste and spent nuclear fuel. To evaluate its mobility in the engineered barrier system of a repository, the sorption of radium on calcium-silicate hydrates (CSH), which form the major hydration phases in cementitious materials, was studied. Radium sorption to CSH was found to be very fast, leading to steady-state between solid and liquid phase after less than four days. The dependency of the equilibrium distribution ratios on radium concentration, the calcium to silicon ratio in CSH, liquid to solid ratio, and temperature was investigated, and estimates of apparent activation energy, reaction enthalpy, entropy, and Gibbs energy of the sorption process were derived. Radium sorption on CSH can be described by linear isotherms with Rd values mostly in the order of 104 L/kg. Radium sorption was found to be an exothermic and spontaneous reaction probably governed by chemical reaction rather than diffusion. As expected, the presence of ethylene diamine tetraacetic acid (EDTA) at low concentrations led only to a small decrease in radium sorption, due to the strong competition of dissolved calcium for EDTA complexation. A comparison of the sorption behavior of various alkaline earth elements used as chemical analogs for radium confirmed the significant difference in the Rd values with sorption on CSH decreasing in the order Ra > Ba > Sr

Comparative study of radium and strontium behaviour in contact with cementitious materials

A comparative study of the sorption behaviour of radium and strontium was performed on various cementitiousmaterials including crushed hardened cement pastes (HCP) and concretes as well as a synthesised calcium silicatehydrate (CSH) phase.Rd values obtained for the Ra and Sr uptake on commercial cement materials were in the range of50–380 L kg-1 and 10–30 L kg-1, respectively. No significant difference between the distribution ratios of theisotopes 226Ra and 223Ra was observed in the studied liquid to solid (L/S) ratio range, although different isothermswere determined. The Rd values for Ra were found to increase with increasing L/S ratio. The cause of thiseffect is obviously the non-linearity of the sorption isotherm, here of the convex type. In contrast, Sr uptakeseemed to be largely unaffected by variation of L/S ratios; this indicates an isotherm of almost linear type.Sorption experiments with the CSH phase confirmed the distinctive differences in the sorption behaviour betweenRa and Sr as expected, with Rd values significantly higher for Ra. Similarly, the difference between realcementitious materials and the pure CSH phase was confirmed, indicating that the sorption of alkaline earthelements is mainly due to uptake by CSH.The kinetics of Ra and Sr uptake on cementitious materials were evaluated by a set of models describing thesorption in heterogeneous systems based on different rate-controlling processes. The FD (film diffusion) model inthe case of Ra, and the ID (diffusion in inert layer) model in the case of Sr provided the best fits.The influence of temperature on the kinetics of radium sorption was studied, suggesting change in the shape ofisotherm with increasing temperature. Evaluation of sorption kinetic data yielded values of the apparent activationenergy of the uptake process.Complementary through diffusion experiments using compacted crushed HCP confirmed and extended thefindings obtained by evaluation of the batch sorption experiments performed with Ra and Sr