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 1. Radium sorption was found to be an exothermic and spontaneous reaction probably governed by chemical reaction rather than diffusion. As expected, the presence of 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 analogues for radium confirmed the significant difference in the Rd values with sorption on CSH decreasing in the order Ra > Ba > Sr.

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