Comparison between cesium and sodium retention on calcium silicate hydrate (C S H) phases

Abstract Batch sorption experiments and sorption modelling were combined to analyse the mechanisms of Cs and Na retention on calcium silicate hydrate phases, C S H, and to identify similitudes/differences between the two elements. The role played by the C S H's Ca/Si ratio and the tracer concentration on alkali ions retention was experimentally analysed. For both elements, sorption decreased when the Ca/Si ratio increased, but only in the case of Cs sorption was clearly not linear. Distribution coefficients (K d ) for Cs were higher than those of Na at trace concentration and low Ca/Si ratio but at high alkali concentrations, K d values for Cs and Na were always comparable. This was a first indication that low-density but high selective sorption sites exist on the C S H surface, accessible for Cs and not for Na. Electrophoretic measurements were carried out to evaluate the variation of the C S H surface charge (ζ-potential) upon alkali addition and to support modelling studies. Under similar experimental conditions, the change produced by Cs or Na additions on the ζ-potential of the gels was always similar. Different sorption models were proposed to fit the experimental data, based on the adsorption data, on the variation of C S H surface potential upon Cs/Na sorption and on previous tests showing the competitive effects of Na and Cs on Ba uptake. Their adequacy will be discussed as well as the differences/similitudes of sorption behaviour for both elements.