Comparison between cesium and sodium retention on calcium silicate hydrate (CSH) phases

Abstract

Batch sorption experiments and sorption modelling were combined to analyse the mechanisms of Cs and Na retention on calcium silicate hydrate phases, CeSeH, and to identify similitudes/differences between the two elements. The role played by the CSH'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 (Kd) for Cs were higher than those of Na at trace concentration and low Ca/Si ratio but at high alkali concentrations, Kd values for Cs and Na were always comparable. This was a first indication that low-density but high selective sorption sites exist on the CSH surface, accessible for Cs and not for Na. Electrophoretic measurements were carried out to evaluate the variation of the CSH 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 CSH 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.