Adsorption of caesium on raw Ca-bentonite in high saline solutions: Influence of concentration, mineral composition, other radionuclides and modelling

Abstract

Concentration depended retention of Cs+ was analysed in alkaline and very high saline solutions (0.1–5 M), mainly NaCl but also in an artificial cement pore water (ACW), representing conditions in a potential high level nuclear waste disposal in northern Germany. Adsorbent agent was Calcigel, a raw Ca-bentonite proposed as a clay based buffer material, which was used as supplied. Batch experiments were carried out in a wide concentration range (5–250,000 nmol L−1) and subsequently, the aqueous phase was analysed via mass spectrometry with inductively coupled plasma (ICP-MS). The adsorption still depended on the Cs+ concentration in every medium and decreased with increasing electrolyte concentration, in general. In nanomolar concentration range, the retention was above 50% but dropped rapidly to only approximately 10% when ionic strength exceeded 1 M. The amount of Cs+ leached out of the clay appeared to be commensurate with electrolyte concentration. Ion exchange modelling was done using specific ion interaction theory to calculate activity coefficients. Two surface complexations were chosen to describe different adsorption sites on Calcigel. One with high selectivity for Cs+ (logKCs = 8.14) and very low capacity, such as frayed edges sites in micaceous minerals, the other one based on cationic exchange capacity of Calcigel and rather unselective for Cs+ (logKCs = 1.96). With refined selectivity constants, it was possible to describe experimental data quite perfect. However, determined values for constants differ from those found to be best for lower ionic strength. In addition, adsorption on raw Ca-bentonite was compared with that on Na-montmorillonite as a single mineral. Lower dependence on concentration was determined on single mineral in 0.1 M NaCl (79% to 53% adsorption on Na-montmorillonite, 89% to 36% on Ca-bentonite), but at higher ionic strengths the differences between the investigated clays diminished. The Cs+ adsorption on Calcigel was also analysed in presence of Eu3+, UO22+ and I−, representing potential competing ions attendant in high level nuclear waste. Here, almost no relevant influence could be detected, except at highest investigated concentration and ionic strength >1 M. At an initial concentration [Cs+]i = 250 μmol L−1, the adsorption declined remarkably in presence of other ions from approximately 10% adsorption to at least 0–5%.