Building the bridge between U(VI) and Ca-bentonite - Influence of concentration, ionic strength, pH, clay composition and competing ions.

In the context of a high-level nuclear waste disposal, the retention of U(VI) on non-pre-treated Ca-bentonite as potential technical barrier is studied. The objective of this study is to reveal the retention behaviour of U(VI) under extreme geochemical conditions, such as hyperalkaline pH range as well as high salinity at the same time, and taking into account other relevant parameters. This should lead to a better understanding of necessary safety precautions for avoiding a release of U(VI) in the environment. Batch experiments were conducted to determine the influence of the initial U(VI) concentration, salinity, pH value, clay composition and the presence of other elements (Ca(II), I-, Cs(I), Eu(III)). After the sorption experiments, the remaining U(VI) concentration in solution was determined via mass spectrometry with inductively coupled plasma. U(VI) can be immobilised from 10% to 100% under all investigated conditions. Precipitation plays a role in the U(VI) retention but only at higher concentrations (≥10-5 mol L-1). The retention is reversible especially with decreasing pH (<10.5) as the aquo complex Ca2UO2(CO3)3(aq) is formed. Ca(II) strongly enhances the U(VI) adsorption onto Ca-bentonite in the hyperalkaline pH range, probably due to the formation of Ca(II)-bridges. The best retention could be observed on natural bentonite compared to pure montmorillonite and altered bentonite. From a waste cocktail containing important elements of the repository inventory (Cs(I), Eu(III), U(VI) and iodide), only Eu(III) as homologous element to trivalent actinoids competes with U(VI) for binding sites, especially at low metal concentrations, but also facilitates the precipitation at higher concentrations.

Simultaneous quantification of iodine and high valent metals via ICP-MS under acidic conditions in complex matrices.

The determination of iodine as a main fission product (especially the isotopes I-129 and I-131) of stored HLW in a disposal beside its distribution as a natural ingredient of many different products like milk, food and seawater is a matter of particular interest. The simultaneous ICP-MS determination of iodine as iodide together with other elements (especially higher valent metal ions) relevant for HLW is analytically very problematic. A reliable ICP-MS quantification of iodide must be performed at neutral or alkaline conditions in contrast to the analysis of metal ions which are determined in acidic pH ranges. Herein, we present a method to solve this problem by changing the iodine speciation resulting in an ICP-MS determination of iodide as iodate. The oxidation from iodide to iodate with sodium hypochlorite at room temperature is a fast and convenient method with flexible reaction time, from one hour up to three days, thus eliminating the disadvantages of quantifying iodine species via ICP-MS. In the analysed concentration range of iodine (0.1-100µgL-1) we obtain likely quantitative recovery rates for iodine between 91% and 102% as well as relatively low RSD values (0.3-4.0%). As an additional result, it is possible to measure different other element species in parallel together with the generated iodate, even high valent metals (europium and uranium beside caesium) at recovery rates in the same order of magnitude (93-104%). In addition, the oxidation process operates above pH 7 thus offering a wide pH range for sample preparation. Even analytes in complex matrices, like 5M saline (NaCl) solution or artificial cement pore water (ACW) can be quantified with this robust sample preparation method.