Incorporation of Europium and Nickel in calcite studied by Rutherford Backscattering Spectrometry

This study aims at elucidating the mechanisms leading to the incorporation of cations (Eu and Ni) into carbonates (CaCO3). These minerals are present in the French Callovo Oxfordian shales where the radioactive waste should be disposed of. Europium is a long lived fission product that can also be used as analogue of trivalent actinides. Nickel is a long lived activation product. Therefore, for safety reasons, the evaluation of the retention capabilities of calcite with respect to these radionuclides has to be checked. Calcite powders or calcite single crystals (some mm sized) have been put into contact with inactive Europium or Nickel enriched solutions. The concentrations ranged from 10-3 to 10-5 mol/L for Eu and 10-3 mol/L for Ni and the sorption durations ranged from one week to one month. In order to elucidate the incorporation mechanisms of these elements on calcite, Rutherford Backscattering Spectrometry (RBS) experiments have been carried out using an alpha particle millibeam at the 4 MV Van de Graaff accelerator of IPNL. This technique is well adapted to discriminate incorporation processes such as: (i) adsorption or co precipitation at the mineral surfaces or, (ii) incorporation into the mineral structure (through diffusion for instance). For the single crystals, complementary Scanning Electron Microscopy (SEM) observations of the mineral surfaces at low voltage have also been carried out. Moreover, for Europium incorporation, using its fluorescence properties, the results have been compared to those obtained by Time-resolved laser fluorescence spectroscopy (TRLFS). Results on single crystals show different sorption behavior for Ni and Eu. Ni accumulates at the calcite surface whereas Eu is also incorporated at a greater depth. Eu seems therefore to be incorporated into two different states in calcite: (i) heterogeneous surface accumulation and (ii) incorporation at depth greater than 160 nm after 1 month of sorption.