Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

1 Publication
Interaction between europium and calcite: macroscopic and spectroscopic studies
Sabău, A.; Marmier, N.; Jordan, N.; Lomenech, C.; Barkleit, A.; Toulhoat, N.; Pipon, Y.; Moncoffre, N.; Brendler, V.; Giffaut, E.;
Interactions between cations and natural or synthetic calcite may include incorporation processes, resulting in the irreversibility of some sorption reactions. Understanding and quantification of poorly to non reversible trapping mechanisms can be considered as a significant improvement in the description of a geological barrier or a backfill material performance in the safety assessment. To investigate these irreversible processes, we have decided to work on the Eu-CO2-NaCl-CaCO3 system at pH 8.3, buffered by calcite under air and corresponding to the typical pH range of natural interstitial groundwaters. Europium was chosen as analogue for trivalent actinides and due to its fluorescence properties enabling its study by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Our study combines macroscopic batch investigations and the use of spectroscopic tools to comprehensively characterize this system.
At first, a material appropriate for sorption experiments had to be selected, based on characterization studies. From a variety of sources, we focused on a calcite from SOLVAY (SOCAL U1-R) with a particle size of 0.2 µm for TRLFS investigations, mainly due to its large BET specific surface area (18.4 m2/g). We also used a calcite from OMYA (BL 200), with a bigger particle size (56 µm) and a lower specific surface area (0.66 m2/g) for Rutherford Backscattering Spectrometry (RBS) measurements, due to the specific requirements of this technique. Purity of the calcite samples and absence of polymorphic CaCO3 compounds (i.e. vaterite and aragonite) were confirmed by XRD, DRIFT or SEM.
The second step of the work was to investigate the irreversible Eu(III) uptake mechanisms onto calcite. Batch experiments, done under atmospheric conditions (pCO2 = 10−3.5), were performed under different element concentration (10−5, 10−4 and 10−3 M) and contact time (4 hours, 1 day, 1 week and 1 month), which are assumed to be the most important parameters controlling the transition between adsorption and incorporation into the solid. In addition, this allowed to monitor the potential changes in mechanisms with time. The concentrations of Eu and Ca left in the supernatant were determined by ICP-MS and ICP-OES, respectively.
Europium species formed in solution and at the solid/solution interface were identified by TRLFS. Reference blank (Eu3+ in solution and Eu3+ precipitated) spectra were compared with the sorbed samples. When elements such as europium are sorbed onto a mineral surface by inner-sphere complexation, some of the H2O molecules in the first coordination sphere are displaced. Such changes should impact the fluorescence lifetime of Eu species.
The spectrum presented in Fig. 1 corresponds to Eu-reacted calcite after a contact time of 1 month and an initial concentration of Eu of 10-4 M. Europium was found to be completely sorbed onto calcite, whose dissolution was negligible. Two lifetimes were identified. The lower value, so far not reported in the literature, can thus not unambiguously be attributed to a specific Eu species. However, the higher second lifetime is correlated with the complete loss of the europium hydration sphere, suggesting either a surface precipitate or an incorporation.
RBS measurements (Fig. 2) were carried out using the 4 MV Van De Graaff accelerator facility of IPNL with a 4 MeV incident alpha beam. They were performed at the same Eu concentration as the TRLFS results (Fig. 1), in order to obtain a high resolution signal. RBS results confirmed those obtained by TRLFS, by showing an accumulation of europium onto the calcite surface.
It can be concluded that TRLFS and RBS are appropriate and complementary techniques to study the Eu-CO2-NaCl-CaCO3 system. The results obtained so far indicate that there is either a surface precipitate following a first sorption step or that incorporation into the bulk crystal lattice already started. Both processes may occur simultaneously, further experiments shall reveal this.
Keywords: europium; calcite; TRLFS; RBS; sorption; incorporation
  • Poster
    Clays in Natural and Engineered Barriers for Radioactive Waste Confinement, 22.-25.10.2012, Montpellier, France

Publ.-Id: 17139 - Permalink