Study of Europium and Nickel interaction with calcite - batch experiments and spectroscopic characterization


Study of Europium and Nickel interaction with calcite - batch experiments and spectroscopic characterization

Sabău, A.; Jordan, N.; Lomenech, C.; Marmier, N.; Brendler, V.; Barkleit, A.; Surblé, S.; Toulhoat, N.; Pipon, Y.; Moncoffre, N.; Giffaut, E.

Interactions between cations and natural or synthetic calcite may include incorporation processes, resulting in the irreversibility of some sorption reactions. Calcite is present in soil and sediment materials, and in particular in the Callovo-Oxfordian clay samples from of the French underground laboratory of Bure (France), studied in the context of an underground repository for radioactive waste. Europium has been chosen to be investigated by TRLFS due to its fluorescent properties and because it can serve as an analogue for trivalent actinides. Nickel is toxic as a heavy metal as well as in its radioactive form. Few experimental studies have been made to define its interaction with soil and sediment minerals in general and only a handful of articles report investigations of Ni interaction with calcite. To investigate these irreversible processes, we have chosen to work on the Eu-CO2-NaCl-CaCO3 and Ni-CO2-NaCl-CaCO3 systems at pH 8.4, buffered by calcite under atmospheric conditions.
Our study combines macroscopic batch experiments with spectroscopic investigations (Time Resolved Laser Fluorescence Spectroscopy - TRLFS and Rutherford Backscattering Spectrometry - RBS) to comprehensively characterize these systems.

First, appropriate material for sorption experiments were selected based on characterization studies. Eventually, a calcite powder from SOLVAY (SOCAL U1-R) with a particle size of 0.2 μm for TRLFS investigations was chosen, mainly due to its large BET specific surface area (i.e. 18.4 m2/g). In addition, a calcite powder from OMYA (BL 200), with a bigger particle size (56 μm) and a lower specific surface area (0.7 m2/g) was used for Rutherford Backscattering Spectrometry (RBS) measurements, due to the specific requirements of this technique.
For both powders, Diffuse Reflectance Infra-Red Fourier Transform Spectroscopy (DRIFT), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and elementary analysis confirmed the absence of polymorphic CaCO3 compounds (i.e. vaterite and aragonite).
In order to get a better understanding of incorporation of cations in the structure of calcite, we compared our results obtained on powders with studies on millimetric calcite single crystals from Alfa Aesar, performed under the same experimental conditions as for powders.
The sample preparation consists in open reactor experiments under atmospheric conditions (pCO2 = 10-3.5 atm) in 0.1 M NaCl media. The studied concentrations range from 10-6 to 10-3 M. The experiments were carried out for contact times ranging from 4 hours to 6 months for europium and from 4 hours up to 3 months for nickel. For europium, ICP-AES/ICP-MS analysis of the supernatants showed a strong retention by calcite whatever the initial concentration, contrary to nickel where the retention is depending on the initial concentration.

The second step of the work involved efforts to better understand the time-dependence of Eu and Ni sorption and respective mechanisms.
For each concentration of europium investigated by TRLFS, two species are identified and their fluorescence lifetime increases as the initial concentration decreases and time goes on, corresponding to a gradual loss of water molecules surrounding the europium. For higher concentrations, the species identified appear to correspond to a (co-) surface precipitate and possibly an inner-sphere surface complex with two water molecules retained in the hydration sphere. For lower concentrations, the longer lifetimes observed for one of the two species suggest the incorporation of europium in calcite [1,2].
Rutherford Backscattering Spectrometry (RBS) experiments have been carried out using an alpha particle millibeam at the 4MV Van de Graaff accelerator of IPNL and also on nuclear microprobe of CEA-Saclay. This technique is well adapted to discriminate sorption processes such as: (i) adsorption or co precipitation at the mineral surfaces or (ii) incorporation into the mineral structure (through diffusion for instance). The interpretation of the results shows different sorption behaviors 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, which confirms the hypothesis of the surface precipitate, and (ii) incorporation up to depths greater than 160 nm after 1 month of sorption. Complementary Scanning Electron Microscopy (SEM) observations of the mineral surfaces at low voltage have also been carried out, which confirmed the heterogeneities detected by RBS measurements.

[1] Fernandes, M. M.; Schmidt, M.; Stumpf, T.; Walther, C.; Bosbach, D.; Klenze, R.; Fanghänel, Th., Journal of Colloid and Interface Science (2008), 321(2), 323-331.
[2] Piriou, B; Fedoroff, M.; Jeanjean, J.; Bercis, L., Journal of Colloid and Interface Science (1997), 194, 440-447.

Keywords: Europium; Nickel; calcite; incorporation; diffusion; TRLFS; RBS

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    14th International Conference on the Chemistry and Migration Behaviour of Actinides and Fission Products in the Geosphere, 08.-13.09.2013, Brighton, United Kingdom

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