Sorption competition of trivalent metals on corundum (α-Al2O3) studied on the macro- and microscopic scale


Sorption competition of trivalent metals on corundum (α-Al2O3) studied on the macro- and microscopic scale

Virtanen, S.; Eibl, M.; Meriläinen, S.; Rossberg, A.; Lehto, J.; Rabung, T.; Huittinen, N.

Sorption of trivalent actinides and lanthanides onto the surface of geological materials relevant for nuclear waste disposal is a topic that has been widely studied in recent years. However, the sorption properties of metals are often investigated by studying the sorption behaviour of a single metal at a time, thus, these experiments do not account for potential effects of sorption competition in the presence of multiple dissolved elements or compounds. Bradbury and Baeyens (2005) performed extensive investigations of the sorption competition between various metal cations on the clay mineral montmorillonite. By investigating the competition of metals with similar and dissimilar chemical behaviour (e.g. tendency to hydrolysis and valence state), the authors concluded that metal cations with dissimilar chemical properties do not affect the uptake of one another by the clay mineral, whereas metals with similar chemistries do. Thus, if the data obtained in single metal sorption experiments are used in the safety assessment of nuclear disposal, careful considerations of the chemical environment in the near- or far-field of nuclear waste repository is needed to avoid the possible overestimation of radionuclide sorption.
In this study, we have combined batch sorption and spectroscopic experiments that were performed with Eu(III), Cm(III) and Am(III) in in the absence and presence of Y(III) as competing cation. The objective was to investigate how the sorption behaviour of trivalent actinides and lanthanides is affected by the presence of another trivalent metal. Following the findings of Bradbury and Baeyens (2005) our hypothesis is that the addition of higher concentrations of trivalent Y(III) together with a chemically similar trivalent metal, Eu(III), Cm(III) or Am(III), would affect the sorption behaviour of that metal.
Batch sorption experiments were performed with Eu(III) at different pH (pH-edges) and concentrations (isotherms). The competing metal Y(III) was added before Eu(III) to the mineral suspension in concentrations ranging from 1×10-6 M to 1×10-4 M. In the Eu(III) pH-edge experiments, the sorption of 1×10-5 M Eu(III) was investigated on 0.5 g/l corundum at varying pH, with and without Y(III). In the Eu(III) isotherm experiments, the initial Eu(III) concentration was varied between 1×10-9 M – 1×10-4 M and Y(III) was used in the competing isotherm samples at a constant pH of 7. Batch experiments showed that the addition of Y(III) did decrease the sorption of Eu(III) (Figure 1) on a macroscopic scale. However, as the main emphasis of this study was the possible changes happening at the molecular level as a results of sorption competition, spectroscopic methods were also employed. Time-resolved laser fluorescence spectroscopy (TRLFS) enables the investigations of Cm(III) sorption speciation directly on the mineral surface. We investigated the changes in the speciation of 1×10-7 M Cm(III) in 0.5 g/l corundum suspensions at varying pH under non-competing and competing conditions using 1×10-4 M Y(III). The results indicate changes in the Cm(III) sorption species distribution, thus, confirming our findings in the batch sorption experiments showing that 1×10-4 M Y(III) suppresses Cm sorption complex formation on the mineral surface depending on the solution pH (Figure 2). Cm(III) luminescence spectra of only Cm(III) and of Cm(III) together with Y(III), show that the fraction of aqueous Cm species is substantially greater with high concentrations of Y(III) present. Only when the pH is increased above 7, the first Cm sorption species appears, resulting in a shift of the observed emission peak maximum. X-ray absorption spectroscopy (XAS) was applied to identify the formed trivalent actinide sorption complexes. We investigated the sorption of 6×10-6 M or 2×10-5 M Am(III) on the corundum surface at pH 8.5 in the absence and presence of 2×10-5 or 2×10-4 M Y(III). The treatment of the XAS-data is still ongoing and results will be discussed more closely in the conference presentation.

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