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Spectroscopic and modeling study of the sorption of Ln³⁺ (Eu) and An³⁺ (Am, Cm) on Ca-feldspars
Deep geological repositories are considered as a safe disposal strategy for radioactive waste due their ability to isolate toxic components from the biosphere over hundreds of thousands of years. Minor actinides and Pu dominate the radiotoxicity of spent nuclear fuel over these long time scales. Due to the expected reducing conditions in the underground repository, the trivalent oxidation state is dominant for Am and Cm, and will also be relevant for Pu. For investigations of the mobility of the trivalent actinides Am(III) and Cm(III), the less toxic trivalent rare earth elements, in particular Eu(III), are commonly used.
In Germany and many other countries, crystalline rock is being considered as a possible host rock. Therefore, there is a need for understanding the sorption behavior of radionuclides on this material. Crystalline rock (e.g. granite), consists mainly of quartz, feldspars, and mica. Recently, the retention of trivalent actinides by K-feldspar was investigated from a thermodynamic and structural point of view. Here, we extend this study towards Ca-feldspars (plagioclases), which may show a different sorption behavior due to their different elemental composition, crystal structure, and surface charge behavior.
DESCRIPTION OF THE WORK
Synthetic Ca-feldspar and natural plagioclases of different Ca amounts were used for zeta potential measurements and batch sorption experiments under different geochemical conditions ([M³⁺] = 52 nM – 10 μM; solid-liquid ratio = 1 – 3 g/L, I = 0,1 M NaCl, pH = 3 – 9) to quantify the uptake of Am(III) and Eu(III). For analysis of the sorption structure of trivalent f-elements on the molecular level, time-resolved laser-induced spectroscopy (TRLFS) using Cm(III) as a luminescent probe was carried out on synthetic Ca-feldspar. The obtained data were used to develop a surface complexation model (SCM) and to derive surface complexation parameters for the spectroscopically identified surface complexes.
RESULTS AND DISCUSSION
Zeta potential investigations of all Ca-feldspars show a decrease of the potential for pH = 2 – 4 due to surface site deprotonation. In contrast to the previously reported trend for K-feldspar, the zeta potential increases for pH = 4 – 7, with a stronger increase with higher Ca²⁺ concentration in the crystal lattice of the investigated plagioclases, even reaching positive values in the case of the synthetic Ca-feldspar. This effect can be traced to dissolved Al³⁺: Due to differences in solubility, Al³⁺ concentration in solution increases with increasing Ca²⁺ in the crystal lattice. Experiments on K-feldspar with added Al³⁺ reveal a connection between its concentration and the increase of the zeta potential.
All observed Ca-feldspars show a strong sorption uptake of trivalent f-elements for pH > 6. K- and Ca-feldspars seem to have a similar sorption behavior for low [M³⁺]. In contrast, Ca-feldspar has a slightly stronger sorption affinity when the metal concentrations is increased. This leads to a steeper sorption edge with increasing Ca²⁺ concentration in the crystal lattice of the mineral.
Spectroscopic studies with Cm(III) on synthetic Ca-feldspar reveal three sorption complexes: one inner sphere complex (IS) and its two hydrolysis forms, which have the same band positions as previously determined for K-feldspar. Therefore, it can be concluded that the structure of the formed IS complexes is independent on the feldspar type. Differences are only observed for the quantitative contributions of the surface complexes. In particular, hydrolysis of the IS complex is stronger in the case of the Ca-feldspar.
Batch sorption data and the information about spectroscopically identified surface complexes were then combined to develop a SCM for Ca-feldspar that describes the experimental data. The formation constants of the surface complexes were determined to be −8.37; −10.81, and −16.35, respectively and are very similar to those of the K-feldspar.
From the applied multi-method approach, we conclude that the sorption of trivalent f-elements on K- and Ca-feldspar is most likely comparable for relevant, natural conditions. Therefore, it may be possible not to distinguish between the two minerals in reactive transport simulations, which will reduce calculation resources needed for a reliable risk assessment of repositories for radioactive waste.
 J. Neumann et al., “A comprehensive study of the sorption mechanism and thermodynamics of f-element sorption onto K-feldspar,” Journal of Colloid Interface Science, vol. 591, pp. 490–499 (2021)
 Neumann and Lessing et al., “Structural and modeling study of the retention of trivalent f-elements (Am, Cm, Eu) by natural and synthetic Ca-feldspars”, in preparation.
Keywords: Sorption; Trivalent metal ions; Actinides; Ca-feldspar; Surface complexation model; TRLFS; Zeta potential
TransRet2020, 12.-13.10.2021, Karlsruhe, Deutschland