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Impact of background electrolyte composition on the interfacial formation of Th(IV) NPs on mica (001)

Neumann, J.; Qiu, C.; Eng, P.; Stumpf, T.; Schmidt, M.


A mechanistic understanding of the formation of actinide nanoparticles (NPs) and its impact on the mobility of radionuclides in the environment is important for a reliable risk assessment of repositories for radioactive waste. Previous studies using surface x-ray diffraction (SXD) reported an unexpected impact of electrolyte composition on the sorption of Th(IV) on the muscovite (001) basal plane. Th uptake decreased following an unexpected trend: LiClO4 > KClO4 > NaClO4. A significantly higher coverage than needed for surface charge compensation (0.25 Th/AUC, AUC = 46.72 Ų, area of mica (001) unit cell) was observed for LiClO4 (4.9 Th/AUC), suggesting the formation of Th-NPs [1]. It remained unclear, if the electrolyte affects a reaction at the mineral surface or in solution.
We combined SXD and in situ AFM to address this question. At low [Th] (0.1 mM), the investigated electrolytes include LiCl and KCl, in comparison with the reported Th uptakes for the respective perchlorate electrolytes, and the series is extended to NH4Cl and CsCl. The results are compared to reported value for NaCl [2]. The interfacial structures show an extremely broad distribution of Th electron density up to 50 Å from the surface for LiCl and KCl. A decrease of Th uptake within the alkali series is found (Figure 1). A strong linear correlation (R2 = 0.9962) between Th uptake and ionic radius of the alkali metal ion is found, indicating that sorption competition between Th4+ and the electrolyte cation is the origin of the observed effect. The value for NaCl is a clear outlier in this series, showing a much lower uptake of Th than expected according to the trend.
Perfect agreement between the number of formed particles per area, obtained by in situ AFM, and Th uptake, observed by SXD, is found. Particles show a vertical size of ~1 – 2 nm and lateral dimensions of ~10 – 20 nm, indicating that retention occurs by the formation of NPs at the mineral-solution interface (heterogeneous nucleation), which is strongly influenced by the electrolyte.
Additionally, SXD was performed at higher [Th] = 3 mM, where the formation of Th oligomers in solution is expected. Under these conditions, LiCl (2.0 Th/AUC), NaCl (1.4 Th/AUC), and KCl (1.7 Th/AUC) show similar Th uptake, indicating a much smaller impact of electrolyte composition. The obtained interfacial structures are dominated by a high Th loading at a distinct distance (~ 6.5 Å) from the muscovite surface. Therefore, the main retention mechanism at high [Th] is suggested to be the (electrolyte-independent) formation of Th oligomers in solution and their subsequent sorption on the mineral surface.

[1] - M. Schmidt et al., Geochim. Cosmochim. Acta. 165, 280–293 (2015).
[2] - M. Schmidt et al., Geochim. Cosmochim. Acta. 88, 66–76 (2012).

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