Effect of Background Electrolyte Composition on the Interfacial Formation of Th(IV) Nanoparticles on the Muscovite (001) Basal Plane

Understanding the impact of actinide nanoparticle (NP) formation is important to assess radionuclide mobility in the environment. We combined surface X-ray diffraction (SXRD) and in situ AFM to investigate the previously reported unusual electrolyte effects on Th uptake on mica. At low [Th] (0.1 mM), interfacial structures show a broad Th electron density (∼50 Å). A linear decrease of Th uptake with decreasing hydration enthalpy of the electrolyte cation (Li +, K +, NH 4 +, and Cs + ) indicates a competitive effect between Th and the electrolyte cation. Na + is a clear outlier from this trend. In situ AFM imaging confirms the results. Particles show a vertical size of ∼1–2 nm and larger lateral dimensions of ∼10–20 nm, which is typical for particles formed at interfaces (heterogeneous nucleation). At high [Th] = 1 and 3 mM, all investigated electrolytes (ACl, A = Li +, Na +, K + ) show similar Th uptake, indicating a much smaller impact of electrolyte composition. The interfacial structures are dominated by a high Th loading at a distinct distance (∼6.5 Å) from the surface. Therefore, the main retention mechanism at high [Th] is suggested to be the sorption of Th NPs aggregated from Th oligomers present in solution (homogeneous nucleation).