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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+, NH4 +, 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).

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The environmental fate of metal ions is influenced by their interactions with natural organic and inorganic ligands, which modify the ions' structure and charge and thus influence their interactions with mineral phases. We investigate the impact of ubiquitous sulfate on the retention of trivalent f-element cations (M(III) = Am, Eu, Y) by muscovite. We combine ex situ α spectrometry and in situ surface X-ray diffraction (i.e., crystal truncation rod and resonant anomalous X-ray reflectivity) to determine M(III) coverages and interfacial structures at the molecular level. M(III) cations adsorb as two distinct outer-sphere (OS) complexes (i.e., adsorbed and extended OS complexes) whose coverages vary with increasing sulfate concentration, [SO 42− ]. When [SO 4 2− ] ≤ 0.4 mM, M(III) coverages increase with increasing [SO 4 2− ] and exceed the amounts needed for surface charge compensation of muscovite by a factor of ∼3. This overcompensation is likely controlled by ion−ion correlations at the mineral/water interface rather than adsorption of MSO 4 + , which has a lower thermodynamic stability in the solutions and weaker electrostatic attraction to the mica surface than M 3+ . For higher [SO 42− ], MSO 4 + and M(SO 4 ) 2 − dominate solution speciation, leading to a strong decrease of the M(III) coverage due to their lower sorption affinity and weaker ion−ion correlations compared to M 3+ . These results indicate that interactions between electrolyte anions and metal ions at charged interfaces need to be explored for a more realistic prediction of contaminant transport in the environment.