Effect of background electrolyte composition on the formation of Th(IV) nanoparticles on the muscovite (001) basal plane

Reliable long-term predictions regarding the safety of a nuclear waste disposal facility must be based on a sound understanding of the fundamental processes controlling radionuclide mobility in a subsurface environment. In particular, reactions at the water/mineral interface must be characterized on the molecular level.[1] Several actinides (An) show a tendency to form An-oxo-nanoparticles[2], which may be enhanced in the presence of mineral surfaces and even drive redox reactions.[3-6] As these reaction may, both, enhance and reduce the mobility of the actinides, it is of utmost importance to understand their mechanism and which parameters control the nanoparticle formation in environmental systems.
Recently, we have reported an unusual variability in the reactivity of ThIV on the basal plane of muscovite mica dependent on the composition of the background electrolyte.[7] In this study, based on surface X-ray diffraction [SXD; crystal truncation rod diffraction (CTR) and resonant anomalous X-ray reflectivity (RAXR)] and alpha spectrometry, it was observed that ThIV sorption from NaClO4 solution was significantly lower [< d.l. (~0.04 ThIV per area of the muscovite unit cell AUC)] than from NaCl solution (θNaCl = 0.4 Th/AUC) under otherwise identical conditions.[8] The study also revealed that the adsorbed quantity of ThIV was significantly higher in LiClO4 medium (θLiClO4 = 4.9 Th/AUC), than in NaClO4 with KClO4 intermediate between Li and Na (θKClO4 ~ 0.1 Th/AUC). In the case of LiClO4 it could be shown by RAXR, that sorption occurs in the form of small particles a few nm in size.
Here, we present a study using SXD in combination with alpha spectrometry and atomic force microscopy (AFM) aiming to identify the basis of the previously observed, unexpected effects. To probe whether anion and cation effect occur independently, ThIV sorption was studied in the presence of LiCl and KCl ([Th] = 0.1 mM, pH = 3.3, I = 0.1 M). ThIV uptake is strongest in the presence of LiCl (θLiCl = 8.8 Th/AUC), while sorption in the presence of KCl is weaker (θKCl = 3.6 Th/AUC) but still exceeds the surface occupancy previously found in NaCl media.[8] For all cations ThIV sorption is stronger when Cl- is the counterion compared to ClO4-, confirming that the cation effect is indeed independent of the background electrolyte’s anion. The influence of aqueous speciation on the sorption processes was determined using electro-spray-ionization time-of-flight mass spectrometry (ESI-TOF-MS), which finds a speciation dominated by the ThIV aquo ion in all media, indicating that any electrolyte effects must occur at the water/mineral interface. We investigated the influence of the presence of oligomers on the sorption process, by repeating experiments at higher initial [Th] = 3.0 mM. As expected ThIV sorption is significantly increased. ThIV adsorbs at a preferential height of ~6.5 Å, which can be identified as the preferred size of Th-nanochains on the mica basal plane by AFM (Fig. 1). Uptake from LiCl media is still larger than from NaCl, but only by ~32% compared to 2100% at the lower ThIV concentration. This suggests that the electrolyte cation influences the formation or aggregation of ThIV oligomers at the interface, and its influence is diminished when these are initially present.

Fig. 1. Total electron density and ThIV electron density as a function of distance from the mineral surface determined by SXD upon sorption from NaCl, KCl, and LiCl, respectively. Upper curves (grey, light blue, dark red) are total electron densities determined by CTR, lower curves (black, dark blue, light red) are ThIV electron density distributions from RAXR.

References
[1] H. Geckeis, et al., Chem. Rev., 113, 1016 (2013).
[2] K. E. Knope, et al., Chem. Rev., 113, 944 (2012).
[3] S. Hellebrandt, et al., Langmuir, 32, 10473 (2016).
[4] A. E. Hixon, et al., Environmental Science: Processes & Impacts, 20, 1306 (2018).
[5] M. Schmidt, et al., Env. Sci. Tech., 47, 14178 (2013).
[6] C. Walther, et al., Chem. Rev., 113, 995 (2013).
[7] M. Schmidt, et al., Geochim. Cosmochim. Acta, 165, 280 (2015).
[8] M. Schmidt, et al., Geochim. Cosmochim. Acta, 88, 66 (2012).