SAXS study of the formation and structure of polynuclear thorium(IV) colloids and thorium dioxide nanoparticles

Stable actinide colloids and nanoparticles are of interest because of their potential to affect the transportation of radionuclides in the near-field of a nuclear waste repository. At high concentrations, thorium(IV) can precipitate to form intrinsic colloids. In the present study, polynuclear thorium colloids and thorium dioxide crystallites, formed by the condensation of hydrolyzed Th⁴⁺ solutions (3 mM; initial pH 5.5) aged for up to 18 months, were studied using small-angle X-ray scattering. Scattering profiles were fitted using a unified Guinier/power-law model (Beaucage model) to extract the radii of gyration and Porod exponents. Analysis of the scattering profiles from a dispersion aged for 5 months indicated that both polymer coils and more compacted structures (radius of gyration R_g ≃ 10 nm) were present, which translated in the Kratky plots as a plateau and a peak maximum, respectively. After 18 months, the SAXS data were consistent with the presence of agglomerates of ThO₂ particles suspended in aqueous solution (pH 3.2; [Th] = 1.45 mM). The measured radius of gyration (R_g) of the agglomerates was 5.8 nm, whereas the radius of the ThO₂ particles was 2.5 nm.