Formation of stabilized vaterite nanoparticles via the introduction of uranyl into groundwater†

In the context of deep geological disposal of nuclear wastes, this work reports the formation of vaterite colloids in aqueous mixtures of Beishan groundwater and uranyl nitrate. The thermodynamic equilibrium conditions of Beishan groundwater were altered by the presence of ternary uranyl solution species, e.g., Ca₂UO₂(CO₃)₃(aq) and CaUO₂(CO₃)₃²⁻. This led to the formation of spheroid-like vaterite colloids with a primary size of 3–4 nm and a secondary size of tens of nanometers, evidenced by synchrotron small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Stopped-flow SAXS measurements revealed that the formation and aggregation of vaterite nanoparticles occurred in less than 100 seconds. Vaterite colloids remained stable with respect to transformation to other stable polymorphs of CaCO₃ in groundwater over the course of one year, due to the synergistic effects of UO₂²⁺, Mg²⁺, and SO₄²⁻. The presence of stable nano-sized vaterite nanoparticles with negative surface charges may increase the potential migration risks associated with U(VI). These results contribute to predicting and understanding the geochemical fate of radionuclides, as well as safety assessment of a nuclear waste repository.