Pseudo-сolloidal species of actinides in contaminated aquifers. Insights from experimental and modeling approaches.

The release of actinides into the environment poses a significant risk of their uncontrolled migration in various physical and chemical forms. A key migration mechanism for all actinides is their pseudocolloidal transport, which occurs through sorption onto suspended particles of soils, clays, iron oxides and hydroxides, and other colloids surface in groundwater. This phenomenon facilitates their potential transport over considerable distances, creating additional environmental risks. The behavior of these pseudocolloidal particles is currently not well understood and is highly dependent on a complex set of site-specific physicochemical conditions, such as pH, Eh (redox potential), ionic strength, and the composition of the aqueous solution. In this work, for the first time, using the example of three aquifers with complex pollution, in zones with different redox conditions, an analysis of the distribution of actinides in fractions of different dispersion was carried out, the main phases capable of providing pseudocolloidal transport of actinides were determined. Thermodynamic calculation on the basis of physicochemical parameters of groundwater samples allowed to propose an approach to assessing the risk of colloidal and pseudo-colloidal phase formation based on saturation index values DOC content and PHIEP values. Verification of colloidal particle composition analysis on filters allowed to detect both large UO2 aggregates and morphologically diverse pseudo-colloidal particles (clay, Fe, Ce, Zn, Th, and Au-Ag-Cu). The transition from oxidizing to reducing conditions may play a significant role in the formation of colloidal and pseudo-colloidal particles. The selection of deep disposal sites of liquid radioactive waste on the territory of the Siberian Chemical Plant, operated since the 60s of the last century, as objects of study allowed us to obtain unique material showing the importance of taking into account in pseudocolloidal actinide transport not only clay and iron particles, but also noble metals, uranium, thorium, zinc, chromium and other scattered elements.