Assessing the impact of environmental conditions on ²⁴¹ Am sorption on natural argillaceous rocks and their sorbing constituents

Sorption studies of radionuclides on clay minerals and argillaceous rocks are typically conducted in simplified, inert background electrolytes. However, these conditions do not reflect the complexity of natural systems, where pore water composition can significantly influence radionuclide behaviour. This study investigates the sorption of trivalent americium Am(III) onto possible host rocks for the geological disposal of radioactive waste in Belgium, including Boom Clay, Ypresian Clay and two metapelites, as well as their key mineral constituents (illite, montmorillonite, chlorite, muscovite). Experiments were conducted under three conditions: reference water (RW; 0.015 M NaClO 4 ), synthetic freshwater (FW; 0.015 M NaHCO 3 ) and synthetic saline water (SW). Am(III) showed strong sorption across all minerals in RW (log R d ∼3.9–5.8). Carbonate ligands in FW had minimal or even positive effects on sorption, suggesting ternary surface complexation. Saline conditions modestly reduced sorption (∼1 log unit). The presence of dissolved organic matter (DOM) in RW significantly decreased sorption (∼2 log units), though this effect was less pronounced in FW and neutralized in SW. In Boom Clay sorption was even increased in SW compared to FW, likely due to DOM–radionuclide complexes aggregating. These findings highlight the importance of more realistic geochemical conditions in predicting radionuclide behaviour in geological disposal systems.