RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

This study investigates the retention of trivalent actinides (Cm³⁺) and their lanthanide analogues (Eu³⁺) on monoclinic zirconia (ZrO₂), a solid phase known to form on the zircaloy cladding material surrounding spent nuclear fuel (SNF) rods. Two zirconia solids with varying carbon content were utilized. The influence of carbon impurities on the ZrO₂ surface charge was investigated via zeta-potential measurements. Batch data was collected for various Eu³⁺ concentrations, while the Cm³⁺ surface speciation was studied on the molecular level with laser spectroscopy (TRLFS). The spectroscopic sorption data was modelled using the Diffuse Double Layer (DDL) model. The ZrO₂ surface charge measurements yielded a pH_IEP of 6 which was influenced by the presence of inorganic and organic carbon species. The pH-dependent uptake of Eu³⁺ showed a maximum sorption above pH 5.5, with no impact of the carbon concentration. The speciation of the trivalent metal, however, was different in the presence of intrinsic organic carbon in the sample, resulting in the formation of an organic Cm³⁺-complex on the surface. This ternary complex was absent on the ZrO₂ material with low carbon content. Here, the surface speciation was dominated by Cm³⁺ and Cm³⁺-hydrolysis complexes which could be well described by our DDL model.