A spectroscopic study of trivalent cation (Cm3+ and Eu3+) sorption on monoclinic zirconia (ZrO2)

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 modeled using the Diffuse Double Layer (DDL) model. The ZrO2 surface charge measurements yielded a pHIEP of 6 which was influenced by the presence of inorganic and organic carbon species. The pHdependent 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.