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

Hypothesis:

Zirconia (ZrO₂) formed by corrosion of zircalloy, can immobilize radioactive contaminants (e.g. actinides) in repositories for spent nuclear fuel (SNF). The presence of organic and inorganic carbon at the highly reactive ZrO₂ surface impacts the adsorption of these metal ions and their surface speciation.
Experiments:
Sorption of Eu³⁺ and Cm³⁺ on zirconia was studied in batch-sorption experiments, and via laser spectroscopy (TRLFS). 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 pH-dependent Cm³⁺ surface speciation was studied with TRLFS. The spectroscopic sorption data was modeled using the Diffuse Double Layer (DDL) model.
Findings:
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 sorption 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. The sorption data was well described by our DDL model.