Redox dependent interfacial reactivity of hexavalent radionuclides

A solid comprehension of the geochemical behavior of radionuclides on a molecular level is essential to make reliable long-term predictions about the safety of a nuclear waste repository. The mobility of radionuclides in the environment and thus their hazard potential will also be controlled by the reactivity at the water/mineral interface. In order to understand these processes analytical methods shall ideally be both surface specific and sensitive. X-ray reflectivity techniques, particularly resonant anomalous X-ray reflectivity (RAXR) and crystal truncation rod (CTR) measurements have proved to be a successful combination to investigate geochemical interfacial regimes (Fenter 2002).
Plutonium is one of the most important radionuclides in term of nuclear waste disposal due to its long half-life period and high radiotoxicity. That’s why it has been subject of different studies over the last decades. While these studies could show an enhancement of the mobility of plutonium in the presence of colloidal matter, the formation of Pu(IV)-nanoparticles is still content of ongoing research (Walther & Deneke 2013). Recently, Schmidt et al. suggested a surface-catalyzed formation due to an enhanced concentration of Pu(III) at the surface in equilibrium with a small amount of Pu(IV). Part of the current study was to proof the viability of this mechanism, but also to investigate the interfacial reactivity of Pu’s various oxidation states.
The interaction of UO₂ ²⁺ and PuO₂ ²⁺ with muscovite mica and the effect on the actinides’ different redox properties were investigated using a combination of surface X ray diffraction, alpha spectrometry and grazing-incidence X-ray adsorption near-edge structure (GI-XANES) spectroscopy. Although, U(VI) often is used as a homologue for Pu(VI), this study show a completely different behavior of Pu(VI) and U(VI). Starting with a Pu(VI) solution, Pu(IV)-nanoparticles were formed and adsorbed on the mineral surface. The suggested formation mechanism is similar to that of Pu(III). No such adsorption or nanoparticle formation was observed for U. Our results reveal major differences between Pu and U concerning redox and adsorption behavior, influencing their mobility in the environment. Regarding the prediction of the fate of these contaminants’ in aqueous systems their different interfacial behavior is of importance. This in turn significantly effects the quality of predictions of the allocation of these contaminants in aqueous systems.