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Differential sorption behavior of U(VI) and Pu(VI) dependent on their redox chemistry
Hellebrandt, S.; Knope, K. E.; Lee, S. S.; Lussier, A. J.; Stubbs, J. E.; Eng, P. J.; Soderholm, L.; Fenter, P.; Schmidt, M.;
In a recent paper (Schmidt et al. 2013) our group suggested the surface-catalyzed formation of Pu(IV)-oxo-nanoparticles due to an enhanced concentration of Pu(III) at the surface of muscovite mica in equilibrium with a small amount of Pu(IV). The study took three possible pathways for the reaction into account: (1) Pu(III) adsorbs on the muscovite surface, where the oxidation to Pu(IV) takes place. (2) The oxidation of Pu(III) to Pu(IV) happens in solution, whereupon Pu(IV) adsorbs on the surface. In both cases (1) and (2) the increased Pu(IV) concentration leads to oligomerization and afterwards the formation of Pu(IV)-oxo-nanoparticles. Another pathway (3) is the formation of Pu(IV)-oxo-nanoparticles in solution which subsequently adsorb at the mica surface. This pathway was considered less likely, due to a clear enhancement of the reaction in the presence of the interface, which cannot be explained by this process. Motivation of the current study was to test the viability of these mechanisms, but also to investigate the interfacial reactivity of Pu’s various oxidation states.
The mobility of radionuclides in the environment and thus their hazard potential will be controlled by their reactivity at the water/mineral interface. Thus, it is necessary to understand how Pu behave in contact with mineral surfaces on a molecular level, to make reliable long-term predictions about the safety of a nuclear waste repository. 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 proven 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 (Kersting et al., 1999 and Novikov et al., 2006), the formation of Pu(IV)-nanoparticles is still content of ongoing research (e.g. Kersting 2013, Walther & Deneke 2013).
In the current study a comparison of the interaction of UO22+ and PuO22+ ([Pu] = 0.1 mmol L-1, [U] = 1 mM mmol L-1, I(NaCl) = 0.1 mol L-1, pH 3.2 ± 0.2) with muscovite mica and the effect of 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. RAXR data of a Pu(VI) solution in contact with muscovite show a broad Pu distribution, which cannot be explained by simple ionic adsorption of PuO22+, indicating the formation of Pu(IV)-oxo-nanoparticles. Alpha spectrometry confirms these findings; the occupancy was determined to be ~ 8.3 Pu/AUC (where AUC = 46.72 Å2 is the unit cell area). This means the mechanism of the redox partner independent formation of Pu(IV)-nanoparticles previously observed for Pu(III) can be confirmed for Pu(VI) as well.
UO22+ shows clearly different performance. No RAXR signal was observable, indicating no adsorption of UO22+. The persistence of the hexavalent oxidation state of U was confirmed by GI-XANES spectroscopy. Furthermore, Alpha spectrometry and GI-XANES spectroscopy showed very weak signals or no signal at all, in agreement with the RAXR findings. Assuming that the sorption behavior of UO22+ and PuO22+ is equivalent excluding their redox chemistry, no Pu(VI) should be present at the surface. Therefore, the previously proposed mechanism (1) cannot contribute significantly to the observed formation of Pu(IV)-oxo-nanoparticles from Pu(VI) solution. To distinguish mechanisms (2) and (3) UV/Vis spectroscopy was performed similar to our previous study. No Pu(IV) was detectable, even if measured over a longer periode of time than available for the X-ray reflectivity experiment. Hence mechanism (3) also appears to be implausible. Apparently, the observed formation of Pu(IV)-nanoparticles follows mechanism (2). Because of the redox properties of Pu, an equilibrium of Pu(IV), Pu(V) and Pu(VI) will be present in solution. Thus available Pu(IV) will adsorb on the muscovite (001) basal plane. The tetravalent oxidation state of interfacial Pu was confirmed by GI XANES spectroscopy. Since a threshold is reached polymerization occurs as a consequence of hydrolysis, through an olation (Knope et al., 2015) or oxolation (Knope & Soderholm, 2013) mechanism.
Keywords: Plutonium, Pu, Uranium, U, Redox, Sorption, Muscovite, Nanoparticles
  • Lecture (Conference)
    Plutonium Futures - The Science 2016, 18.-22.09.2016, Baden-Baden, Deutschland

Publ.-Id: 24932 - Permalink