Cooperative effects of adsorption, reduction, and polymerization observed for hexavalent actinides on the muscovite basal plane

Reliable long-term predictions about the safety of a potential nuclear waste repository must be based on a sound, molecular-level comprehension of the geochemical behavior of the radionuclides. Especially, their reactivity at the water/mineral interface will control their mobility and thus hazard potential.1 Understanding the geochemical behavior of plutonium has been particularly challenging, due to its multitude of accessible oxidation states and its capability to form nanoparticles or eigencolloids. Despite the generally accepted importance of Pu(IV)-nanoparticles for Pu’s chemical4-6 and environmental behavior,2, 3, 7, 8 the mechanism of their formation is still the subject of ongoing research.9 Several previous studies have identified Pu(IV) nanoparticles to be the final state of adsorbed Pu, starting from both higher10-12 and lower oxidation states,13 on both redox active10, 12 and redox inactive substrates.11, 13 In our own previous work we suggested a mechanism, in which the enhanced concentration of Pu(III) at the interface, in combination with the presence of minor quantities of Pu(IV) in equilibrium, drives the formation of these nanoparticles in an effectively surface-catalyzed reaction.13 This mechanism would be independent of Pu’s initial oxidation state assuming there is adequate amounts of Pu(IV) present in equilibrium. In order to be able to understand these processes analytical techniques that allow selectively probing the mineral/water interface and elucidating processes at the interface under in situ conditions are required. X-ray reflectivity techniques, such as crystal truncation rod (CTR) measurements and resonant anomalous x-ray reflectivity (RAXR) have proven to be valuable tools for geochemical studies concerning reactions in the interfacial regime14, especially for complex reactions of the actinides.13, 15
To further elucidate the interfacial reactivity of Pu in its various oxidation states, and to test the viability of the mechanisms discussed above for Pu(III), we study the reactivity of hexavalent PuO22+ at the muscovite (001) basal plane and compare it to the behavior of ostensibly analogous UO22+ ([Pu] = 0.1 mmol L-1, [U] = 1 mmol L-1, pH = 3.2 ± 0.2, I(NaCl) = 0.1 mol L-1) using resonant anomalous X-ray reflectivity (RAXR) and crystal truncation rods (CTR), as well as grazing-incidence X-ray adsorption near-edge structure (GI-XANES) spectroscopy and alpha spectrometry. The RAXR data indicate that adsorbed Pu has a broad distribution that extends up to 60 Å from the surface. Independent quantification of the adsorption of Pu by alpha spectrometry finds a coverage of 8.3 Pu/AUC (where AUC = 46.72 Ų is the surface unit cell area). The observed broad structure and large coverage cannot be explained by ionic adsorption of PuO22+, indicating adsorption of Pu(IV) oxo nanoparticles. GI-XANES confirms that most Pu at the interface was tetravalent. These observations corroborate a redox-partner independent mechanism for the interfacial formation of Pu(IV) oxo nanoparticles put forward previously. Uranium exhibits clearly different behavior. No discernible RAXR signal was detected, indicating no adsorption of UO22+. GI-XANES and alpha spectrometry also showed very weak signal, in agreement with the RAXR findings, and in the case of GI-XANES indicating predominantly hexavalent U. Our results reveal significant differences between Pu and U in terms of mineral uptake, greatly impacting their geochemical mobility and potentially useful for predicting the fate of these contaminants’ in the aqueous environment.

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