Transformation of Uranyl Peroxide Studtite, [(UO₂)(O₂)(H₂O)₂](H₂O)₂, to Soluble Nanoscale Cage Clusters

Abstract

The dissolution behavior of uranyl peroxide studtite, [(UO2)­(O2)­(H2O)2]­(H2O)2, was examined under a wide range of alkaline aqueous environments with and without the addition of hydrogen peroxide. In the absence of added H2O2, studtite dissolved in aqueous solutions with a tetraethylammonium hydroxide to uranium molar ratio greater than 0.5, and the resulting species in solution characterized by Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) is the uranyl peroxide nanocluster U24, [(UO2)­(O2)­(OH)]24 24–. This is the first demonstration of the formation of uranyl peroxide nanoclusters from studtite in a solution lacking additional hydrogen peroxide. In similar systems containing added hydrogen peroxide (0.01 M – 1.0 M), studtite dissolved in solutions with a TEAOH to uranium ratio greater than 0.1, and the resulting uranyl peroxide species in solution was U28, [(UO2)­(O2)1.5]28 28–.

Stability of Studtite in Aqueous Suspension: Impact of HCO3– and Ionizing Radiation on the Dynamics of Dissolution

“…The postulated consumption of H 2 O 2 in the system would contribute to further dissolution of studtite from a strictly thermodynamical point of view. It should be noted that the formation of soluble uranyl–peroxide nanoclusters has only been observed in alkaline solutions (pH > 12) and in solutions containing very high concentrations of H 2 O 2 at slightly lower pH (>10). − Hence, it is very unlikely that cluster formation contributes to the dissolution of studtite under the conditions used in this work.…”

“…It is interesting to note that even though the solubility of studtite is quite low in general, there is a growing body of evidence showing that nanocluster formation can efficiently solubilize studtite under certain conditions. , Uranyl peroxide nanoclusters can also be formed from other solid uranium compounds than studtite …”

Meta-studtite stability in aqueous solutions. Impact of HCO3−, H2O2 and ionizing radiation on dissolution and speciation

“…Numerous studies have shown that uranyl forms soluble complexes with H 2 O 2 and CO 3 2− in alkaline solutions or nanoclusters at pH ≥ 12. [34][35][36][37][38][39][40] However, the species formed in the dissolution of studtite or meta-studtite in the presence of HCO 3 − (at pH ≈ 9)…”

Studtite Formation Assessed by Raman Spectroscopy and 18O Isotopic Labeling during the Oxidative Dissolution of a MOX Fuel

“…To date, the oxidative dissolution of UO 2 has been widely described in the literature, as has the nature of the uranium secondary phases that are liable to precipitate. , Thus, it is known that in waters exhibiting simple chemistry, intense irradiation fields (α or β/γ) lead to the formation of hydrogen peroxide by water radiolysis and to the precipitation of uranium peroxides. The formation of these peroxides has been the subject of numerous observations both on irradiated fuel − and on UO 2 model systems, , whether submitted to irradiation or not, with or without the addition of hydrogen peroxide to the solution. , The structure and properties of these peroxides (studtite, metastudtite) are well-known, − and they can have different shapes and morphologies (needles, colloids, and nanoclusters) depending on the precipitation conditions. , The resistance to irradiation of these compounds has also been the subject of recent developments. , …”