Rehydration of metastudtite in the alteration kinetics of α– and β–U3O8 in dilute aqueous solutions of hydrogen peroxide

The formation of alteration phases on uranium ore concentrates and used nuclear fuels under oxidizing conditions is key to understanding the potential mobility of radionuclides in the environment and designing optimal storage conditions of materials. However, the time-dependent distribution of alteration phases on α– and β–U₃O₈ under oxidizing conditions has yet to be explored. Here, crystalline powders of α– and β–U₃O₈ were submerged in aqueous solutions of hydrogen peroxide (1.6 × 10⁻¹ to 5.4 × 10⁻⁵ M) with aliquots of solution and solid removed for analysis at 1, 8, 15, 22, 29, 36, 46, 58, 71, and 83 days. Within one day there is significant alteration of U₃O₈ to the uranyl peroxide metastudtite, [(UO₂)(O₂)(H₂O)₂], that is replaced by studtite, [(UO₂)(O₂)(H₂O)₂]·2H₂O, within a week regardless of the polymorph of U₃O₈ or the initial concentration of H₂O₂ in solution, as determined by partial least squares regression (PLSR) of Raman spectra collected from the solids. A dissolution/reprecipitation mechanism is proposed for both the alteration of U₃O₈ to metastudtite and the subsequent alteration of both U₃O₈ and metastudtite to studtite. The two polymorphs of U₃O₈ exhibit similar rates and extents of alteration over time. The rehydration of metastudtite to studtite has not been previously reported and highlights the need for future work to determine the mechanism by which metastudtite is converted to studtite and what other conditions facilitate this rehydration.