Ozone-Facilitated Formation of Uranyl Peroxide in Humid Conditions

Abstract

Metaschoepite, [(UO 2 ) 8 O 2 (OH) 12 ](H 2 O) 10 , maintained in a high relative humidity (RH) environment with air initially transformed into an intermediate phase that subsequently was replaced by the peroxide phase studtite, [(UO 2over the course of 42 days, as observed using Raman and infrared spectroscopy and powder X-ray diffraction. Addition of atmospheric ozone vastly increased the rate and extent of the transformation to studtite but only in a high-RH atmosphere. Owing to its strong affinity for peroxide, uranyl reacted with hydrogen peroxide as it formed and precipitated stable studtite. In this work, we provide a previously unidentified source of hydrogen peroxide and make a case for the re-examination of storage systems where the consequences of atmospheric ozone are not considered.

Investigation of Radiation Effects in the Uranyl Mineral Metaschoepite

“…While this may be attributed to a red shift of the lowest energy uranyl symmetric stretching mode in metaschoepite at 828 cm –1 , previous studies identified the 820 cm –1 peak as a uranyl symmetric stretching mode of studtite . Hydration studies absent added ionizing radiation demonstrated metaschoepite transformation to studtite at high relative humidity that was enhanced by atmospheric ozone . It is plausible that studtite formed on the surface of the bulk irradiated metaschoepite.…”

“…Studtite forms on the surface of metaschoepite at longer hydration times in high RH conditions; its formation after only 1 day indicates that degradation to UO 3 (whether by thermal decomposition or irradiation) accelerates formation of studtite. Peroxide formation as a result of radiolysis was considered, but the lack of studtite formation in the 75% RH bulk irradiated samples indicates that studtite formation in these experiments depends largely on the post-storage atmospheric conditions.…”

“…28 Hydration studies absent added ionizing radiation demonstrated metaschoepite transformation to studtite at high relative humidity that was enhanced by atmospheric ozone. 29 It is plausible that studtite formed on the surface of the bulk irradiated metaschoepite. The presence of more intense 820 cm −1 signals in spectra of material stored at 85% RH as compared to 75% RH indicates water is important in the transformation, as found in previous studies on metaschoepite and hydrated uranyl fluoride.…”

“…These phases are unstable in high relative humidity environments and promote the formation of new phases including studtite. Studies focused on smaller particles showed the extent of transformation to studtite depends on the size of the aggregate, likely due to increased susceptibility toward atmospheric reactions including the formation of hydrogen peroxide from ozone and water vapor …”

“…Studies focused on smaller particles showed the extent of transformation to studtite depends on the size of the aggregate, likely due to increased susceptibility toward atmospheric reactions including the formation of hydrogen peroxide from ozone and water vapor. 29 Degradation of metaschoepite may serve as a platform for understanding how other uranyl phases react under similar conditions. More work is required to understand the influence of radiolysis during irradiation and poststorage conditions.…”