| Uranium(VI) solid phases occur in the environment following alteration of uranium (U) ore deposits and surface contamination of U mill tailings, and are expected to form as aging spent nuclear fuel (SNF) emplaced in a geologic repository chemically reacts with its surroundings. The existence of impurities in natural minerals, as well as structural studies by others, suggest that these phases will serve as hosts for fission products and actinide elements present in radioactive waste, resulting in the formation of solid solutions. These secondary uranyl phases are expected to be the long-term solubility limiting solids for radionuclides that have partitioned into them; however, experimental studies have been lacking to confirm these predictions.; This research concerns the formation of several uranyl phases in the presence of a variety of other metal cations expected in radioactive waste and the environment. Solid phases have been characterized using a combination of X-ray powder diffractometry, electron-microscopy, and elemental analysis. In some cases, dissolution studies were conducted to examine release of metal ions from the solids.; When low-valence cations were included in the synthesis of uranyl silicate phases, the data were consistent with incorporation into the structure at levels not exceeding 10 mol% substitution. At higher levels of substitution, polycrystalline material precipitated in addition to the uranyl silicate phase. Polycrystalline phases had different structures and elemental compositions than the uranyl silicate phase, and often contained higher fractions of the substituted ion. Although neptunium (Np) was excluded from metaschoepite, it associated with the U(VI)-peroxide metastudtite, and dissolution studies of this solid demonstrated Np release that exceeded congruent dissolution of U.; Amorphous solids and solids of low apparent crystallinity may control dissolved concentrations of radionuclides associated with U(VI) solid phases, even when present as minor constituents, and should be considered when evaluating the behavior of SNF in a geologic repository and the geochemical cycling of U and associated trace metals in the environment. |
