Uranate

Abstract: Laboratory experiments were performed to simulate the interaction of contaminated pore fluids with a variety of industrial materials. The objective was to evaluate the materials for use in a chemical barrier under a repository containing uranium mill tailings. Pore water would pass through the barrier, but contaminants would remain fixed in the solid fraction. More than 99% of the dissolved uranium in a synthetic pore fluid (initial uranium concentration of 30.0 mg/L) was extracted by the addition of hydrated lime, fly ash, barium chloride, calcium phosphate, titanium oxide, peat, and lignite. More than 96% of the molybdenum (initial molybdenum concentration of 8.9 mg/L) was extracted by ferrous sulfate, ferric oxyhydroxide, titanium oxide, peat, hematite, calcium chloride, and barium chloride. Some materials were effective only for a limited range of pH values. Extraction was caused by both precipitation (as calcium uranate, calcium molybdate, ferrous molybdate, or barium molybdate) and sorption (on ferric oxyhydroxide, hematite, calcium phosphate, peat, or titanium oxide). Chemicals that precipitate contaminant-bearing minerals are able to control solution chemistry and, therefore, have an advantage over sorbents which are subject to externally determined solution variables such as pH. On the basis of the predicted flux of pore fluid from themore » Monticello (Utah) uranium mill tailings, some industrial materials may be suitable for a chemical barrier at that site. 37 refs., 6 figs., 6 tabs.« less

Abstract: U(VI) doped hematite was synthesized and exposed to two different organic reductants with E0 of 0.23 and 0.70 V. A combination of HAADF-TEM and EXAFS provided evidence that uranium was incorporated in hematite in uranate, likely octahedral coordination. XPS indicated that structurally incorporated U(VI) was reduced to U(V), whereas non-incorporated U(VI) was reduced to U(IV). Specifically, the experiments indicate that U(V) was the dominant oxidation state of uranium in hematite around Eh −0.24 to −0.28 V and pH 7.7–8.6 for at least up to 5 weeks of reaction time. U(V), but not U(IV), was also detected in hematite at Eh +0.21 V (pH 7.1–7.3). The results support the hypothesis, based on previous experimental and theoretical work, that the stability field of U(V) is widened relative to U(IV) and U(VI) in uranate coordination environments where the coordination number of U is less than 8.