Karrie-Ann Kubatko

Crown and Bowl-Shaped Clusters of Uranyl Polyhedra

Bowl (U(16)) and crown-shaped clusters (U(20R) and U(24R)) containing 16, 20, and 24 uranyl peroxide polyhedra self-assemble in alkaline aqueous solution under ambient conditions. Structural analyses of crystallized clusters provided details of their topologies. Each contains uranyl hexagonal bipyramids in which two cis edges are peroxide, with a third edge defined by two OH groups, as well as hexagonal bipyramids in which three edges are peroxide. These are the first open uranyl peroxide clusters reported, and they join a growing family of complex cluster topologies based on uranium that hold promise for nanoscale control of chemistry in nuclear energy cycles.

Thermodynamics of uranyl minerals: Enthalpies of formation of uranyl oxide hydrates

Abstract The enthalpies of formation of seven uranyl oxide hydrate phases and one uranate have been determined using high-temperature oxide melt solution calorimetry: [(UO2)4O(OH)6](H2O)5, metaschoepite; β-UO2(OH)2; CaUO4; Ca(UO2)6O4(OH)6(H2O)8, becquerelite; Ca(UO2)4O3(OH)4(H2O)2; Na(UO2)O(OH), clarkeite; Na2(UO2)6O4(OH)6(H2O)7, the sodium analogue of compreignacite, and Pb3(UO2)8O8(OH)6(H2O)2, curite. The enthalpy of formation from the binary oxides, ΔHf-ox, at 298 K was calculated for each compound from the respective drop solution enthalpy, ΔHds. The standard enthalpies of formation from the elements, ΔH0f, at 298 K are -1791.0 ± 3.2, -1536.2 ± 2.8, -2002.0 ± 3.2, -11389.2 ± 13.5, -6653.1 ± 13.8, -1724.7 ± 5.1, -10936.4 ± 14.5, and -13163.2 ± 34.4 kJ/mol, respectively. These values are useful in exploring the stability of uranyl oxide hydrates in auxiliary chemical systems, such as those expected in U-contaminated environments.

Thermodynamics of uranyl minerals: Enthalpies of formation of rutherfordine, UO2CO3, andersonite, Na2CaUO2(CO3)3(H2O)5, and grimselite, K3NaUO2(CO3)3H2O

Abstract Enthalpies of formation of rutherfordine, UO2CO3, andersonite, Na2CaUO2(CO3)3(H2O)5, and grimselite, K3NaUO2(CO3)3(H2O), have been determined using high-temperature oxide melt solution calorimetry. The enthalpy of formation of rutherfordine from the binary oxides, ΔHr-ox, is .99.1 ± 4.2 kJ/mol for the reaction UO3 (xl, 298 K) + CO2 (g, 298 K) = UO2CO3 (xl, 298 K). The ΔHr-ox for andersonite is .710.4 ± 9.1 kJ/mol for the reaction Na2O (xl, 298 K) + CaO (xl, 298 K) + UO3 (xl, 298 K) + 3CO2 (g, 298 K) + 5H2O (l, 298 K) = Na2CaUO2(CO3)3(H2O)6 (xl, 298 K). The ΔHr-ox for grimselite is .989.3 ± 14.0 kJ/mol for the reaction 1.5 K2O (xl, 298 K) + 0.5Na2O (xl, 298 K) + UO3 (xl, 298 K) + 3CO2 (g, 298 K) + H2O (l, 298 K) = K3NaUO2(CO3)3H2O (xl, 298 K). The standard enthalpies of formation from the elements, ΔHfºf are .1716.4 ± 4.2, .5593.6 ± 9.1, and .4431.6 ± 15.3 kJ/mol for rutherfordine, andersonite, and grimselite, respectively. Energetic trends of uranyl carbonate formation from the binary oxides and ternary carbonates are dominated by the acid-base character of the binary oxides. However, even relative to mixtures of UO2CO3, K2CO3, and Na2CO3 or CaCO3, andersonite and grimselite are energetically stable by 111.7 ± 10.2 and 139.6 ± 16.1 kJ/mol, respectively, suggesting additional favorable interactions arising from hydration and/or changes in cation environments. These enthalpy values are discussed in comparison with earlier estimates

A novel arrangement of silicate tetrahedra in the uranyl silicate sheet of oursinite, (Co0.8Mg0.2)[(UO2)(SiO3OH)]2(H2O)6

Abstract Oursinite is a rare Co-bearing uranyl silicate of the uranophane group. The structure of oursinite, (Co0.8Mg0.2)[(UO2)(SiO3OH)]2(H2O)6, is orthorhombic, space group Cmca, a = 7.0494(5), b = 17.550(1), c = 12.734(1) Å, V = 1575.4(2) Å3, Z = 4. It was solved by direct methods and refined on the basis of F2 for all unique reflections using least-squares techniques to an agreement index (R1) of 2.66%. The structure contains an approximately linear (UO2)2+ uranyl ion that is present as a uranyl pentagonal bipyramid, one symmetrically distinct SiO3OH acid silicate group, and one M2+(OH,H2O)6 octahedron (M is dominated by Co). The uranyl pentagonal bipyramids and silicate tetrahedra are linked by the sharing of edges and vertices, giving a sheet based upon the uranophane anion topology. Adjacent sheets are linked by M2+(OH,H2O)6 octahedra located in the interlayer, and by hydrogen bonds. Each M2+(OH,H2O)6 octahedron contains two OH groups that are apical ligands of silicate tetrahedra in adjacent uranyl silicate sheets. Although several uranophane-group minerals contain sheets that are based upon the uranophane anion topology, the oursinite sheet involves novel orientations of silicate tetrahedra.

The Rb analogue of grimselite, Rb6Na2[(UO2)(CO3)3]2(H2O).

The crystal structure of the Rb analogue of grimselite, rubidium sodium uranyl tricarbonate hydrate, Rb6Na2[(UO2)(CO3)3]2(H2O), consists of a uranyl hexagonal bipyramid that shares three non-adjacent equatorial edges with carbonate triangles, resulting in a uranyl tricarbonate cluster of composition [(UO2)(CO3)3)]. These uranyl tricarbonate clusters form layers perpendicular to [001] and are interconnected by NaO8 polyhedra. The title compound is isostructural with grimselite, with a reduced occupancy of the H2O site (25% versus 50% in grimselite).

Affects of Hydrogen Peroxide on the Stability of Becquerelite

While the majority of studies of alteration of UO 2 and commercial spent nuclear fuel under simulated geological repository conditions have emphasized the importance of uranyl oxide hydrates and uranyl silicates, the influence of peroxide on repository performance has been largely overlooked. There is considerable evidence that uranyl peroxides will be important alteration phases of nuclear waste, and that these phases may impact the long-term performance of a geologic repository such as Yucca Mountain. Here we report the thermodynamics and kinetics of becquerelite, Ca[(UO 2 ) 6 O 4 (OH) 6 ](H 2 O) 8 , in the presence of solutions containing hydrogen peroxide. Thermodynamic calculations reveal that in solutions containing 3.5 × 10 -6 M hydrogen peroxide, studtite is thermodynamically favorable over becquerelite at 298 K. To access the kinetics of this reaction, batch experiments were conducted by the reaction of becquerelite and solutions containing hydrogen peroxide. In the presence of 0.1 M hydrogen peroxide, becquerelite altered to studtite within eight hours.