Ginger E. Sigmon

Symmetry versus Minimal Pentagonal Adjacencies in Uranium‐Based Polyoxometalate Fullerene Topologies

C U soon: Clusters containing 60, 44, and 36 uranyl peroxide hydroxide polyhedra (see picture) adopt fullerene topologies of maximum symmetry. The largest of these, denoted U60, is topologically identical to C(60) with no pentagonal adjacencies and the highest possible symmetry. U44 adopts the topology with maximum symmetry rather than that with the lowest number of pentagonal adjacencies.

Uranyl-peroxide interactions favor nanocluster self-assembly.

Uranyl peroxide polyhedra are known to self-assemble into complex closed clusters with fullerene and other topologies containing as many as 60 polyhedra. Here clusters containing 20 uranyl pentagonal triperoxides have been isolated and characterized that assume the smallest possible fullerene topology consisting only of 12 pentagons. Oxalate has been used to crystallize fragments of larger uranyl peroxide clusters, and these fragments and other known structures indicate that the U-O(2)-U dihedral angle is inherently bent. Such bending is thought to be essential in directing the self-assembly of uranyl peroxide polyhedra into closed clusters.

Uranium Pyrophosphate/Methylenediphosphonate Polyoxometalate Cage Clusters

Despite potential applications in advanced nuclear energy systems, nanoscale control of uranium materials is in its infancy. In its hexavalent state, U occurs as (UO(2))(2+) uranyl ions that are coordinated by various ligands to give square, pentagonal, or hexagonal bipyramids. Creation and design of nanostructured uranyl materials requires interruption of the tendency of uranyl bipyramids to share equatorial edges to form infinite sheets that occur in extended structures. Where a bidentate peroxide group bridges uranyl bipyramids, the configuration is inherently bent, fostering formation of cage clusters. Here the bent configurations of four- and five-membered rings of uranyl peroxide hexagonal bipyramids are bridged by pyrophosphate or methylenediphosphonate, creating eight chemically complex cage clusters with specific topologies. Chemical complexity in such clusters provides opportunities for the tuning of cage sizes, pore sizes, and properties such as aqueous solubility. Several of these are topological derivatives of simpler clusters that contain only uranyl bipyramids, whereas others exhibit new topologies.

Uranyl peroxide enhanced nuclear fuel corrosion in seawater

The Fukushima-Daiichi nuclear accident brought together compromised irradiated fuel and large amounts of seawater in a high radiation field. Based on newly acquired thermochemical data for a series of uranyl peroxide compounds containing charge-balancing alkali cations, here we show that nanoscale cage clusters containing as many as 60 uranyl ions, bonded through peroxide and hydroxide bridges, are likely to form in solution or as precipitates under such conditions. These species will enhance the corrosion of the damaged fuel and, being thermodynamically stable and kinetically persistent in the absence of peroxide, they can potentially transport uranium over long distances.

Removal of oil droplets from contaminated water using magnetic carbon nanotubes

Water contaminated by oil and gas production poses challenges to the management of Americas water resources. Here we report the design, fabrication, and laboratory evaluation of multi-walled carbon nanotubes decorated with superparamagnetic iron-oxide nanoparticles (SPIONs) for oil-water separation. As revealed by confocal laser-scanning fluorescence microscopy, the magnetic carbon nanotubes (MCNTs) remove oil droplets through a two-step mechanism, in which MCNTs are first dispersed at the oil-water interface and then drag the droplets with them out of water by a magnet. Measurements of removal efficiency with different initial oil concentration, MCNT dose, and mixing time show that kinetics and equilibrium of the separation process can be described by the Langmuir model. Separation capacity qt is a function of MCNT dose m, mixing time t, and residual oil concentration Ce at equilibrium: [Formula in text] where qmax, kw, and K are maximum separation capacity, wrapping rate constant, and equilibrium constant, respectively. Least-square regressions using experimental data estimate qmax = 6.6(± 0.6) g-diesel g-MCNT(-1), kw = 3.36(± 0.03) L g-diesel(-1) min(-1), and K = 2.4(± 0.2) L g-diesel(-1). For used MCNTs, we further show that over 80% of the separation capacity can be restored by a 10 min wash with 1 mL ethanol for every 6 mg MCNTs. The separation by reusable MCNTs provides a promising alternative strategy for water treatment design complementary to existing ones such as coagulation, adsorption, filtration, and membrane processes.

Rapid self-assembly of uranyl polyhedra into crown clusters.

Clusters built from 32 uranyl peroxide polyhedra self-assemble and crystallize within 15 min after combining uranyl nitrate, ammonium hydroxide, and hydrogen peroxide in aqueous solution under ambient conditions. These novel crown-shaped clusters are remarkable in that they form so quickly, have extraordinarily low aqueous solubility, form with at least two distinct peroxide to hydroxyl ratios, and form in very high yield. The clusters, which have outer diameters of 23 Å, topologically consist of eight pentagons and four hexagons. Their rapid formation and low solubility in aqueous systems may be useful properties at various stages in an advanced nuclear energy system.

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.

Uranyl peroxide closed clusters containing topological squares.

Four self-assembling clusters of uranyl peroxide polyhedra have been formed in alkaline aqueous solutions and structurally characterized. These clusters consist of 28, 30, 36 and 44 uranyl polyhedra and exhibit complex new topologies. Each has a structure that contains topological squares, pentagons and hexagons. Analysis of possible topologies within boundary constraints indicates a tendency for adoption of higher symmetry topologies in these cases. Small angle X-ray scattering data demonstrated that crystals of one of these clusters can be dissolved in ultrapure water and that the clusters remain intact for at least several days.

Syntheses, structures, and IR spectroscopic characterization of new uranyl sulfate/selenate 1D-chain, 2D-sheet and 3D-framework

Abstract Three uranyl sulfates, (C6H20N4)[(UO2)2(SO4)4 · (H2O)2](H2O)6 (TETAUS), (C15H14N3)[(UO2)(SO4)2](NO3) · (H2O)2 (TPUS), and K2[(UO2)(SO4)2(H2O)] · H2O (KUS), and two uranyl selenates, K(H3O)[(UO2)2(SeO4)3 · (H2O)](H2O)6 (KUSe) and (H3O)2[(UO2)2(SeO4)3 · (H2O)] (USe), were synthesized by slow evaporation of aqueous solutions at room temperature. TETAUS crystallizes in space group P-1, a = 6.7186(5) Å, b = 9.2625(7) Å, c = 13.1078(9) Å, α = 72.337(2)°, β = 89.198(2)°, γ = 70.037(1)°, V = 726.89(9) Å3, Z = 1. TPUS is triclinic, P-1, a = 6.9732(7) Å, b = 13.569(1) Å, c = 13.641(1) Å, α = 111.809(2)°, β = 102.386(2)°, γ = 93.833(2)°, V = 1150.0(2) Å3, Z = 2. KUS is orthorhombic, Cmca, a = 12.171(2) Å, b = 16.689(3) Å, c = 10.997(2) Å, V = 2233.8(6) Å3, Z = 8. These uranyl sulfates are built from infinite one-dimensional uranyl sulfate chains with different topologies. KUSe is monoclinic, P21/n, a = 14.715(1) Å, b = 10.1557(7) Å, c = 15.833(1) Å, β = 114.415(1)°, V = 2154.5(3) Å3, Z = 4. Its structure is based on a two-dimensional uranyl selenate sheet. USe crystallizes in space group P21/c, a = 10.6124(2) Å, b = 14.7717(3) Å, c = 13.7139(3) Å, β = 96.989(1)°, V = 2133.86(8) Å3, Z = 4, with a complex three-dimensional uranyl selenate framework containing channels extending in three directions.

Raman Spectroscopic and ESI-MS Characterization of Uranyl Peroxide Cage Clusters

Strategies for interpreting mass spectrometric and Raman spectroscopic data have been developed to study the structure and reactivity of uranyl peroxide cage clusters in aqueous solution. We demonstrate the efficacy of these methods using the three best-characterized uranyl peroxide clusters, {U24}, {U28}, and {U60}. Specifically, we show a correlation between uranyl-peroxo-uranyl dihedral bond angles and the position of the Raman band of the symmetric stretching mode of the peroxo ligand, develop methods for the assignment of the ESI mass spectra of uranyl peroxide cage clusters, and show that these methods are generally applicable for detecting these clusters in the solid state and solution and for extracting information about their bonding and composition without crystallization.