Solid-State Transformation of Uranyl Peroxide Materials through High-Level Irradiation.

The solid-state transformation of sodium uranyl triperoxide (Na4(UO2)(O2)3·9H2O, NaUT) to sodium uranyl tricarbonate (Na4(UO2)(CO3)3) by radiolysis was observed for the first time. The exposure of NaUT to 3 MGy gamma irradiation resulted in partial breakdown of the peroxides forming a mixed peroxide and carbonate species. The effects of He-ion irradiation on NaUT were also investigated up to 225 MGy using both hydrated argon and dry argon. The complete conversion to the uranyl tricarbonate phase by 56 MGy was done using hydrated argon, while dry argon did not fully convert showing the importance of water in the system. He-ion irradiated NaUT samples all convert to the tricarbonate phase with time in air post radiation exposure. This transition was monitored via Raman spectroscopy, infrared spectroscopy (IR), and powder X-ray diffraction (PXRD) to further confirm the identity of the final product as the sodium uranyl tricarbonate, cejkaite. This transformation outlines a mechanism for the mobility of uranyl in natural environments and in the Hanford tanks.

Quantification of zwitterion betaine in betaine bis(trifluoromethylsulfonyl)imide and its influence on liquid-liquid equilibrium with water.

Ionic liquids (ILs) have been proposed as extractants for separation of metals, including rare earth elements. In particular, protonated betaine bis(trifluoromethylsulfonyl)imide ([HBet][TFSI]) exhibits liquid-liquid phase behavior with water that can be tuned by complexation with various metals. Here we show that previously undetected neutral zwitterionic betaine formed during the IL synthesis can affect the phase behaviour.

Evidence of a Uranium-Paddlewheel Node in a Catecholate-Based Metal-Organic Framework.

The interactions between uranium and non-innocent organic species are an essential component of fundamental uranium redox chemistry. However, they have seldom been explored in the context of multidimensional, porous materials. Uranium-based metal-organic frameworks (MOFs) offer a new angle to study these interactions, as these self-assembled species stabilize uranium species through immobilization by organic linkers within a crystalline framework, while potentially providing a method for adjusting metal oxidation state through coordination of non-innocent linkers. We report the synthesis of the MOF NU-1700, assembled from U4+ -paddlewheel nodes and catecholate-based linkers. We propose this highly unusual structure, which contains two U4+ ions in a paddlewheel built from four linkers-a first among uranium materials-as a result of extensive characterization via powder X-ray diffraction (PXRD), sorption, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA), in addition to density functional theory (DFT) calculations.

Gamma-Ray-Induced Formation of Uranyl Peroxide Cage Clusters.

Aqueous solutions of lithium uranyl triperoxide, Li4[UO2(O2)3] (LiUT), were irradiated with gamma rays at room temperature and found to form the uranyl peroxide cage cluster, Li24[(UO2)(O2)(OH)]24 (Li-U24). Raman spectroscopy and 18O labeling were used to identify the Raman-active vibrations of LiUT. With these assignments, the concentration of LiUT was tracked as a function of radiation dose. A discrepancy between monomer removal and cluster formation suggests that the reaction proceeds by the assembly of an intermediate. Non-negative matrix factorization was used to separate Raman spectra into components and resulted in the identification of a unique intermediate species. Much of the conversion appears to be driven by water radiolysis products, particularly the hydroxyl radical. This differs from the 18O-labeled copper-catalyzed formation of U24, which progresses at a steady rate with no observation of intermediates. Li-U24 in solution decomposes at high radiation doses resulting in a solid insoluble product similar to Na-compreignacite, Na2(UO2)6O4(OH)6·7H2O, which contains uranyl oxyhydroxy sheets.

Radiation-Induced Solid-State Transformations of Uranyl Peroxides.

Single-crystal X-ray diffraction studies of pristine and γ-irradiated Ca2[UO2(O2)3]·9H2O reveal site-specific atomic-scale changes during the solid-state progression from a crystalline to X-ray amorphous state with increasing dose. Following γ-irradiation to 1, 1.5, and 2 MGy, the peroxide group not bonded to Ca2+ is progressively replaced by two hydroxyl groups separated by 2.7 Å (with minor changes in the unit cell), whereas the peroxide groups bonded to Ca2+ cations are largely unaffected by irradiation prior to amorphization, which occurs by a dose of 3 MGy. The conversion of peroxide to hydroxyl occurs through interaction of neighboring lattice H2O molecules and ionization of the peroxide O-O bond, which produces two hydroxyls, and allows isolation of the important monomer building block, UO2(O2)2(OH)24-, that is ubiquitous in uranyl capsule polyoxometalates. Steric crowding in the equatorial plane of the uranyl ion develops and promotes transformation to an amorphous phase. In contrast, γ-irradiation of solid Li4[(UO2)(O2)3]·10H2O results in a solid-state transformation to a well-crystallized peroxide-free uranyl oxyhydrate containing sheets of equatorial edge and vertex-sharing uranyl pentagonal bipyramids with likely Li and H2O in interlayer positions. The irradiation products of these two uranyl triperoxide monomers are compared via X-ray diffraction (single-crystal and powder) and Raman spectroscopy, with a focus on the influence of the Li+ and Ca2+ countercations. Highly hydratable and mobile Li+ yields to uranyl hydrolysis reactions, while Ca2+ provides lattice rigidity, allowing observation of the first steps of radiation-promoted transformation of uranyl triperoxide.

Unusual Metal-Organic Framework Topology and Radiation Resistance through Neptunyl Coordination Chemistry.

A Np(V) neptunyl metal-organic framework (MOF) with rod-shaped secondary building units was synthesized, characterized, and irradiated with γ rays. Single-crystal X-ray diffraction data revealed an anionic framework containing infinite helical chains of actinyl-actinyl interaction (AAI)-connected neptunyl ions linked together through tetratopic tetrahedral organic ligands (NSM). NSM exhibits an unprecedented net, demonstrating that AAIs may be exploited to give new MOFs and new topologies. To probe its radiation stability, we undertook the first irradiation study of a transuranic MOF and its organic linker building block using high doses of γ rays. Diffraction and spectroscopic data demonstrated that the radiation resistance of NSM is greater than that of its linker building block alone. Approximately 6 MGy of irradiation begins to induce notable changes in the long- and short-range order of the framework, whereas 3 MGy of irradiation induces total X-ray amorphization and changes in the local vibrational bands of the linker building block.

Unprecedented Radiation Resistant Thorium-Binaphthol Metal-Organic Framework.

A thorium-organic framework (TOF-16) containing hexameric secondary building units connected by functionalized binaphthol linkers was synthesized, characterized, and irradiated to probe its radiation resistance. Radiation stability was examined using γ-rays and 5 MeV He2+ ions to simulate α particles. γ-irradiation of TOF-16 to an unprecedented 4 MGy dose resulted in no apparent bulk structural damage visible by X-ray diffraction. To further probe radiation stability, we conducted the first He2+ ion irradiation study of a metal-organic framework (MOF). Diffraction data indicate onset of crystallinity loss upon approximately 15 MGy of irradiation and total loss of crystallinity upon exposure to approximately 25 MGy of He2+ ion irradiation. The high radiation resistance observed suggests MOFs can withstand radiation exposure at doses found in nuclear waste streams and highlights the need for a systematic approach to understand and eventually design frameworks with exceptional radiation resistance.

Stability of Solid Uranyl Peroxides under Irradiation.

The effects of radiation on a variety of uranyl peroxide compounds were examined using γ-rays and 5 MeV He ions, the latter to simulate α-particles. The studied materials were studtite, [(UO2)(O2)(H2O)2](H2O)2, the salt of the U60 uranyl peroxide cage cluster, Li44K16[(UO2)(O2)(OH)]60·255H2O, the salt of U60Ox30 uranyl peroxide oxalate cage cluster, Li12K48[{(UO2)(O2)}60(C2O4)30]·nH2O, and the salt of the U24Pp12 (Pp = pyrophosphate) uranyl peroxide pyrophosphate cage cluster, Li24Na24[(UO2)24(O2)24(P2O7)12]·120H2O. Irradiated powders were characterized using powder X-ray diffraction, Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and UV-vis spectroscopy. A weakening of the uranyl bonds of U60 was found while studtite, U60Ox30, and U24Pp12 were relatively stable to γ-irradiation. Studtite and U60 are the most affected by α-irradiation forming an amorphous uranyl peroxide as characterized by Raman spectroscopy and powder X-ray diffraction while U60Ox30 and U24Pp12 show minor signs of the formation of an amorphous uranyl peroxide.

Characterization of Phosphate and Arsenate Adsorption onto Keggin-Type Al30 Cations by Experimental and Theoretical Methods.

Keggin-type aluminum oxyhydroxide species such as the Al30 (Al30O8(OH)56(H2O)26(18+)) polycation can readily sequester inorganic and organic forms of P(V) and As(V), but there is a limited chemical understanding of the adsorption process. Herein, we present experimental and theoretical structural and chemical characterization of [(TBP)2Al2(µ4-O8)(Al28(µ2-OH)56(H2O)22)](14+) (TBP = t-butylphosphonate), denoted as (TBP)2Al30-S. We go on to consider the structure as a model for studying the reactivity of oxyanions to aluminum hydroxide surfaces. Density functional theory (DFT) calculations comparing the experimental structure to model configurations with P(V) adsorption at varying sites support preferential binding of phosphate in the Al30 beltway region. Furthermore, DFT calculations of R-substituted phosphates and their arsenate analogues consistently predict the beltway region of Al30 to be most reactive. The experimental structure and calculations suggest a shape-reactivity relationship in Al30, which counters predictions based on oxygen functional group identity.

Interplay of condensation and chelation in binary and ternary Th(IV) systems.

Th(IV) readily undergoes hydrolysis and condensation in aqueous solutions to form polynuclear molecular species and the system becomes increasingly complicated when organic chelators or other metals are present in solution, leading to the formation of complexes with vastly different structural topologies. Five compounds containing binary and ternary Th(IV) complexes have been synthesized and structurally characterized using single-crystal X-ray diffraction, including Na4[Th6O2(C10O7N2H14)6]·20.5H2O (Th6hedta), [Th(C9O6NH12)(H2O)(NO3)]·1.5H2O (Th(ntp)), [Th2Al8(OH)14(H2O)12(C6O5NH8)4](NO3)6·17.5H2O (Th2Al8heidi), (C4N2H12) [Th2Fe2(OH)2(H2O)2(C6O7H4)2(C6O7H5)2]·6H2O (Th2Fe2cit), (C4N2H12) [ThFe2O(H2O)3(C11O9N2H13)2]·6H2O (ThFe2dhpta). Additional chemical characterization by infrared spectroscopy and thermogravimetric analysis provides information on the chelation by the organic ligands and thermal stability. These molecular complexes can be utilized to understand aqueous speciation in mixed-metal solutions and also provide information regarding contaminant adsorption on iron(III) and aluminum(III) oxide surfaces.

Synthesis and characterization of homo- and heteronuclear molecular Al3+ and Th4+ species chelated by the ethylenediaminetetraacetate (edta) ligand.

Four molecular species chelated by edta have been synthesized from aqueous solutions and characterized by single-crystal X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis. [Th(H2O)4(edta)] (Th1) crystallized in monoclinic space group P2(1)/c with unit cell parameters of a = 8.5275(5) Å, b = 12.0635(7) Å, c = 15.8825(9) Å, and ß = 105.340(2)°. Monoclinic space group C2 was identified for [Al2(edta)(OH)2(H2O)2] (Al2) with a = 11.1089(10) Å, b = 9.4830(8) Å, c = 7.6116(7) Å, and ß = 112.026(3)°. [Th4(H2O)4Al10(H2O)8(OH)28(edta)4]·Cl2(H2O)29 (Th4Al10) formed the triclinic space group P1 with a = 9.3172(8) Å, b = 16.6099(14) Å, c = 19.5080(14) Å, α = 102.314(3)°, ß = 95.615(3)°, and γ = 92.473(3)°. [Th2(H2O)2Al6(H2O)10(OH)14(edta)2]·(NO3)4(H2O)18 (Th2Al6) also crystallized in triclinic space group P1[combining macron] with unit cell parameters a = 10.9899(12) Å, b = 11.6107(12) Å, c = 14.3350(17) Å, α = 73.012(4)°, ß = 87.411(4)°, and γ = 75.717(4)°. Small molecular species are observed in the Th1 and Al2 compounds, while the metal cations hydrolyze to create nanometer-sized heterometallic clusters in the Th4Al10 and Th2Al6 materials. The topological characteristics of each species are described and related to aqueous speciation in Th(4+), Al(3+)-edta systems.

Synthesis and structural characterization of heterometallic thorium aluminum polynuclear molecular clusters.

Aluminum can undergo hydrolysis in aqueous solutions leading to the formation of soluble molecular clusters, including polynuclear species that range from 1 to 2 nm in diameter. While the behavior of aluminum has been extensively investigated, much less is known about the hydrolysis of more complex mixed-metal systems. This study focuses on the structural characteristics of heterometallic thorium-aluminum molecular species that may have important implications for the speciation of tetravalent actinides in radioactive waste streams and environmental systems. Two mixed metal (Th(4+)/Al(3+)) polynuclear species have been synthesized under ambient conditions and structurally characterized by single-crystal X-ray diffraction. [Th(2)Al(6)(OH)(14)(H(2)O)(12)(hedta)(2)](NO(3))(6)(H(2)O)(12) (ThAl1) crystallizes in space group P2(1)/c with unit cell parameters of a = 11.198(1) Å, b = 14.210(2) Å, c = 23.115(3) Å, and ß = 96.375° and [Th(2)Al(8)(OH)(12)(H(2)O)(10)(hdpta)(4)](H(2)O)(21) (ThAl2) was modeled in P1 with a = 13.136(4) Å, b = 14.481(4) Å, c = 15.819(4) Å, α = 78.480(9)°, ß = 65.666(8)°, γ = 78.272(8)°. Infrared spectra were collected on both compounds, confirming complexation of the ligand to the metal center, and thermogravimetric analysis indicated that the thermal degradation of these compounds resulted in the formation of an amorphous product at high temperatures. These mixed metal species have topological relationships to previously characterized aluminum-based polynuclear species and may provide insights into the adsorption of tetravalent actinides on colloidal or mineral surfaces.