Transition metal dopants can be incorporated in metal organic frameworks to change the physical properties of the material. Metal organic nanotubes are a less well studied form of hybrid material, and in this study, transition metals were substituted into U(VI) metal organic nanotubes (UMON) to investigate changes with water uptake, solvent selectivity, and hydrostability. Single-crystal X-ray analysis, UV/vis spectroscopy, and electron microprobe analysis confirmed the substitution of (VO)2+, Co(II), Ni(II), Fe(II), and Cu(II), with the highest amount of incorporation by Cu(II). Water uptake and release by the substituted materials were similar to that of the pure UMON sample, with the exception in the Cu(II)-UMON samples, where less water present in the nanotubular cavities and additional heating were necessary for dehydration. A detailed investigation of the Cu(II)-UMON material indicated that the overall selectivity of the material was maintained and the hydrostability was drastically enhanced with incorporation. In the presence of ammonia, the pure and doped UMON material degraded to secondary phases.

Development of novel materials for water purification and storage is reliant on our understanding of the interaction of water within solid-state materials and inspired by the high selectivity present in certain biological systems. The present study investigates the influence of zwitterionic ligands on water properties within hybrid materials through the synthesis and characterization of four novel coordination polymers built from the uranyl unit and an imidazolium dicarboxylate (Imd) linker. One of the compounds (UIM-5) hosts an infinite water chain of edge sharing hexamers and octamers of hydrogen bonded water molecules. The other three compounds (UIM-2A, 2B, 2C) make up a polymorphic system in which the guest water molecules are found in varying degrees of isolation. We further explore the expansive literature on uranyl hybrid materials to identify common water nets and possible structural features that may control water structure and properties.

•5 heterometallic uranyl PDC compounds were synthesized and characterized by X-ray diffraction and spectroscopic techniques.•Uranyl PDC 1-D chain complexes that were linked via transition metal cations were further linked through π- πinteractions.•Structural data denotes shorter uranyl bonds in the presence of the transition metals and this is confirmed by Raman shifts.•Solid-state UV/Vis spectroscopy confirms the presence of transition metals in this system.The incorporation of transition metals into hybrid uranyl materials can result in more diverse structural topologies and variations in physical and chemical properties. To explore the impact of transition metals on the uranyl cation, five uranium containing bimetallic chain compounds, [(UO2)M(PDC)2(H2O)4]·4(H2O) (PDC = 2,6 pyridinedicarboxylate; M = Ni2+, Co2+, Fe2+, Zn2+, and Cu2+) were synthesized by evaporation of aqueous solutions at room temperature. The uranyl cation is complex by two PDC ligands and the transition metal cations bond to the complex to form a one-dimensional chain topology. The presence of the transition metal leads to the presence of a stronger uranyl oxo bonds as shown by the single-crystal X-ray diffraction data and the Raman spectra. Solid state diffuse reflectance UV/Visible spectra confirmed the presence of the transition metals in the structure by the broad bands that appeared at relevant wavelengths.Synthesis and characterization of [(UO2)M(2,6 pyridinedicarboxylate)(H2O)4]·4(H2O) [M = Ni2+, Co2+, Fe2+, Zn2+, and Cu2+] compounds reveals impact of transition metals in uranyl hybrid systems.Download high-res image (350KB)Download full-size image

Raman spectroscopy is emerging as a powerful tool for identifying hexavalent uranium speciation in situ; however, there is no straightforward protocol for identifying uranyl species in solution. Herein, uranyl samples are evaluated using Raman spectroscopy, and speciation is monitored at various solution pH values and anion compositions. Spectral quality is evaluated using two Raman excitation wavelengths (532 and 785 nm) as these are critical for maximizing signal-to-noise and minimizing background from fluorescent uranyl species. The Raman vibrational frequency of uranyl shifts according to the identity of the coordinating ions within the equatorial plane and/or solution pH; therefore, spectral barcode analysis and rigorous peak fitting methods are developed that allow accurate and routine uranium species identification. All in all, this user’s guide is expected to provide a user-friendly, straightforward approach for uranium species identification using Raman spectroscopy.

Uranyl hybrid compounds are complex materials due to variability in coordination geometry, flexibility in ligand chelation, and metal hydrolysis, which leads to difficulty in controlling the secondary building units. The presence of transition metals in uranyl hybrid materials adds to the complexity, but also leads to an increase in the dimensionality of the topology from infinite chains and 2-D sheets, to 3-D framework lattices. In this study, five uranyl malate compounds were synthesized at room temperature: ((C4H12N2)[(UO2)2(C4H3O5)2]·4H2O (UMal1), (C4H12N2)[(UO2)2(C4H3O5)2] (UMal1-b), [(UO2)(C4H3O5)Cu(C10H8N2)Cl(H2O)]·2H2O (UCuMal1), [(UO2)2(C4H3O5)2Cu(C5H5N)2(H2O)2]·2H2O (UCuMal2), [(UO2)2(C4H3O5)2Cu(C5H5N)2(H2O)2]·2H2O (UCuMal3)). These compounds were characterized using single-crystal X-ray diffraction, thermogravimetric analysis and Raman spectroscopy. All five compounds contain an identical uranyl malate secondary building unit that could be further linked through the Cu(II) cation. In this system, the identity of the ligands bonded to the Cu(II) cation impacted dimensionality and could be the key to designing materials with a known uranyl building unit.Five uranyl malate compounds were synthesized and characterized by single-crystal X-ray diffraction, TGA, and Raman Spectroscopy. The presence of Cu(II) in uranyl hybrid materials contributes to increasing the dimensionality of the material.

The structural chemistry of Group 13 polyoxometalates lags far behind related negatively charged transition metal species and limits the development of advanced materials. A novel heterometallic cluster [Ga2Al18O8(OH)36(H2O)12]8+ (Ga2Al18) has been isolated using a supramolecular approach and structurally characterized using single-crystal X-ray diffraction. Ga2Al18 represents the Wells–Dawson structure polycations and variations in the structural topology may be related to the initial stabilization of the Keggin isomer. DFT calculations on the related ε-Keggins (GaAl12 and Al13), Ga2Al18, and theoretical Al2Al18 clusters reveal similar features of electronic structure, suggesting additional heteroatom substitution in other isostructural clusters should be possible.

Uranyl citrate forms trimeric species at pH > 5.5, but exact structural characteristics of these important oligomers have not previously been reported. Crystallization and structural characterization of the trimers suggests the self-assembly of the 3:3 and 3:2 U:Cit complexes into larger sandwich and macrocyclic molecules. Raman spectroscopy and ESI-MS have been utilized to investigate the relative abundance of these species in solution under varying pH and citrate concentrations. Additional dynamic light scattering experiments indicate that self-assembly of the larger molecules does occur in aqueous solution.

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.

Keggin-type aluminum oxyhydroxide species such as the Al30 (Al30O8(OH)56(H2O)2618+) 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.

Porous hybrid materials such as metal–organic nanotubes are of interest due to synthetic tunability, possibility for 1-D flow, and confinement of solvent molecules. In the current study, the stability and solvent selectivity of two hybrid materials with nanotubular arrays, (pip)0.5[(UO2)(HIDA)(H2IDA)]·2H2O (UIDA; IDA = iminodiacetate) and [(UO2)(PDC)(H2O)] (UPDC; PDC = pyridine dicarboxylate) were analyzed using X-ray diffraction, gas chromatography/mass spectrometry, thermogravimetric analysis, and infrared spectroscopy. The fine details of the structural characterization, such as the presence of solvent molecules, were found to be important to the overall properties of the materials as evidenced by the increased stability of the UIDA compound when formed from a solvent mixture containing acetone. Careful analysis of the UPDC compound indicated that the ligated solvent molecule can be exchanged, which may impact the hydration state of the material. Overall, the UIDA compound displays complete selectivity to water, but the UPDC compound adsorbs THF, methanol, ethanol, and cyclohexane.

Organic acids are important metal chelators in environmental systems and tend to form soluble complexes in aqueous solutions, ultimately influencing the transport and bioavailability of contaminants in surface and subsurface waters. This is particularly true for the formation of uranyl citrate complexes, which have been utilized in advanced photo- and bioremediation strategies for soils contaminated with nuclear materials. Given the complexity of environmental systems, the formation of ternary or heterometallic uranyl species in aqueous solutions are also expected, particularly with Al(III) and Fe(III) cations. These ternary forms are reported to be more stable in aqueous solutions, potentially enhancing contaminant mobility and uptake by organisms, but the exact coordination geometries of these soluble molecular complexes have not been elucidated. To provide insight into the nature of these species, we have developed a series of geochemical model compounds ([(UO2)2Al2(C6H4O7)4]6− (U2Al2), [(UO2)2Fe2(C6H4O7)4]6− (U2Fe2-1) and [(UO2)2Fe2(C6H4O7)4(H2O)2]6− (U2Fe2-2) and [(UO2)2Fe4(OH)4(C6H4O7)4]8− (U2Fe4)) that were characterized by single-crystal X-ray diffraction and vibrational spectroscopy. Mass spectroscopy was then employed to compare the model compounds to species present in aqueous solutions to provide an enhanced understanding of the ternary uranyl citrate complexes that could be relevant in natural systems.

Hybrid materials, such as metal organic nanotubes (MON), are of interest because of their chemical tunability and permanent porosity. While an increasing number of compounds is being reported, very little is known about their thermodynamic stability. Herein, the energetics of a MON, (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O (UMON, C10H21N3UO12) (ida = iminodiacetate), that possesses unique water exchange and uptake has been investigated by acid solution calorimetry, thermal analysis, and water adsorption calorimetry. The enthalpy of formation of UMON, C10H21N3UO12 (ΔHf,rxn), from the dense components (uranium oxide (UO3), piperazine (C4H10N2), and iminodiacetic acid (C4H7NO4) was −55.3 ± 0.9 kJ/mol, which was similar to values for other metal organic framework materials. The dehydration enthalpy to form an anhydrous UMON and gaseous H2O at 37 °C from thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC) experiments was 57.8 ± 1.9 kJ/mol of water. This value is somewhat higher than the vaporization enthalpy of water (44 kJ/mol) and suggests modest bonding interactions of H2O with the inner walls of the nanotubes. Water adsorption calorimetry of (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O indicated that the water molecules are confined inside the UMON material in two thermally distinct positions. The ice-like arrangement of the confined water molecules inside the nanotube impacts the energetics of the material and adds to the stabilization of the structure.

Supramolecular assembly of U(VI) materials can be limited by the passivation of the uranyl oxo group and the propensity of the metal center to hydrolyze, resulting in the formation of extended two-dimensional (2D) structures. To overcome these barriers, the use of charge-assisted H-bonding was explored using amino acids (glycine [Gly] and l-alanine [Ala]), resulting in the formation of three novel compounds {[(UO2)3(Gly)2(O)2(OH)2](H2O)6 (1), [(UO2)5(Gly)4(O)3(OH)3](NO3)(H2O)12 (2), and [(UO2)3(Ala)2O(OH)3](NO3)(H2O)3 (3)} that have been characterized by X-ray diffraction, elemental analysis, TGA, and vibrational spectroscopy. Hydrolysis of the uranyl cation (UO22+) chelated by bridging zwitterionic amino acids results in the formation of infinite chains when synthesized from mildly acidic aqueous solutions. While positively charged chains form densely packed structures, the neutral UO2-glycine chains support a nanoporous (internal diameter ∼1.35 nm) supramolecular architecture through multifurcated charge-assisted hydrogen bonding. These interactions occur directly between the protonated amine of glycine and the uranyl’s oxo moiety, representing a unique supramolecular synthon for the assembly of hybrid porous uranyl materials. The zwitterionic glycine ligands also assist in the helical assembly of water molecules that are hydrogen bonded to the interior walls of the nanopores, resulting in the formation of an empty 0.85 nm channel through the pore space.

Nanotubular materials have unique water transport and storage properties that have the potential to advance separations, catalysis, drug delivery, and environmental remediation technologies. The development of novel hybrid materials, such as metal–organic nanotubes (MONs), is of particular interest, as these materials are amenable to structural engineering strategies and may exhibit tunable properties based upon the presence of inorganic components. A novel metal–organic nanotube, (C4H12N2)0.5[(UO2)(Hida)(H2ida)]·2H2O (UMON) (ida = iminodiacetate), that demonstrates the possibilities of these types of hybrid compounds has been synthesized via a supramolecular approach. Single-crystal X-ray diffraction of the compound revealed stacked macrocyclic arrays that contain highly ordered water molecules with structural similarities to the “ice channels” observed in single-walled carbon nanotubes. Nanoconfinement of the water molecules may be the cause of the unusual exchange properties observed for UMON, including selectivity to water and reversible exchange at low temperature (37 °C). Similar properties have not been reported for other inorganic or hybrid compounds and indicate the potential of MONs as advanced materials.

The interplay of hydrolysis and chelation by organic ligands results in the formation of novel uranium species in aqueous solutions. Many of these molecular complexes have been identified by spectroscopic and potentiometric techniques, but a detailed structural understanding of these species is lacking. Identification of possible uranyl hydrolysis products in the presence of organic functional groups has been achieved by the crystallization of molecular species into a solid-state compound, followed by structural and chemical characterization of the material. The structures of three novel molecular complexes containing either iminodiacetate (ida) (Na3[(UO2)3(OH)3(ida)3]·8H2O (1)) or malate (mal) (K(pip)2[(UO2)3O(mal)3]·6H2O (2a) (pip = C4N2H12), (2b) (pip)3[(UO2)3O(mal)3]·H2O, and (pip)6[(UO2)11(O)4(OH)4(mal)6(CO3)2]·23H2O (3)) ligands have been determined by single-crystal X-ray diffraction and have been chemically characterized by IR, Raman, and NMR spectroscopies. A major structural component in compounds 1 and 2 is a trimeric 3:3 uranyl ida or mal species, but different bridging groups between the metal centers create variations in the structural topologies of the molecular units. Compound 3 contains a large polynuclear cluster with 11 U atoms, which is composed of trimeric and pentameric building units chelated by mal ligands and linked through hydroxyl groups and carbonate anions. The characterized compounds represent novel structural topologies for U6+ hydrolysis products that may be important molecular species in near-neutral aqueous systems.

Keggin-type molecular clusters formed from the partial hydrolysis of aluminum in aqueous solutions have the capacity to adsorb a variety of inorganic and organic contaminants. The adsorptive capability of Keggin-type polyaluminum species, such as Al13 and Al30, lead to their wide usage as precursors for heterogeneous catalysts and clarifying agents for water purification applications, but a molecular-level understanding of adsorption process is lacking. Two model Al30 clusters, whose surface has been modified with chelated metals (Al3+ and Zn2+) have been synthesized and structurally characterized by single-crystal X-ray diffraction. Al32IDA [(Al(IDA)H2O)2(Al30O8(OH)60(H2O)22)](2,6-NDS)4(SO4)2Cl4(H2O)40, IDA = iminodiacetic acid, 2,6-NDS = 2,6 napthalene disulfonate) crystallize in the triclinic space group, P1̅ with a = 13.952(2) Å, b = 16.319(3) Å, c = 23.056(4) Å, α = 93.31(1)°, β = 105.27(1)°, and γ = 105.52(1)°. Zn2Al32 [(Zn(NTA)H2O)2(Al(NTA)(OH)2)2(Al30(OH)60(O)8(H2O)20](2,6-NDS)5(H2O)64, (NTA = nitrilotriacetic acid), also crystallizes in P1̅ with unit cell parameter refined as a = 16.733(7) Å, b = 18.034(10) Å, c = 21.925(11) Å, α = 82.82(2)°, β = 70.96(2)°, and γ = 65.36(2)°. The chelated metal centers adsorb to the surface of the Al30 clusters through hydroxyl bridges located at the central belt region of the molecule. The observed binding sites for the metal centers mirror the reactivity predicted by previously reported molecular dynamic simulations and can be identified by the acidity and hydration factor of the water group that participates in the adsorption process.

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 P21/c with unit cell parameters of a = 8.5275(5) Å, b = 12.0635(7) Å, c = 15.8825(9) Å, and β = 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 β = 112.026(3)°. [Th4(H2O)4Al10(H2O)8(OH)28(edta)4]·Cl2(H2O)29 (Th4Al10) formed the triclinic space group P with a = 9.3172(8) Å, b = 16.6099(14) Å, c = 19.5080(14) Å, α = 102.314(3)°, β = 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 P with unit cell parameters a = 10.9899(12) Å, b = 11.6107(12) Å, c = 14.3350(17) Å, α = 73.012(4)°, β = 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 Th4+, Al3+-edta systems.

The adsorption of contaminants onto metal oxide surfaces with nanoscale Keggin-type structural topologies has been well established, but identification of the reactive sites and the exact binding mechanism are lacking. Polyaluminum species can be utilized as geochemical model compounds to provide molecular level details of the adsorption process. An Al30 Keggin-type species with two surface-bound Cu2+ cations (Cu2Al30-S) has been crystallized in the presence of disulfonate anions and structurally characterized by single-crystal X-ray diffraction. Density functional theory (DFT) calculations of aqueous molecular analogues for Cu2Al30-S suggest that the reactivity of Al30 toward Cu2+ and SO42– shows opposite trends in preferred adsorption site as a function of particle topology, with anions preferring the beltway and cations preferring the caps. The bonding competition was modeled using two stepwise reaction schemes that consider Cu2Al30-S formation through initial Cu2+ or SO42– adsorption. The associated DFT energetics and charge density analyses suggest that strong electrostatic interactions between SO42– and the beltway of Al30 play a vital role in governing where Cu2+ binds. The calculated electrostatic potential of Al30 provides a theoretical interpretation of the topology-dependent reactivity that is consistent with the present study as well as other results in the literature.

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 (Th4+/Al3+) polynuclear species have been synthesized under ambient conditions and structurally characterized by single-crystal X-ray diffraction. [Th2Al6(OH)14(H2O)12(hedta)2](NO3)6(H2O)12 (ThAl1) crystallizes in space group P21/c with unit cell parameters of a = 11.198(1) Å, b = 14.210(2) Å, c = 23.115(3) Å, and β = 96.375° and [Th2Al8(OH)12(H2O)10(hdpta)4](H2O)21 (ThAl2) was modeled in P1̅ with a = 13.136(4) Å, b = 14.481(4) Å, c = 15.819(4) Å, α = 78.480(9)°, β = 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.

The hydrolysis of aluminum and formation of polynuclear species, such as the Keggin-type polycations, impacts the chemical and physical properties of the resulting aluminum oxide and hydroxide materials. Despite years of study, only a handful of Keggin-type species have been identified, hampering efforts toward a molecular-level understanding of the mechanisms of condensation. To improve the crystallization of Keggin-type polyaluminum cations, a supramolecular approach using 2,6-napthalene disulfonate (2,6-NDS) was proposed herein for the isolation of novel compounds. The present study describes the successful synthesis and structural characterization of three Keggin-type polyaluminum compounds, including (Na(Al(μ4-O4)Al12(μ-OH)24(H2O))12(2,6NDS)4(H2O)13.5 (δ-Al13), (Al2(μ4-O8)(Al28(μ2-OH)56(H2O)26)(2,6NDS)8Cl2(H2O)40 (Al30), and a new polycation, (Al2(μ4-O8)(Al24(μ2-OH)50(H2O)20)(2,6NDS)6(H2O)12.4 (Al26). Additional chemical characterization of the compounds, particularly 27Al-NMR, suggests that identifying the Al26 polycation in aqueous solutions may be difficult due to structural similarities to the δ-Al13 moiety. The structural characterization of novel Keggin-type aluminum polycations is important for a complete understanding of aluminum hydrolysis in aqueous solutions, and organosulfonates represent a viable approach for the crystallization of new polynuclear species.

•210Po accumulates in lake bottom sediments downstream of water treatment facility.•210Po approximately 10-fold higher than parent 238U.•Increased levels of 210Po appear to be a natural phenomenon.•More studies are needed to understand the bioaccumulation pathways of 210Po.Coal is an integral part of global energy production; however, coal mining is associated with numerous environmental health impacts. It is well documented that coal-mine waste can contaminate the environment with naturally-occurring radionuclides from the uranium-238 (238U) decay series. However, the behavior of the final radionuclide in the 238U-series, i.e., polonium-210 (210Po) arising from coal-mine waste-water discharge is largely unexplored. Here, results of a year-long (2014–2015) field study, in which the concentrations of 210Po in sediments and surface water of a lake that receives coal-mine waste-water discharge in West Virginia are presented. Initial measurements identified levels of 210Po in the lake sediments that were in excess of that which could be attributed to ambient U-series parent radionuclides; and were indicative of discharge site contamination of the lake ecosystem. However, control sediment obtained from a similar lake system in Iowa (an area with no coal mining or unconventional drilling) suggests that the levels of 210Po in the lake are a natural phenomenon; and are likely unrelated to waste-water treatment discharges. Elevated levels of 210Po have been reported in lake bottom sediments previously, yet very little information is available on the radioecological implications of 210Po accumulation in lake bottom sediments. The findings of this study suggest that (Monthly Energy Review, 2016) the natural accumulation and retention of 210Po in lake sediments may be a greater than previously considered (Chadwick et al., 2013) careful selection of control sites is important to prevent the inappropriate attribution of elevated levels of NORM in lake bottom ecosystems to industrial sources; and (Van Hook, 1979) further investigation of the source-terms and potential impacts on elevated 210Po in lake-sediment ecosystems is warranted.

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 P21/c with unit cell parameters of a = 8.5275(5) Å, b = 12.0635(7) Å, c = 15.8825(9) Å, and β = 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 β = 112.026(3)°. [Th4(H2O)4Al10(H2O)8(OH)28(edta)4]·Cl2(H2O)29 (Th4Al10) formed the triclinic space group P with a = 9.3172(8) Å, b = 16.6099(14) Å, c = 19.5080(14) Å, α = 102.314(3)°, β = 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 P with unit cell parameters a = 10.9899(12) Å, b = 11.6107(12) Å, c = 14.3350(17) Å, α = 73.012(4)°, β = 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 Th4+, Al3+-edta systems.

Uranyl citrate forms trimeric species at pH > 5.5, but exact structural characteristics of these important oligomers have not previously been reported. Crystallization and structural characterization of the trimers suggests the self-assembly of the 3:3 and 3:2 U:Cit complexes into larger sandwich and macrocyclic molecules. Raman spectroscopy and ESI-MS have been utilized to investigate the relative abundance of these species in solution under varying pH and citrate concentrations. Additional dynamic light scattering experiments indicate that self-assembly of the larger molecules does occur in aqueous solution.

The structural chemistry of Group 13 polyoxometalates lags far behind related negatively charged transition metal species and limits the development of advanced materials. A novel heterometallic cluster [Ga2Al18O8(OH)36(H2O)12]8+ (Ga2Al18) has been isolated using a supramolecular approach and structurally characterized using single-crystal X-ray diffraction. Ga2Al18 represents the Wells–Dawson structure polycations and variations in the structural topology may be related to the initial stabilization of the Keggin isomer. DFT calculations on the related ε-Keggins (GaAl12 and Al13), Ga2Al18, and theoretical Al2Al18 clusters reveal similar features of electronic structure, suggesting additional heteroatom substitution in other isostructural clusters should be possible.

Organic acids are important metal chelators in environmental systems and tend to form soluble complexes in aqueous solutions, ultimately influencing the transport and bioavailability of contaminants in surface and subsurface waters. This is particularly true for the formation of uranyl citrate complexes, which have been utilized in advanced photo- and bioremediation strategies for soils contaminated with nuclear materials. Given the complexity of environmental systems, the formation of ternary or heterometallic uranyl species in aqueous solutions are also expected, particularly with Al(III) and Fe(III) cations. These ternary forms are reported to be more stable in aqueous solutions, potentially enhancing contaminant mobility and uptake by organisms, but the exact coordination geometries of these soluble molecular complexes have not been elucidated. To provide insight into the nature of these species, we have developed a series of geochemical model compounds ([(UO2)2Al2(C6H4O7)4]6− (U2Al2), [(UO2)2Fe2(C6H4O7)4]6− (U2Fe2-1) and [(UO2)2Fe2(C6H4O7)4(H2O)2]6− (U2Fe2-2) and [(UO2)2Fe4(OH)4(C6H4O7)4]8− (U2Fe4)) that were characterized by single-crystal X-ray diffraction and vibrational spectroscopy. Mass spectroscopy was then employed to compare the model compounds to species present in aqueous solutions to provide an enhanced understanding of the ternary uranyl citrate complexes that could be relevant in natural systems.