Actinide Sulfate Structures from Caustic Solvents.

Four unique actinide sulfates were synthesized using solvothermal techniques with strong acids. The first plutonium(III) sulfate structure, Pu(HSO4)3, was synthesized and is isostructural with analogous lanthanide-based frameworks. A similar synthesis approach yielded crystals of NpNa0.5(HSO4)15(SO4)1.5, which has a comparable framework to the Pu(III) compound, but the neptunium metal is tetravalent and sodium is incorporated into the structure, as confirmed by chemical analysis. Anhydrous neptunium sulfate, Np(SO4)2, is reported and is isotypic with U(SO4)2. Finally, (H3O)2(UO2)(SO4)2, which contains a uranyl sulfate sheet structure, was synthesized and characterized. The corresponding sheet anion topology has previously been reported with various oxyanions, but this is the first report that contains sulfate. The sheets are charge balanced by hydronium cations in the interstitial space. This compound readily degrades and forms crystals of the synthetic analogue to the uranium mineral shumwayite, which is likely thermodynamically favorable. All four of these actinide sulfate compounds were synthesized in extremely acidic media, resulting in interesting and unique structures.

Structural characterization of a new samarium-sodium heterometallic coordination polymer.

Lanthanide-containing materials are of inter-est in the field of crystal engin-eering because of their unique properties and distinct structure types. In this context, a new samarium-sodium heterometallic coordination polymer, poly[tetra-kis-(µ2-2-formyl-6-meth-oxy-phenolato)samarium(III)sodium(I)], {[SmNa(C8H7O3)4]·solvent}n (Sm-1), was synthesized and crystallized via slow evaporation from a mixture of ethanol and aceto-nitrile. The compound features alternating SmIII and NaI ions, which are linked by ortho-vanillin (o-vanillin) ligands to form a mono-periodic chain-like coordination polymer. The chains propagate along the [001] direction. Residual electron density of disordered solvent mol-ecules in the void space could not be reasonably modeled, thus the SQUEEZE function was applied. The structural, vibrational, and optical properties are reported.

Plutonium and Cerium Perrhenate/Pertechnetate Coordination Polymers and Frameworks.

Spent nuclear fuel (SNF) contains transuranic and lanthanide species, which are sometimes recovered and repurposed. One particularly problematic fission product, 99TcO4-, hampers this recovery via coextraction with high valence metals, perhaps by complexation during aqueous reprocessing of SNF. There is limited molecular-level knowledge concerning the coordination chemistry between TcO4- or its well-known surrogate ReO4- and transuranic/lanthanide species. In the current study, we investigated the coordination of ReO4-/TcO4- with plutonium and cerium cations by structural and chemical characterization of a series of isolated extended solids. In this study, Ce represents both trivalent lanthanides and is considered a surrogate for Pu, respectively, in its common trivalent and tetravalent oxidation states. The structural elucidation of the seven isolated crystalline solids revealed that ReO4-/TcO4- directly connects to PuIV, PuVIO22+, CeIII, and CeIV in the terminal and bridging coordination modes, leading to 1-, 2-, and 3-dimensional frameworks. For example, ReO4- coordination to Pu(IV) formed a 1D chain or 2D framework, isostructural with previously isolated Th(IV) compounds. However, PuVIO22+ alternating with ReO4- led to a unique 1D chain, different from the prior-reported U(VI)/Np(VI)-ReO4-/TcO4- structures. Coordination of ReO4-/TcO4- with Ce(III) promotes the assembly of 3D frameworks. Finally, attempted synthesis of a Ce(IV)-ReO4- compound resulted in a 2D framework with a mixed-valence CeIII/IV. The highly acidic reaction conditions supported the reduction of both CeIV and TcVII, challenging isolation of compounds featuring these species. Only one TcO4-containing structure was obtained in this study (CeIII-TcO4 3D framework), vs the six total Ce/Pu-ReO4 compounds. Our three Pu-ReO4 crystal structures are the first reported and translated to atomic-level information about Pu-TcO4 coordination in nuclear fuel reprocessing scenarios, in addition to broadening our knowledge of bonding trends in the early, high-valence actinides.

Pu(VI) Oxalate Crystal Structure and Evidence of Photoreduction to Pu(IV) Oxalate.

We report the first crystal structure of a Pu(VI)-oxalate compound. This compound, [PuO2(C2O4)(H2O)]·2(H2O) (1), crystallizes in space group P21/c with a = 5.5993(3) Å, b = 16.8797(12) Å, c = 9.3886(6) Å, and ß = 98.713(6)°. It is isostructural with the previously reported U(VI) compound, [UO2(C2O4)(H2O)]·2(H2O). Each plutonyl ion (PuO22+) is coordinated in the equatorial plane by two side-on bidentate oxalates, creating an infinite chain along [001]. A coordinated water molecule and twisting of the oxalates lead to a distorted pentagonal bipyramidal geometry of the Pu. A photochemical degradation was observed for 1, which resulted in the formation of a secondary crystalline phase. The absorption spectrum of this secondary phase confirmed the presence of Pu(IV), but it did not match the spectrum of Pu(C2O4)2·6H2O, which is considered to be the primary product of Pu-oxalate precipitation. While compound 1 has previously been proposed to exist in solution, this is the first time it has been isolated via crystallization. Although redox interactions between Pu and oxalate have been documented in the literature, the present study is the first observation of a photochemical reduction of Pu(VI)-oxalate. As a result, this study has expanded on the limited understanding of the Pu(VI)-oxalate system, which is important for nuclear fuel cycle applications.

A spectrophotometric study of the impact of pH and metal-to-ligand ratio on the speciation of the Pu(VI)-oxalate system.

The oxalate ligand is prevalent throughout the nuclear fuel cycle. While the Pu(III)- and Pu(IV)-oxalate systems are well studied due to their use in plutonium metal and PuO2 production, the effect of oxalate on Pu(VI) remains understudied. Absorption spectroscopy was employed to probe the solution behavior of the Pu(VI)-oxalate system as a function of pH (1, 3, 7) and metal-to-ligand ratio (M/L; 10 : 1-1 : 10). Peak changes in the UV-vis-NIR spectra were associated with the formation of multiple Pu(VI)-oxalate species with increasing oxalate concentration. Some insight into identification of species present in solution was gained from the limited Pu(VI)-oxalate literature and comparisons with the assumed isostructural U(VI)-oxalate system. A peak in the UV-vis-NIR spectrum at 839 nm, which corresponds to the formation of a 1 : 1 PuO2(C2O4)(aq) complex, was observed and used to determine the formation constant (log ß° = 4.64 ± 0.06). A higher coordinated Pu(VI)-oxalate peak at 846 nm was tentatively assigned as the 1 : 2 complex PuO2(C2O4)22- and a preliminary formation constant was determined (log ß° = 9.30 ± 0.08). The predominance of both complexes was shown in speciation diagrams calculated from the formation constants, illustrating the importance of considering the Pu(VI)-oxalate system in the nuclear fuel cycle.

The crystal chemistry of plutonium(IV) borophosphate.

In this work, we report the synthesis and characterization of a plutonium(IV) borophosphate, Pu(H2O)3[B2(OH)(H2O)(PO4)3] (1). The basic building unit of 1 has a B : P ratio of 2 : 3 with an equal number of BO4 and PO4 groups that assemble into 12-membered rings and take on a sheet topology due to presence of hydroxyl groups or a water molecule on one vertex of each BO4 tetrahedron. This unique borophosphate anion topology is not observed in other members of the borophosphate family; it is the plutonium(IV) metal centers, rather than borate or phosphate groups, that link the sheets to form an extended framework. The presence of boron in 1 was confirmed using single crystal X-ray diffraction, electron microprobe analysis, and infrared spectroscopy. Peaks corresponding to the tetrahedral BO45- and tetrahedral PO43- anions were all identified in the fingerprint region (500-1500 cm-1) of the infrared spectrum. Additionally, peaks in the higher wavenumber region corresponded to crystalline water and B-OH vibrations, providing further evidence for the water molecules surrounding plutonium in the structure and the protonation of the BO4 tetrahedron, respectively. This compound represents the first Pu(IV) borophosphate structure and a novel borophosphate anion topology. Furthermore, the long time-frame required for crystallization of 1 and the suspected leaching of boron from the borosilicate vial used during synthesis indicate that 1 could serve as a model for the crystalline materials that are expected to form during the corrosion of vitrified nuclear waste.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Invited for the cover of this issue is the group of Amy Hixon at the University of Notre Dame. The image depicts the newly identified structure of a PuIV oxalate sheet compared to the historically assumed structure. Read the full text of the article at 10.1002/chem.202301164.

Eu(III) and Am(III) adsorption on aluminum (hydr)oxide minerals: surface complexation modeling.

Americium is a highly radioactive actinide element found in used nuclear fuel. Its adsorption on aluminum (hydr)oxide minerals is important to study for at least two reasons: (i) aluminum (hydr)oxide minerals are ubiquitous in the subsurface environment and (ii) bentonite clays, which are proposed engineered barriers for the geologic disposal of used nuclear fuel, have the same ≡AlOH sites as aluminum (hydr)oxide minerals. Surface complexation modeling is widely used to interpret the adsorption behavior of heavy metals on mineral surfaces. While americium sorption is understudied, multiple adsorption studies for europium, a chemical analog, are available. In this study we compiled data describing Eu(III) adsorption on three aluminum (hydr)oxide minerals-corundum (α-Al2O3), γ-alumina (γ-Al2O3) and gibbsite (γ-Al(OH)3)-and developed surface complexation models for Eu(III) adsorption on these minerals by employing diffuse double layer (DDL) and charge distribution multisite complexation (CD-MUSIC) electrostatic frameworks. We also developed surface complexation models for Am(III) adsorption on corundum (α-Al2O3) and γ-alumina (γ-Al2O3) by employing a limited number of Am(III) adsorption data sourced from literature. For corundum and γ-alumina, two different adsorbed Eu(III) species, one each for strong and weak sites, were found to be important regardless of which electrostatic framework was used. The formation constant of the weak site species was almost 10,000 times weaker than the formation constant for the corresponding strong site species. For gibbsite, two different adsorbed Eu(III) species formed on the single available site type and were important for the DDL model, whereas the best-fit CD-MUSIC model for Eu(III)-gibbsite system required only one Eu(III) surface species. The Am(III)-corundum model based on the CD-MUSIC framework had the same set of surface species as the Eu(III)-corundum model. However, the log K values of the surface reactions were different. The best-fit Am(III)-corundum model based on the DDL framework had only one site type. Both the CD-MUSIC and the DDL model developed for Am(III)-γ-alumina system only comprised of one site type and the formation constant of the corresponding surface species was ~ 500 times stronger and ~ 700 times weaker than the corresponding Eu(III) species on the weak and the strong sites, respectively. The CD-MUSIC model for corundum and both the DDL and the CD-MUSIC models for γ-alumina predicted the Am(III) adsorption data very well, whereas the DDL model for corundum overpredicted the Am(III) adsorption data. The root mean square of errors of the DDL and CD-MUSIC models developed in this study were smaller than those of two previously-published models describing Am(III)-γ-alumina system, indicating the better predictive capacity of our models. Overall, our results suggest that using Eu(III) as an analog for Am(III) is practical approach for predicting Am(III) adsorption onto well-characterized minerals.

Insight into the Structural Ambiguity of Actinide(IV) Oxalate Sheet Structures: A Case for Alternate Coordination Geometries.

Plutonium(IV) oxalate hexahydrate (Pu(C2 O4 )2 ⋅ 6 H2 O; PuOx) is an important intermediate in the recovery of plutonium from used nuclear fuel. Its formation by precipitation is well studied, yet its crystal structure remains unknown. Instead, the crystal structure of PuOx is assumed to be isostructural with neptunium(IV) oxalate hexahydrate (Np(C2 O4 )2 ⋅ 6 H2 O; NpOx) and uranium(IV) oxalate hexahydrate (U(C2 O4 )2 ⋅ 6 H2 O; UOx) despite the high degree of unresolved disorder that exists when determining water positions in the crystal structures of the latter two compounds. Such assumptions regarding the isostructural behavior of the actinide elements have been used to predict the structure of PuOx for use in a wide range of studies. Herein, we report the first crystal structures for PuOx and Th(C2 O4 )2 ⋅ 6 H2 O (ThOx). These data, along with new characterization of UOx and NpOx, have resulted in the full determination of the structures and resolution of the disorder around the water molecules. Specifically, we have identified the coordination of two water molecules with each metal center, which necessitates a change in oxalate coordination mode from axial to equatorial that has not been reported in the literature. The results of this work exemplify the need to revisit previous assumptions regarding fundamental actinide chemistry, which are heavily relied upon within the current nuclear field.

Crystallization of a Neptunyl Oxalate Hydrate from Solutions Containing NpV and the Uranyl Peroxide Nanocluster U60 Ox30.

Uranyl peroxide nanoclusters are an evolving family of materials with potential applications throughout the nuclear fuel cycle. While several studies have investigated their interactions with alkali and alkaline earth metals, no studies have probed their interactions with the actinide elements. This work describes a system containing U60 Ox30 , [((UO2 )(O2 ))60 (C2 O4 )30 ]60- , and neptunium(V) as a function of neptunium concentration. Ultra-small and small angle X-ray scattering were used to observe these interactions in the aqueous phase, and X-ray diffraction was used to observe solid products. The results show that neptunium induces aggregation of U60 Ox30 when the neptunium concentration is≤10 mM, whereas (NpO2 )2 C2 O4 ⋅ 6H2 O(cr) and studtite ultimately form at 15-25 mM neptunium. The latter result suggests that neptunium coordinates with the bridging oxalate ligands in U60 Ox30 , leaving metastable uranyl peroxide species in solution. This is an important finding given the potential application of uranyl peroxide nanoclusters in the recycling of used nuclear fuel.

Leveraging Nitrogen Linkages in the Formation of a Porous Thorium-Organic Nanotube Suitable for Iodine Capture.

We report the synthesis, characterization, and iodine capture application of a novel thorium-organic nanotube, TSN-626, [Th6O4(OH)4(C6H4NO2)7(CHO2)5(H2O)3]·3H2O. The classification as a metal-organic nanotube (MONT) distinguishes it as a rare and reduced dimensionality subset of metal-organic frameworks (MOFs); the structure is additionally hallmarked by low node connectivity. TSN-626 is composed of hexameric thorium secondary building units and mixed O/N-donor isonicotinate ligands that demonstrate selective ditopicity, yielding both terminating and bridging moieties. Because hard Lewis acid tetravalent metals have a propensity to bind with electron donors of rival hardness (e.g., carboxylate groups), such Th-N coordination in a MOF is uncommon. However, the formation of key structural Th-N bonds in TSN-626 cap some of the square antiprismatic metal centers, a position usually occupied by terminal water ligands. TSN-626 was characterized by using complementary analytical and computational techniques: X-ray diffraction, vibrational spectroscopy, N2 physisorption isotherms, and density functional theory. TSN-626 satisfies design aspects for the chemisorption of iodine. The synergy between accessibility through pores, vacancies at the metal-oxo nodes, and pendent N-donor sites allowed a saturated iodine loading of 955 mg g-1 by vapor methods. The crystallization of TSN-626 diversifies actinide-MOF linker selection to include soft electron donors, and these Th-N linkages can be leveraged for the investigation of metal-to-ligand bonding and unconventional topological expressions.

Pu(iii) and Cm(iii) in the presence of EDTA: aqueous speciation, redox behavior, and the impact of Ca(ii).

The impact of calcium on the solubility, redox behavior, and speciation of the An(iii)-EDTA (An = Pu or Cm) system under reducing, anoxic conditions was investigated through batch solubility experiments, X-ray absorption spectroscopy (XAS), density functional theory (DFT), and time-resolved laser fluorescence spectroscopy (TRLFS). Batch solubility experiments were conducted from undersaturation using Pu(OH)3(am) as the solid phase in contact with 0.1 M NaCl-NaOH-HCl-EDTA-CaCl2 solutions at [EDTA] = 1 mM, pHm = 7.5-9.5, and [CaCl2] ≤20 mM. Additional samples targeted brine systems represented by 3.5 M CaCl2 and WIPP simulated brine. Solubility data in the absence of calcium were well-described by Pu(iii)-EDTA thermodynamic models, thus supporting the stabilization of Pu(iii)-EDTA complexes in solution. Cm(iii)-EDTA TRLFS data suggested the stepwise hydrolysis of An(iii)-EDTA complexes with increasing pH, and current Pu(iii)-EDTA solubility models were reassessed to evaluate the possibility of including Pu(iii)-OH-EDTA complexes and to calculate preliminary formation constants. Solubility data in the presence of calcium exhibited nearly constant log m(Pu)tot, as limited by total ligand concentration, with increasing [CaCl2]tot, which supports the formation of calcium-stabilized Pu(iii)-EDTA complexes in solution. XAS spectra without calcium showed partial oxidation of Pu(iii) to Pu(iv) in the aqueous phase, while calcium-containing experiments exhibited only Pu(iii), suggesting that Ca-Pu(iii)-EDTA complexes may stabilize Pu(iii) over short timeframes (t ≤45 days). DFT calculations on the Ca-Pu(iii)-EDTA system and TRLFS studies on the analogous Ca-Cm(iii)-EDTA system show that calcium likely stabilizes An(iii)-EDTA complexes but can also potentially stabilize An(iii)-OH-EDTA species in solution. This hints towards the possible existence of four major complex types within Ca-An(iii)-EDTA systems: An(iii)-EDTA, An(iii)-OH-EDTA, Ca-An(iii)-EDTA, and Ca-An(iii)-OH-EDTA. While the exact stoichiometry and degree of ligand protonation within these complexes remain undefined, their formation must be accounted for to properly assess the fate and transport of plutonium under conditions relevant to nuclear waste disposal.

Snapshots of Ce70 Toroid Assembly from Solids and Solution.

Crystallization at the solid-liquid interface is difficult to spectroscopically observe and therefore challenging to understand and ultimately control at the molecular level. The Ce70-torroid formulated [CeIV70(OH)36(O)64(SO4)60(H2O)10]4-, part of a larger emerging family of MIV70-materials (M = Zr, U, Ce), presents such an opportunity. We elucidated assembly mechanisms by the X-ray scattering (small-angle scattering and total scattering) of solutions and solids as well as crystallizing and identifying fragments of Ce70 by single-crystal X-ray diffraction. Fragments show evidence for templated growth (Ce5, [Ce5(O)3(SO4)12]10-) and modular assembly from hexamer (Ce6) building units (Ce13, [Ce13(OH)6(O)12(SO4)14(H2O)14]6- and Ce62, [Ce62(OH)30(O)58(SO4)58]14-). Ce62, an almost complete ring, precipitates instantaneously in the presence of ammonium cations as two torqued arcs that interlock by hydrogen boding through NH4+, a structural motif not observed before in inorganic systems. The room temperature rapid assemblies of both Ce70 and Ce62, respectively, by the addition of Li+ and NH4+, along with ion-exchange and redox behavior, invite exploitation of this emerging material family in environmental and energy applications.

Impact of Ca(II) on the aqueous speciation, redox behavior, and environmental mobility of Pu(IV) in the presence of EDTA.

The impact of calcium on the solubility and redox behavior of the Pu(IV)-EDTA system was investigated using a combination of undersaturation solubility studies and advanced spectroscopic techniques. Batch solubility experiments were conducted in 0.1 M NaCl-NaOH-HCl-EDTA-CaCl2 solutions at constant [EDTA] = 1∙10-3 M, 1 ≤ pHm ≤ 11, and 1∙10-3 M ≤ [CaCl2] ≤ 2∙10-2 M. Additional samples targeted brine systems represented by 3.5 M CaCl2 and WIPP simulated brine. Redox conditions were buffered with hydroquinone (pe + pH ≈ 9.5) with selected samples prepared in the absence of any redox buffer. All experiments were performed at T = 22 °C under Ar atmosphere. In-situ X-ray absorption spectroscopy indicated that PuO2(ncr,hyd) was the solubility-controlling phase during the lifetime of all experiments and that aqueous plutonium was present in the +IV oxidation state across all experimental conditions except at pHm ≈ 1, where a small fraction of Pu(III) was also identified. Current thermodynamic models overestimate Pu(IV)-EDTA solubility in the absence of calcium by approximately 1-1.5 log10-units and do not describe the nearly pH-independent, increased solubility observed with increased calcium concentrations. The ternary Pu(IV)-OH-EDTA system without calcium was reevaluated using solubility data obtained in this work and reported in the literature. An updated thermodynamic model including the complexes Pu(OH)(EDTA)-, Pu(OH)2(EDTA)2-, and Pu(OH)3(EDTA)3- was derived. Solubility data collected in the presence of calcium follows a pH-independent trend (log m(Pu)tot vs. pHm), which can only be explained by assuming the formation of a quaternary complex, tentatively defined as CaPu(OH)4(EDTA)2-, in solution. The significant enhancement of plutonium solubility observed in the investigated brine systems supports the formation of a quaternary complex that is not outcompeted by Ca(EDTA)2-, even in concentrated CaCl2 solutions. Although the exact stoichiometry of the complex may need to be revisited, this new quaternary complex has a pronounced impact on plutonium predominance diagrams over a broad range of pH, pe, and calcium concentrations that are relevant to nuclear waste disposal.

Influence of ethylenediaminetetraacetic acid on the long-term oxidation state distribution of plutonium.

Spectrophotometry was used to study the effect of EDTA on plutonium oxidation state distribution as a function of time, pH, and ligand-to-metal ratio (L/M) under anoxic conditions. Novel Pu(V)-EDTA absorption bands were identified at 571, 993, 1105, and 1150 nm with molar absorption coefficients of 15 ± 1, 6 ± 1, 10 ± 1, and 10 ± 1 cm-1M-1, respectively. Pu(V)-EDTA spectral changes occurred at L/M < 1, indicating only PuVO2(EDTA)3- formed with logK = 3.6 ± 0.3. Time-resolved experiments showed EDTA drastically increased the Pu(V/VI) reduction rate, which we propose is driven by amine lone-pair electron donation and the oxidative decarboxylation of EDTA. Oxidation of Pu(III)-EDTA to Pu(IV)-EDTA occurred on a slower time scale (110-237 days) than previously reported (<15 min) and is hypothesized to be radiolysis driven. Pu(V/VI)-EDTA and Pu(III)-EDTA both approached Pu(IV)-EDTA stabilization over time, yet Pu(V/VI)-EDTA solubility data was ≥ 1.0 log10 units higher than predicted by Pu(IV)-EDTA solubility models, indicating that current thermodynamic models are incomplete. Ultimately, the data show EDTA preferentially stabilizes Pu(IV) over time regardless of initial oxidation state, but Pu(V)-EDTA can persist under environmentally-relevant conditions, emphasizing the need to continue investigating redox reactions, speciation, and behavior of these complexes to support the transuranic waste disposal and surface remediation/containment efforts.

Quantifying surface roughness on UO2 fuel pellets using optical techniques.

Rapid, non-destructive nuclear forensic techniques can aid in signature development and provide valuable information for provenance assessments. Using optical profilometry and digital microscopy, we studied the surface roughness of fuel pellets to probe its usefulness as a forensic signature and its relationship to a given producer's grinding techniques. Arithmetic average areal (Sa) surface roughness measurements provide a rapid, non-destructive technique, producing efficient measurements with smaller standard uncertainties relative to 2D, arithmetic average profile (Ra) surface roughness measurements. Digital microscopy proved to be the superior technique over optical profilometry, in part due to its higher image quality, faster data acquisition capabilities, and multi-purpose potential in physical surface characterization. Using digital microscopy, fuel pellet Sa surface roughness varies in commercial reactor fuel pellets from 1.54±0.17µm to 2.11±0.12µm and does not appear to depend solely on the use of wet versus dry grinding techniques. Populations of pellets produced at three different commercial reactor fuel production facilities were distinguishable on the basis of Sa. Complementary to other key forensic characteristics, such as dimensions and enrichment, Sa measurements provide a promising nuclear forensic signature for sintered UO2 fuel pellets.

Advancement of Actinide Metal-Organic Framework Chemistry via Synthesis of Pu-UiO-66.

We report the synthesis and characterization of the first plutonium metal-organic framework (MOF). Pu-UiO-66 expands the established UiO-66 series, which includes transition metal, lanthanide, and early actinide elements in the hexanuclear nodes. The thermal stability and porosity of Pu-UiO-66 were experimentally determined, and multifaceted computational methods were used to corroborate experimental values, examine inherent defects in the framework, decipher spectroscopic signatures, and elucidate the electronic structure. The crystallization of a plutonium chain side product provides direct evidence of the competition that occurs between modulator and linker in MOF syntheses. Ultimately, the synthesis of Pu-UiO-66 demonstrates adept control of Pu(IV) coordination under hydrolysis-prone conditions, provides an opportunity to extend trends across isostructural UiO-66 frameworks, and serves as the foundation for future plutonium MOF chemistry.

Experimental and Quantum Mechanical Characterization of an Oxygen-Bridged Plutonium(IV) Dimer.

We report the synthesis and characterization of K4 {[PuCl2 (NO3 )3 ]2 (µ2 -O)}⋅H2 O, which contains the first known µ2 -oxo bridge between two PuIV metal centers. Adding to its uniqueness is the Pu-(µ2 -O) bond length of 2.04 Å, which is the shortest of other analogous compounds. The Pu-(µ2 -O)-Pu bridge is characterized by the mixing of s-, d-, and p-orbitals from Pu with the p-orbitals of O; the 5f-orbitals do not participate in bonding. Natural bond orbital analysis indicates that Pu and O interact through one 3c-2e σPu-O-Pu and two 3c-2e πPu-O-Pu bonding orbitals and that the electron density is highly polarized on the µ2 -O. Bond topology properties analysis indicates that the Pu-(µ2 -O) bond shares both ionic and covalent character. Quantum mechanical calculations also show that the dimer has multiconfigurational ground states, where the nonet, septet, quintet, triplet, and singlet are close in energy. This work demonstrates the interplay between experimental and computational efforts that is required to understand the chemical bonding of Pu compounds.

Evidence for non-electrostatic interactions between a pyrophosphate-functionalized uranyl peroxide nanocluster and iron (hydr)oxide minerals.

The terminal oxygen atoms of the pyrophosphate groups in the uranyl peroxide nanocluster U24Pp12 ([(UO2)24(O2)24(P2O7)12]48-) are not fully satisfied by bond valence considerations and can become protonated. This functionality could allow for specific interactions with mineral surfaces, as opposed to the electrostatically-driven interactions observed between non-functionalized uranyl peroxide nanoclusters and mineral surfaces. The sorption of U24Pp12 to goethite and hematite was studied using batch sorption experiments as a function of U24Pp12 concentration, mineral concentration, and pH. A suite of spectroscopic techniques, scanning electron microscopy, and electrophoretic mobility measurements were used to examine the minerals before and after reaction with U24Pp12, leading to a proposed conceptual model for U24Pp12 interactions with goethite. The governing rate laws were determined and compared to those previously determined for a non-functionalized uranyl peroxide nanocluster. The rate of uranyl peroxide nanocluster sorption depends on the charge density and functionalized component of the uranyl peroxide cage. Electrophoretic mobility and attenuated total reflectance Fourier transform infrared spectroscopy analyses show that an inner-sphere complex forms between the U24Pp12 cluster and the goethite surface through the terminal pyrophosphate groups, leading to a proposed conceptual model in which U24Pp12 interacts with the triply-coordinated reactive sites on the (110) plane of goethite. These results demonstrate that the behavior of U24Pp12 at the iron (hydr)oxide-water interface is unique relative to interactions of the uranyl ion and non-functionalized uranyl peroxide nanoclusters.

Kinetics of neptunium sorption and desorption in the presence of aluminum (hydr)oxide minerals: Evidence for multi-step desorption at low pH.

Kinetics analyses of sorption and desorption provide important insight into reaction mechanisms occurring at the mineral-water interface. They are also needed to determine when equilibrium is achieved, identify intermediate chemical species, and inform models describing neptunium mobility. Neptunium sorption to and desorption from four different aluminum (hydr)oxides - bayerite (α-Al(OH)3), gibbsite (γ-Al(OH)3), corundum (α-Al2O3), and γ-alumina (γ-Al2O3) - were investigated as a function of mineral concentration (5 - 170 m2 L-1), neptunium concentration (10-9 - 10-7 M), and pH (5.5 - 10.5). Neptunium sorption was characterized by a two-step reaction with an initial fast sorption step occurring within minutes followed by a slower equilibrium process, which was attributed to initial sorption of neptunium to a small number of strong sorption sites followed by sorption of neptunium to a larger number of weak sorption sites. The kinetics data were modeled using the linear and non-linear forms of the pseudo-first and pseudo-second order rate equations and the goodness of fit parameters were compared. Non-linear pseudo-second order rate constants described neptunium sorption to aluminum (hydr)oxides most accurately and were used to determine the reaction orders with respect to mineral concentration and [H+]. Neptunium desorption experiments demonstrated that the desorption mechanism changed as a function of pH and that the forward and reverse reactions were not equivalent. At pH ≥ 7.5, desorption reached steady-state within an hour and was accurately described by the non-linear pseudo-second order rate equations. A desorption plateau was observed at pH 5.5 that could not be described by either pseudo-first or -second order kinetics, suggesting the possibility of a multi-step desorption reaction. The comparatively slow desorption kinetics observed here suggests that sorbed neptunium could be slowly released back into the aqueous phase and act as a continuous source of contamination to the environment.