Reduction of Np(VI) and Pu(VI) by Organic Chelating Agents

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 β = 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 β° = 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 β° = 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.

Influencing Bonding Interactions of the Neptunyl (V, VI) Cations with Electron-Donating and -Withdrawing Groups

Neptunium makes up the largest percentage of minor actinides found in spent nuclear fuel, yet separations of this element have proven difficult due to its rich redox chemistry. Developing new reprocessing techniques should rely on understanding how to control the Np oxidation state and its interactions with different ligands. Designing new ligands for separations requires understanding how to properly tune a system toward a desired trait through functionalization. Emerging technologies for minor actinide separations focus on ligands containing carboxylate or pyridine functional groups, which are desirable due to their high degree of functionalization. Here, we use DFT calculations to study the interactions of carboxylate and polypyridine ligands with the neptunyl cation [Np(V/VI)O2]+/2+. A systematic study is performed by varying the electronic properties of the carboxylate and polypyridine ligands through the inclusion of different electron-withdrawing and electron-donating R groups. We focus on how these groups can affect geometric properties, electronic structure, and bonding characterization as a function of the metal oxidation state and ligand character and discuss how these factors can play a role in neptunium ligand design principles.

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 ≤ pH_m ≤ 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 pH_m ≈ 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. pH_m), 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 Pu^VO₂(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.

Exploring the kinetics of actinyl-EDTA reduction by ferrous iron using quantum-mechanical calculations

The reduction of An(vi) (An = U, Np, and Pu) to An(iv) significantly decreases its solubility and mobility. This reaction can be hindered by complexation with inorganic (e.g., carbonate) or organic ligands. Ethylenediaminetetraacetic acid (EDTA) is one such organic ligand that forms stable complexes with actinides. Therefore, it may enhance the mobility of actinides. However, the redox kinetics and mechanisms of actinyl (An(v/vi)O2+/2+)-EDTA are not well characterized yet and are thus studied here using quantum-mechanical calculations. The principle is to approach the actinyl-EDTA and Fe2+ (reductant) in small incremental steps and calculate the system energy at each distance. The overall reaction is then delineated into sub-processes (encounter frequency in bulk solution, formation of outer-sphere complex, transition from outer- to inner-sphere complex, and electron transfer), and reaction rates are determined for each sub-process. The formation of outer-sphere complexes occurs rapidly in microseconds to seconds over a wide range of actinyl concentrations (pM to μM); in contrast, the transition to the inner-sphere complex is relatively slow (milliseconds to a few seconds). Immediate electron transfer to form the pentavalent actinide is observed along the reaction path for Np(vi) and Pu(vi), but not for U(vi). Surprisingly, in acidic conditions, one of the carboxylic groups gets protonated in EDTA of [UO2(edta)]2- rather than one of the amino groups. This process-based series of calculations can be applied to any redox reaction and allows the prediction of changes to the rate law and rate-limiting step in a more fundamental way for different environments.

Plutonium Oxidation States in the Waste Isolation Pilot Plant Repository

The Waste Isolation Pilot Plant (WIPP), a deep geologic repository located 660 meters underground in bedded salt, is designed to isolate U.S. defense-related transuranic waste from the accessible environment. Plutonium isotopes are the most important radionuclides in WIPP waste. Plutonium solubility in WIPP brines (ionic strengths from 5.3 to 7.4) is strongly dependent on its oxidation state, with much lower solubilities associated with Pu(III) and Pu(IV) than with the higher Pu(V) and Pu(VI) oxidation states. The large quantity of metallic iron in WIPP waste and waste containers is expected to undergo anoxic corrosion, producing strongly reducing conditions and high hydrogen gas pressures after repository closure and brine intrusion. Because reducing conditions will prevail in the WIPP repository, the most important long-term oxidation states will be Pu(III) and Pu(IV). We performed a literature review to evaluate the effects of WIPP chemical and physical processes (not colloidal) on plutonium oxidation states that included reactions with reducing agents such as iron solids and aqueous species and radiolysis of solids and aqueous species. The results of this review indicate that equilibrium between Pu(III) solids and aqueous species will control dissolved plutonium concentrations in WIPP brines. We also performed geochemical modeling calculations using the ThermoChimie database to support this assessment of plutonium oxidation states in the long-term WIPP repository. Control of plutonium solubilities by Pu(III) solid instead of Pu(IV) solid may lead to higher predicted plutonium concentrations in brines potentially released to the ground surface by an inadvertent drilling intrusion into the long-term WIPP repository. The results of this study demonstrate that Pu(III) solid solubilities provide a reasonable upper bound for dissolved plutonium concentrations in WIPP brines.

Impacts of oxo interactions within actinyl metal organic materials: highlight on thermal expansion behaviour

Physical properties of actinyl materials are influenced by the presence of oxo functional groups. Herein, we report large thermal expansion coefficients for a uranyl metal organic nanotube that switch from positive to negative upon dehydration. Different behaviour is observed in the neptunyl system due to variations in the oxo interactions.

Interactions of Plutonium with Pseudomonas sp. Strain EPS-1W and Its Extracellular Polymeric Substances

Safe and effective nuclear waste disposal, as well as accidental radionuclide releases, necessitates our understanding of the fate of radionuclides in the environment, including their interaction with microorganisms. We examined the sorption of Pu(IV) and Pu(V) to Pseudomonas sp. strain EPS-1W, an aerobic bacterium isolated from plutonium (Pu)-contaminated groundwater collected in the United States at the Nevada National Security Site (NNSS) in Nevada. We compared Pu sorption to cells with and without bound extracellular polymeric substances (EPS). Wild-type cells with intact EPS sorbed Pu(V) more effectively than cells with EPS removed. In contrast, cells with and without EPS showed the same sorption affinity for Pu(IV). In vitro experiments with extracted EPS revealed rapid reduction of Pu(V) to Pu(IV). Transmission electron microscopy indicated that 2- to 3-nm nanocrystalline Pu(IV)O₂ formed on cells equilibrated with high concentrations of Pu(IV) but not Pu(V). Thus, EPS, while facilitating Pu(V) reduction, inhibit the formation of nanocrystalline Pu(IV) precipitates.

IMPORTANCE

Our results indicate that EPS are an effective reductant for Pu(V) and sorbent for Pu(IV) and may impact Pu redox cycling and mobility in the environment. Additionally, the resulting Pu morphology associated with EPS will depend on the concentration and initial Pu oxidation state. While our results are not directly applicable to the Pu transport situation at the NNSS, the results suggest that, in general, stationary microorganisms and biofilms will tend to limit the migration of Pu and provide an important Pu retardation mechanism in the environment. In a broader sense, our results, along with a growing body of literature, highlight the important role of microorganisms as producers of redox-active organic ligands and therefore as modulators of radionuclide redox transformations and complexation in the subsurface.

Collection of lanthanides and actinides from natural waters with conventional and nanoporous sorbents

Effective collection of trace-level lanthanides and actinides is advantageous for recovery and recycling of valuable resources, environmental remediation, chemical separations, and in situ monitoring. Using isotopic tracers, we have evaluated a number of conventional and nanoporous sorbent materials for their ability to capture and remove selected lanthanides (Ce and Eu) and actinides (Th, Pa, U, and Np) from fresh and salt water systems. In general, the nanostructured materials demonstrated a higher level of performance and consistency. Nanoporous silica surface modified with 3,4-hydroxypyridinone provided excellent collection and consistency in both river water and seawater. The MnO(2) materials, in particular the high surface area small particle material, also demonstrated good performance. Other conventional sorbents typically performed at levels below the nanostructured sorbents and demonstrate a larger variability and matrix dependency.

Redox behavior of Ce(IV)/Ce(III) in the presence of nitrilotriacetic acid: A surrogate study for An(IV)/An(III) redox behavior

Using cyclic voltammetry, we investigated the redox behavior of Ce(IV)/Ce(III), which is a surrogate for An(IV)/An(III) (An=actinides), in a solution of nitrilotriacetic acid (NTA) at 25 °C. The cyclic voltammogram of Ce in a 0.1 M NTA solution at pH 6 showed a reversible one-electron redox reaction for Ce(IV)/Ce(III) at 0.51 V vs. Ag/AgCl. This redox potential was much lower than that obtained in 1 M nitric acid, indicating that Ce(IV) was preferentially stabilized by complexation with NTA. The redox potential in the NTA solution was independent of the Ce concentration from 2 to 20 mM, NTA concentration from 5 to 200 mM and pH between 3 and 7. These results indicated that no polymerization and no additional coordination of NTA and OH− to the Ce(III)-NTA complex took place during the redox reaction. As the speciation calculation of Ce(III) in the NTA solution showed that the predominant species was CeIII(nta)2 3− (H3nta=NTA), the redox reaction of the Ce-NTA complex was expressed by the following: CeIV(nta)2 2−+e−⇋CeIII(nta)2 3−. The logarithm of the stability constant of CeIV(nta)2 2− was calculated to be 38.6±0.8 for I=0 from the redox potential shift of Ce(IV)/Ce(III) in the NTA solution. The value was in good accordance with the stability constant of the NpIV(nta)2 2− complex, demonstrating that the aqueous coordination chemistry of Ce(IV) with NTA is quite similar to that of An(IV). These results strongly suggest that a negative shift of the Pu(IV)/Pu(III) redox potential in the NTA solution should make Pu(IV) more stable than Pu(III) even in a reducing environment.

Reduction of plutonium(VI) in brine under subsurface conditions

The redox stability of PuO2 2+ was investigated in brine under subsurface conditions. In simulated brines, when no reducing agent was present, 0.1 mM concentrations of plutonium(VI) were stable as regards to reduction for over two years, which was the duration of the experiments performed. In these systems, the plutonyl existed as a carbonate or hydroxy-chloride species. The introduction of reducing agents (e.g. steel coupons, and aqueous Fe2+) typically present in a subsurface repository, however, led to the destabilization of the plutonium(VI) complexes and the subsequent reduction to Pu(IV) under most conditions investigated. X-ray Absorption Near-Edge Spectroscopy (XANES) confirmed that the final oxidation state in these systems was Pu(IV). This reduction lowered the overall steady state concentration of plutonium in the brine by 3−4 orders of magnitude. These results show the importance of considering repository constituents in evaluating subsurface actinide solubility/mobility and provide further evidence of the effectiveness of reduced iron species in the reduction and immobilization of higher-valent plutonium species.

Chemical speciation of heterogeneously reduced Pu in synthetic brines

X-ray absorption fine structure (XAFS) spectroscopy has been used to determine the speciation of Pu precipitates prepared by the heterogeneous reduction of Pu(VI) with Al and Fe in 5 M NaCl and an ERDA-6 brine, a simulant from the Waste Isolation Pilot Plant in Carlsbad, New Mexico. NaOCl was added to some of these solutions to determine its effect on Pu speciation. Analysis of the PuLIIIspectra showed that all solids consisted of PuO2+x-y(OH)2y·zH2O, compounds with characteristics identical to those prepared by hydrolysis and with Pu-O and Pu-Pu distances identical to those treated at elevated temperature. Additionally, reduction with Al gave compounds with different site distributions than reduction with Fe, and reduction with Al or the addition of NaOCl appeared to suppress the formation of oxo groups and their associated Pu(V) sites.

Prognosticating the humic complexation for redox sensitive actinides through analogy, using the charge neutralisation model

The complexation of redox sensitive elements by humic acid (HA), described through the charge neutralisation model (CNM), has been reviewed in order to have a comprehensive scope. The data acquired in HUMICS program on thorium(IV), and data available in the literature, were reinterpreted according to the CNM, and adapted to uranium(IV), neptunium(IV) and plutonium(IV) through analogy in order to draw a boundary prediction. Otherwise, available data obtained in the framework of the CNM were used, or adapted if necessary, for other redox states when the analogy is justified,i.e.Am3+-Cm3+for Pu3+, NpO2+for PuO2+and UO22+for PuO22+.

The obtained speciation diagrams indicate that, when (HA)=100 mg/L, redox sensitive actinides should be reduced to their +IV state whenEh≤650 mV/SHE for plutonium,Eh≤100 mV/SHE for neptunium andEh≤-20 mV/SHE for U. Plutonium could be present as mixtures of plutonium(III) and (IV) depending on the pH value in reducing ground waters -150 ≤Eh(mV/SHE) ≤ 150. The known reduction of neptunium(V) to neptunium(IV) in Gorleben ground waters seems also well represented, so is the stability of uranium(VI) in humic solution whenEh≥ 100 mV/SHE. Conversely, the known association of plutonium(VI) in marine systems is not satisfactorily represented, so does is the uranium behaviour under reducing conditions. Experiments under well-controlled conditions are still needed to ascertain the plutonium and uranium comportment in the presence of humic acid.

Plutonium(VI) accumulation and reduction by lichen biomass: correlation with U(VI)

The uptake of plutonium(VI) and uranium(VI) by lichen biomass was studied in the foliose lichen Parmotrema tinctorum to elucidate the migration behavior of Pu and U in the terrestrial environment. Pu and U uptake by P. tinctorum averaged 0.040+/-0.010 and 0.055+/-0.015 g gdry (-1), respectively, after 96 h incubation with 4.0 x 10(14) mol 1(-1) Pu solutions of pH 3, 4 and 5. SEM observations showed that the accumulated Pu is evenly distributed on the upper and lower surfaces of P. tinctorum, in contrast to U(VI), which accumulated in both cortical and medullary layers. UV/VIS absorption spectroscopy demonstrates that a fraction of Pu(VI) in the solution is reduced to Pu(V) by the organic substances released from P. tinctorum, and the accumulated Pu on the surface is reduced to Pu(IV), while U(VI) keeps the oxidation state of VI. Since the solubility of Pu(IV) hydroxides is very low, reduced Pu(VI) does not penetrate to the medullary layers, but is probably precipitated as Pu(IV) hydroxides on the cortical lichen surface. It is concluded that the uptake and reduction of Pu(VI) by lichens is important to determine the mobilization and oxidation states of Pu in the terrestrial environment.