Neptunium Species Research Articles

Insights into the Mechanism of Neptunium Oxidation to the Heptavalent State.

Neptunium can exist in multiple oxidation states, including the rare and poorly understood heptavalent form. In this work, we monitored the formation of heptavalent neptunium [Np(VII)O4(OH)2]3- during ozonolysis of aqueous MOH (M=Li, Na, K) solutions using a combined experimental and theoretical approach. All experimental reactions were closely monitored via absorption and vibrational spectroscopy to follow both the oxidation state and the speciation of neptunium guided by the calculated vibrational frequencies for various neptunium species. The mechanism of the reaction partly involves oxidative dissolution of transient Np(VI) oxide/hydroxide solid phases, the identity of which are dependent on the co-precipitating counter-cation Li+/Na+/K+. Additional calculations suggest that the most favorable energetic pathway occurs through the reaction of a [Np(V)O2(OH)4]3- with the hydroxide radical to form [Np(VI)O2(OH)4]2-, followed by an additional oxidation with HO⋅ to create [Np(VII)O4(OH)2]3-.

Neptunium behavior in conditions of deep stratum-collector of liquid radioactive waste

237Np isotope is one of the long-lived and hazardous components of the accumulated liquid radioactive waste (LRW). The potential for environmental contamination when the LRW is directly disposed of in deep geological stratum is a matter of concern. Np(V) is highly mobile in the groundwater, and numerous studies are focused on its interaction with natural geological systems and prospective engineered barriers for the radioactive waste repositories. In this work, we studied the neptunium behavior in simulated conditions of acidic liquid radioactive waste injection to the deep permeable stratum collector. To determine components of the host rock preferentially binding the neptunium, and to estimate the effect of elevated temperature on its mobility, a series of laboratory experiments on sorption and leaching of neptunium was carried out using synthetic acid LRW solution and real stratum-collector sand along with individual component simulants. It was found that neptunium sorption on stratum collector sands in the conditions of acidic waste injection is quite low. Clay minerals turned out to be the sorption-dominating components of the host rock for neptunium. Increase in temperature significantly enhances the retention of neptunium on solid phase. Solid-bound neptunium species are sufficiently stable over the two-year experimental time under ambient conditions.

Effect of manganese on the speciation of neptunium(V) on manganese doped magnetites

Speciation and fate of radionuclides in environment are strongly dependent on their interaction with various mineral surfaces. Presence of impurity atoms in minerals modifies these interactions. 237Np, a long-lived component of nuclear high-level waste, is highly mobile in environment as neptunyl (NpO2+) cation. Role of impurities in defining its interaction with mineral surfaces present in natural and engineered systems is yet to be completely understood. Manganese is a ubiquitous impurity present in naturally occurring iron corrosion product, magnetite. Here, a series of manganese (Mn)-doped magnetites (FM) was synthesized and the evolution of solid-sorbed speciation of Np(V) was investigated as a function of Mn doping (3–11 wt%) in near neutral pH condition. Combining Mn K-edge and Np L3-edge X-ray absorption fine structure spectra with Np M5-edge high energy-resolution X-ray absorption near-edge structure spectra, we found nanoparticulate NpIVO2 solid as the single speciation of Np sorbed on both doped and undoped magnetites. Without changing the surface speciation of Np, Mn-O sites did show preference for sorbing Np(V) vis-á-vis Fe-O sites. Mn participates through Fe(II)/Mn(III) redox couple in reducing sorbed Np(V). Its participation, however, becomes significant only when the doped-magnetite contains a critical minimum concentration (> 6 wt%) of Mn. This study highlights the significant role of impurity atom wherein it interacts with sorbing species through redox process.

Activating Water and Hydrogen by Ligand-Modified Uranium and Neptunium Complexes: A Density Functional Theory Study

Organometallic uranium complexes that can activate small molecules are well-known. In contrast, there are no known organometallic trans-uranium species capable of small-molecule transformations. Using density functional theory, we previously showed that changing actinide-ligand bonds from U-O groups to Np-N- (amide/imido) bonds makes redox small-molecule activation more energetically favorable for Np species. Here, we determine how general this ligand-modulation strategy is for affecting small-molecule activation in Np species. We focus on two reactions, one involving redox transformation of the actinide(s) and the other involving no change in the oxidation state of the actinide(s). Specifically, we considered the hydrogen evolution reaction (HER) from H2O by actinide tris-aryloxide species. We also considered H2 capture and hydride transfer by actinide siloxide and silylamide complexes. For the HER, the barriers for Np(III) systems are much higher than those of U(III). The overall reaction energies are also much worse. An-O → An-N substitutions marginally improve the barriers by 1-4 kcal/mol and more substantially improve the reaction energies by 9-15 kcal/mol. For H2 capture and hydride transfer, the reaction energies for the U and Np species are similar. For both actinides, like-for-like An-O → An-N substitutions lead to improved reaction energies. Interestingly, in a recent report, it seemingly appears that U-O (siloxide) → U-N (silylamide) leads to complete shutdown of reactivity for H2 capture and hydride transfer. This observation is reproduced and explained with calculations. The ligand environments of the siloxide and silylamide that were compared are vastly different. The steric environment of the siloxide is conducive for reactivity while the particular silylamide is not. We conclude that small-molecule activation with organometallic neptunium species is achievable with a guided choice of ligands. Additional emphasis should be placed on ligands that can allow for improved transition state barriers.

Electrochemical, spectroscopic and theoretical studies on redox speciation of neptunium with phenylphosphonic acid

Neptunium (Np) is a key element of concern for the safety aspects regarding the reprocessing and handling of spent nuclear fuel owing to its long half life (t1/2 = 2.14 × 106 years). Np can exist in several oxidation states in aquatic environments depending on the surrounding conditions in which it exists. The +5 oxidation state of Np as NpO2+ is the most stable and least binding moiety that can migrate in aquatic arena to far away positions from its source of origin. Humic acids, which are omnipresent in aquifers, are the major source of binding for the actinides including Np to facilitate their migration and transportation phenomenon. Probing the systematic knowledge of neptunium (Np), such as, speciation and coordination as function of its oxidation states in presence of chelating agents is essential aspect of its solution chemistry. The present study aimed at investigating the redox and complexation of Np with phenylphosphonic acid (PhPO3H2) (PPA) as mimic for binding of aromatic phosphorus functionalities in humic substances by using electrochemical and UV–Visible absorption spectroscopy and density functional calculations. The cyclic voltammetric measurement indicates the complexation of both oxidation states (V & VI) of Np with [PhPO3]2−. The presence and stability region (via Eh-pH plot) of new species of Np-PhPO3 in two different oxidation states (V & VI) in aqueous solution at varying physicochemical conditions are identified. The kinetics for reduction of Np complexes in different oxidation states are explored by evaluation of heterogeneous electron-transfer kinetic (D0, ko and α) parameters by cyclic voltammetric results. The spectral parameters, namely λmax and its molal extinction coefficient (ε) are obtained from UV–Visible spectrophotometric measurements and are found to be 988.2 nm and 302 ± 8 mol−1 L cm−1 respectively. The electrochemical as well as UV–Visible spectrophotometric measurements showed the formation of 1:1 complex only and the stability constant (logβ) obtained for the same from both the methods found to be in agreement with each other. Density Functional Theory (DFT) calculations are carried out to optimize the geometries of Np-PhPO3 complex for the Np in +5 and +6 oxidation states with [PhPO3]2− in mono and bidentate mode to identify the most probable coordination mode for complex formation.

Redox interactions of technetium with neptunium in acid solutions

Redox interaction of reduced technetium forms and technetium(VII) with neptunium(III), neptunium(IV) and neptunium(VI) have been investigated using electrochemical and spectroscopic (Vis–NIR) techniques. The neptunium species most stable in 4 M H2SO4, i.e. Np(IV) ions, do not reduce Tc(VII) in contrast to Np(VI) ions which oxidize Tc(IV) species to Tc(VII). The interaction of pertechnetates with Np(III) leads to formation of Tc(IV) species. The Vis–NIR measurements showed the generation of intermediate Tc(V) and Np(V) forms during the oxidation of Tc(IV) and competitive reduction of Np(VI). Tc(V) and Np(V) forms are characterised by the bands at 460 and 980 nm respectively.

Mobility of Aqueous and Colloidal Neptunium Species in Field Lysimeter Experiments.

Due to its radiotoxicity, long half-life, and potentially high environmental mobility, neptunium transport is of paramount importance for risk assessment and safety. Environmental transport of neptunium through field lysimeters at the Savannah River Site was observed from both oxidized (Np(V)) and reduced (Np(IV)) source materials. While transport from oxidized neptunium sources was expected, the unexpected transport from reduced neptunium sources spurred further investigation into transport mechanisms. Partial oxidation of the reduced neptunium source resulted in significant release and transport into the mobile aqueous phase, though a reduced colloidal neptunium species appears to have also been present, enhancing neptunium mobility over shorter distances. These field and laboratory experiments demonstrate the multiple controls on neptunium vadose zone transport and chemical behavior, as well as the need for thorough understanding of radionuclide source terms for long-term risk prediction.

Speciation of neptunium during sorption and diffusion in natural clay

In argillaceous rocks, which are considered as a potential host rock for nuclear waste repositories, sorption and diffusion processes govern the migration behaviour of actinides like neptunium. For the safety analysis of such a repository, a molecular-level understanding of the transport and retardation phenomena of radioactive contaminants in the host rock is mandatory. The speciation of Np during sorption and diffusion in Opalinus Clay was studied at near neutral pH using a combination of spatially resolved synchrotron radiation techniques. During the sorption and diffusion experiments, the interaction of 8 μM Np(V) solutions with the clay lead to the formation of spots at the clay-water interface with increased Np concentrations as determined by μ-XRF. Several of these spots are correlated with areas of increased Fe concentration. Np L3-edge μ-XANES spectra revealed that up to 85% of the initial Np(V) was reduced to Np(IV). Pyrite could be identified by μ-XRD as a redox-active mineral phase responsible for the formation of Np(IV). The analysis of the diffusion profile within the clay matrix after an in-diffusion experiment for two months showed that Np(V) is progressively reduced with diffusion distance, i.e. Np(IV) amounted to ≈12% and ≈26% at 30 μm and 525 μm, respectively.

Neptunium speciation in soils

The relative content of 237Np in the water-soluble form depends on the soil type and decreases in the following order: sandy loam soil > soddy podzolic soil > chernozem (black soil). The fraction of the most mobile forms of 237Np decreases with an increase in the content of organic carbon in soils. The major fraction of 237Np is associated with the fulvic acid (FA) group (90.3 and 71.0% of the total neptunium content for soddy podzolic soil and chernozem, respectively), with 8.6 and 18.7% of 237Np, respectively, associated with the humic acid (HA) group. In the FA group, the major fraction of 237Np is associated with low-molecular-weight organic compounds and with FA, free or bonded mainly with relatively mobile hydroxides of Fe, Al, Mn, etc. (R2O3·nH2O) (fractions 1a and 1).

Chemical Speciation of Neptunium(VI) under Strongly Alkaline Conditions. Structure, Composition, and Oxo Ligand Exchange

Hexavalent neptunium can be solubilized in 0.5-3.5 M aqueous MOH (M = Li(+), Na(+), NMe4(+) = TMA(+)) solutions. Single crystals were obtained from cooling of a dilute solution of Co(NH3)6Cl3 and NpO2(2+) in 3.5 M [N(Me)4]OH to 5 °C. A single-crystal X-ray diffraction study revealed the molecular formula of [Co(NH3)6]2[NpO2(OH)4]3·H2O, isostructural with the uranium analogue. The asymmetric unit contains three distinct NpO2(OH)4(2-) ions, each with pseudooctahedral coordination geometry with trans-oxo ligands. The average Np═O and Np-OH distances were determined to be 1.80(1) and 2.24(1) Å, respectively. EXAFS data and fits at the Np L(III)-edge on solid [Co(NH3)6]2[NpO2(OH)4]3·H2O and aqueous solutions of NpO2(2+) in 2.5 and 3.5 M (TMA)OH revealed bond lengths nearly identical with those determined by X-ray diffraction but with an increase in the number of equatorial ligands with increasing (TMA)OH concentration. Raman spectra of single crystals of [Co(NH3)6]2[NpO2(OH)4]3·H2O reveal a ν1(O═Np═O) symmetric stretch at 741 cm(-1). Raman spectra of NpO2(2+) recorded in a 0.6-2.2 M LiOH solution reveal a single ν1 frequency of 769 cm(-1). Facile exchange of the neptunyl oxo ligands with the water solvent was also observed with Raman spectroscopy performed with (16)O- and (18)O-enriched water solvent. The combination of EXAFS and Raman data suggests that NpO2(OH)4(2-) is the dominant solution species under the conditions of study and that a small amount of a second species, NpO2(OH)5(3-), may also be present at higher alkalinity. Crystal data for [Co(NH3)6]2[NpO2(OH)4]3·H2O: monoclinic, space group C2/c, a = 17.344(4) Å, b = 12.177(3) Å, c = 15.273 Å, β = 120.17(2)°, Z = 4, R1 = 0.0359, wR2 = 0.0729.

Sensitive redox speciation of neptunium by CE–ICP–MS

Capillary electrophoresis (CE) was used to separate the neptunium oxidation states Np(IV) and Np(V), which are the only oxidation states of Np that are stable under environmental conditions. The CE setup was coupled to an inductively coupled plasma mass spectrometer (Agilent 7500ce) using a Mira Mist CE nebulizer and a Scott-type spray chamber. The combination of the separation capacity of CE with the detection sensitivity of inductively coupled plasma mass spectrometry (ICP-MS) allows identification and quantification of Np(IV) and Np(V) at the trace levels expected in the far field of a nuclear waste repository. Limits of detection of 1 × 10(-9) and 5 × 10(-10) mol L(-1) for Np(IV) and Np(V), respectively, were achieved, with a linear range from 10(-9) to 10(-6) mol L(-1). The method was applied to study the redox speciation of the Np remaining in solution after interaction of 5 × 10(-7) mol L(-1) Np(V) with Opalinus Clay. Under mildly oxidizing conditions, a Np sorption of 31% was found, with all the Np remaining in solution being Np(V). A second sorption experiment performed in the presence of Fe(2+) led to complete sorption of the Np onto the clay. After desorption with HClO(4), a mixture of Np(IV) and Np(V) was found in solution by CE-ICP-MS, indicating that some of the sorbed Np had been reduced to Np(IV) by Fe(2+).

The role of oxidizing radicals in neptunium speciation in γ-irradiated nitric acid

The irradiation of aqueous nitric acid solutions generates transient, reactive species that are known to oxidize neptunium. However, nitrous acid is also a long-lived product of nitric acid irradiation, which reduces neptunium. When we irradiated nitric acid solutions of neptunium and measured its speciation by UV/Vis spectroscopy, we found that at short irradiation times, oxidation of Np(V) to Np(VI) occurred due to reactions with radicals such as •OH, •NO3 and •NO2. However, at higher absorbed doses and after a sufficient amount of nitrous acid was produced, reduction of Np(VI) to Np(V) began to occur, eventually reaching an equilibrium distribution of these species depending on nitric acid concentration. Neptunium(IV) was not produced.

Neptunyl (Np [formula omitted]) interaction with green rust, [formula omitted

Green rust (GR), a member of the Fe(II),Fe(III) layered double hydroxide mineral family, forms in groundwater and during steel corrosion. It has high surface area and is very reactive, especially for redox-sensitive elements such as some actinides. During neutron irradiation of nuclear fuel in a reactor, 237Np develops. Although the abundance of Np in spent nuclear fuel is only about 0.05% by mass, it has a very long half life, 2.14 × 10 6 years, so there is concern about its mobility in the distant future, when radioactive storage sites might be expected to degrade. Under oxidizing conditions, as are expected from radiolysis in a repository, the very mobile neptunyl-ion, Np O 2 + , would be the dominant aqueous neptunium species. In this work, we investigated the interaction of Np O 2 + with green rust sodium sulphate ( GR Na,SO 4 ). The aim of the study was to define the processes involved, to determine the final redox speciation of Np, hence its potential mobility, and to characterise changes in the green rust. The GR Na,SO 4 sorbed and reduced Np O 2 + within minutes. Reduced Np(IV) was primarily found as precipitated nanoparticles at the edges of the GR Na,SO 4 crystal platelets. The position of the particles at the crystal edges suggests initial sorption of Np(V) and subsequent reduction at GR edge sites. In further experiments, Np-containing GR was allowed to oxidise completely in aqueous suspension. Surprisingly, about 50% of the Np remained associated with the oxidised product and about 75% of the associated Np was still present as Np(IV).

EFFECT OF CARBONATE ON THE SOLUBILITY OF NEPTUNIUM IN NATURAL GRANITIC GROUNDWATER

This study investigates the solubility of neptunium (Np) in the deep natural groundwater of the Korea Atomic Energy Research Institute Underground Research Tunnel (KURT). According to a Pourbaix diagram (pH-Eh diagram) that was calculated using the geochemical modeling program PHREEQC 2.0, the redox potential and the carbonate ion concentration both control the solubility of neptunium. The carbonate effect becomes pronounced when the total carbonate concentration is higher than 1.5 × 10 -2 M at E h = -200 mV and the pH value is 10. Given the assumption that the solubility-limiting stable solid phase is Np(OH)₄(am) under the reducing condition relevant to KURT, the soluble neptunium concentrations were in the range of 1 × 10 -9 M to 3 × 10 -9 M under natural groundwater conditions. However, the solubility of neptunium, which was calculated with the formation constants of neptunium complexes selected in an OECD-NEA TDB review, strongly deviates from the value measured in natural groundwater. Thus, it is highly recommended that a prediction of neptunium solubility is based on the formation constants of ternary Np(IV) hydroxo-carbonato complexes, even though the presence of those complexes is deficient in terms of the characterization of neptunium species. Based on a comparison of the measurements and calculations of geochemical modeling, the formation constants for the “upper limit” of the Np(IV) hydroxo-carbonato complexes, namely Np(OH)y(CO₃)z 4-y-2z , were appraised as follows: log K˚ 122 = -3.0±0.5 for Np(OH)₂(CO₃)₂ 2- , log K˚ 131 =-5.0±0.5 for Np(OH)₃(CO₃)-, and log K˚ 141 = -6.0±0.5 for Np(OH)₄(CO₃) 2- .

Kinetics of reduction of hexavalent neptunium by nitrous acid in solutions of nitric acid

Nitrous acid is a key redox controlling factor, affecting the speciation of neptunium in the reprocessing of used nuclear fuel by solvent extraction. The kinetics of the reduction of neptunium(VI) by nitrous acid in solutions of nitric acid was investigated spectrophotometrically by the method of initial rates. The reaction is of first order with respect to Np(VI) while the order with respect to HNO2 is 1.20 ± 0.04. The reaction rate is almost inversely proportional to the hydrogen ion concentration (reaction order −0.92 ± 0.06), indicating that the reaction proceeds primarily through the reaction of neptunium(VI) with the nitrate anion. The experimental value of the rate constant k for the rate law −d[Np(VI)]/dt = k·[Np(VI)]·[HNO2]1.2/[H+] is of (0.159 ± 0.014) M−0.2 s−1 in I = 4 M and at 20 °C. The activation energy is (−57.3 ± 1.6) kJ/mol, which is in agreement with previous data on this reaction in perchloric acid.

Spatially resolved micro-X-ray fluorescence and micro-X-ray absorption fine structure study of a fractured granite bore core following a radiotracer experiment

Spatially resolved X-ray absorption and fluorescence investigation with a micrometer-scale resolution on actinide-containing samples provide information necessary for safety assessment of nuclear waste disposal. In this paper one example of such an experiment is presented. This example entails neptunium speciation in a fractured granite bore core from the Swedish Äspö Hard Rock Laboratory following a radiotracer experiment using µ-XAFS and µ-XRF. In order to probe micro-volumes below the surface in the granite samples and thereby avoid potential changes in the Np speciation during cutting of the bore core, a confocal irradiation–detection geometry is employed. µ-XAFS results for a selected granite bore core cross section with ~ 3 nmol Np/g reveal that Np, originally introduced as Np(V) in the tracer cocktail, is present in the granite in its reduced Np(IV) form. The Np(IV) is often present as particles, tens of µm in size. Elemental distribution maps show the tracer Np to be located in fissures and permeable channels not larger than 100 µm. The Np distribution appears often correlated with Zn also present in some fissures. We observe small granite fissures containing Fe (presumably Fe(II)), where we do not detect any Np. It is feasible that inflowing Np(V) has a shorter residence time in large fractures, while in the smaller fissures migration is slower, leading to longer residence times, i.e., reaction times, where it is reduced to less soluble Np(IV) and becomes thereby immobilized.

Spectroelectrochemical Study of Uranium and Neptunium in LiCl-KCl Eutectic Melt

Experimental set up for spectroelectrochemical measurements was designed. Uranium(III) ions, UCl6 3-, are formed during anodic dissolution of metallic uranium and prolonged contact of U(IV)- containing melts with uranium metal. UCl4 dissolution in 3LiCl- 2KCl melt leads to the formation of UCl6 2- ions. They undergo a two-step electrochemical reduction: U4+→U3+ recharge followed by U deposition. Neptunium(IV) chloride dissolves in 3LiCl-2KCl melt producing NpCl6 2- ions. Electrochemical reduction of Np(IV) on a glassy carbon electrode results in the formation of NpCl6 3- ions and then leads to the deposition of neptunium metal. Electronic absorption spectra of uranium (III, IV, V and VI) and neptunium (III and IV) species in 3LiCl-2KCl eutectic were measured at 450 oC.

Plutonium and neptunium speciation bound to hydrous ferric oxide colloids

Abstract The speciation of plutonium and neptunium in colloid suspension collected from the contaminated aquifer of the “Mayak” site was studied by nano-SIMS, XPS and XANES. At tracer level concentration found in original groundwater sample Pu is sorbed onto hydrous ferric oxide colloids, while at higher concentration relevant to near-field conditions it forms intrinsic PuO 2 × n H 2 O colloids. The formation of such colloids occurs through the reduction of Pu(V) to Pu(IV) upon its sorption.

Interaction of neptunyl with goethite (α-FeOOH), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) in water as probed by X-ray photoelectron spectroscopy

The sorption behavior is studied and the physicochemical neptunium species existing on the surface of goethite (α-FeOOH), maghemite (γ-Fe2O3), and hematite (α-Fe2O3) are determined. Solvent extraction and X-ray photoelectron spectroscopy (XPS) are used to determine the neptunium surface species. The ion and elemental composition of the surface of the minerals and surface neptunyl NpO 2 + complexes is determined using these data. Compounds containing neptunium(IV) or neptunium(VI) ions do not appear; rather, neptunyl (Np(V)O 2 + group is complexed with surface hydroxide groups of α-FeOOH, γ-Fe2O3, and α-Fe2O3. Presumably, the oxygen atoms of iron oxides and water and/or carbonate (CO 3 2- ) or nitrate (NO 3 - ) group lie in the equatorial plane of the neptunyl (NpO 2 + ) group.

Rapid and quantitative electrolytic preparation and speciation of neptunium ions of various oxidation states using multi-step column electrodes

Redox behavior of Np(III)–Np(VI) ions in aqueous perchloric, nitric and sulphuric acid solutions was elucidated by using column electrodes connected in series in a flow system (multi-step column electrode system). Using a glassy carbon fiber working electrode as the column electrode was found to be very useful for the investigation of current–potential relations of not only reversible redox processes, such as Np(VI)/Np(V) or Np(IV)/Np(III) couple but also irreversible processes, such as Np(V)/Np(IV) or Np(V)/Np(III) couple, the latter not observed by conventional voltammetry which uses a glassy carbon or platinum electrode. Quantitative (coulometric) electrolysis could be executed even if the redox process was completely irreversible by the use of the column electrodes. By taking advantage of column electrode electrolysis, a novel method was developed for the rapid preparation of a neptunium species of a desired oxidation state, including unstable species. The multi-step column electrode system was demonstrated to be useful for the coulometric determination and speciation of Np(IV)–Np(VI) in aqueous HNO 3 solution as an example.