Neptunium Ions Research Articles

Gamma Radiation-Induced Degradation of Acetohydroxamic Acid (AHA) in Aqueous Nitrate and Nitric Acid Solutions Evaluated by Multiscale Modelling.

Acetohydroxamic acid (AHA) has been proposed for inclusion in advanced, single-cycle, used nuclear fuel reprocessing solvent systems for the reduction and complexation of plutonium and neptunium ions. For this application, a detailed description of the fundamental degradation of AHA in dilute aqueous nitric acid is required. To this end, we present a comprehensive, multiscale computer model for the coupled radiolytic and hydrolytic degradation of AHA in aqueous sodium nitrate and nitric acid solutions. Rate coefficients for the reactions of AHA and hydroxylamine (HA) with the oxidizing nitrate radical were measured for the first time using electron pulse radiolysis and used as inputs for the kinetic model. The computer model results are validated by comparison to experimental data from steady-state gamma ray irradiations, for which the agreement is excellent. The presented model accurately predicts the yields of the major degradation products of AHA: acetic acid, HA, nitrous oxide, and molecular hydrogen.

Photolysis of Neptunium Ions in Acetic Acid Solutions

Photochemical reactions involving Np(VI) and Np(V) in 0.5–17.3 M CH3COOH (HAc) solutions have been studied. It was found that irradiation with UV light causes reduction of Np(VI) to Np(V). The initial rate of Np(VI) reduction is at a maximum in a 0.5–1 M solution of HAc and decreases with increasing HAc concentration, which is due to the formation of higher acetic acid complexes with Np(VI). After a partial reduction of Np(VI) as a result of irradiation, its concentration continues to decrease in the dark reaction with products formed in the oxidation of HAc. The reduction of Np(V) occurs with an induction period, after which the reaction becomes faster. This is due to the appearance and accumulation of Np(IV), which forms a cation-cation complex with Np(V). The complex absorbs light, which leads to the reaction of Np(V) with HAc. Np(VI) is oxidized in acetic acid solutions by oxygen to Np(V).

Neptunium – Tri-n-butyl phosphate complexes in room temperature ionic liquids: Extraction and spectroelectrochemical studies

The extraction of Np4+ and NpO22+ was investigated from nitric acid feeds using solutions of tri-n-butyl phosphate (TBP) in two room temperature ionic liquids viz. 1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonamide (C4mim•NTf2) and 1-methyl-3-octylimidazolium bis(trifluoromethane)sulfonamide (C8mim•NTf2). For comparison purposes, the extraction of Pu4+ and UO22+ were also studied in select cases in view of their similarity with the tetra- and hexavalent neptunium ions. The nature of the extracted species was ascertained by slope analysis and two-phase extraction constants were determined. While the Np4+ ion bearing species of the type Np(NO3)3(TBP)2+ and Np(NO3)3(TBP)+ were extracted into C4mim•NTf2 and C8mim•NTf2, respectively, species of the type NpO2(NO3)2∙TBP were obtained for NpO22+ ion in both the ionic liquids. The extraction data were compared with those reported before in other solvent systems. Cyclic voltammetric studies were carried out using the Np(IV)-TBP extract in both the ionic liquid medium and the conversion to the penta-valent state (NpO2+) was confirmed by UV–visible spectrophotometric analysis of the extract.

Photolysis of Neptunium Ions in HCOOH Solutions

Spectrophotometric method has been used to examine valence transformations of neptunium ions in formi acid aqueous solutions under the action of light from a mercury lamp. It was shown that Np(VI) ions are reduced in ~10 min to Np(IV) containing a small amount of (Np(V). Np(V) ions in solutions and suspensions are also effectively reduced to Np(IV). Np(III) appears in a photolysis of Np(IV) ions. In 99.7% HCOOH, the Np(IV) → Np(III) conversion occurs nearly fully. Np(III) ions are unstable and are transformed to Np(IV) in storage. The hydrolysis and polymerization of Np(IV) hinder the reduction of Np(IV) in1 M HCOOH.

Comparative evaluation of extraction of tetra- and hexa-valent neptunium ions by CMPO in a room temperature ionic liquid

ABSTRACT Solvent extraction studies were performed to understand the extraction behavior of Np4+ and NpO2 2+ from acidic feeds with CMPO (octyl (phenyl)-N,N-diisobutyl carbamoyl methyl phosphine oxide) dissolved in 1-butyl-3-methylimidazolium bis(trifluoromethane sulfonyl)imide, a water immiscible ionic liquid. Slope analyses on the distribution data revealed the extraction of ML2 type species, where M = Np4+ or NpO2 2+, and L = CMPO. Studies were also carried out with Pu4+ and UO2 2+ under identical conditions. The nature of the extracted species was found to vary with the nature of the ionic species.

Polly L. Arnold

Polly L. Arnold

Rapid, Sensitive and Simultaneous Determination of Plutonium and Neptunium Using Ruthenium Nanoparticles Decorated Glassy Carbon Electrode

Sensitive determination of actinides is of utmost importance to maintain regular surveillance over the environment in the vicinity of nuclear power plants. The noble metal nanoparticles (NPs) exhibit high electrocatalytic activity, and could be employed for the sensitive and rapid quantifications of plutonium (Pu) and neptunium (Np) ions in the aqueous matrices. Therefore, the Ru NPs decorated glassy carbon electrode (RuNPs/GCE) was examined towards the electrocatalytic redox reaction of Pu and Np respectively. Differential pulse voltammetric (DPV) technique was used for rapid and sensitive quantification of Pu and Np. The RuNPs/GC based DPV technique could be used to determine the concentration of Pu and Np in a few minutes. Both Pu and Np gave sensitive oxidation peaks at 470mV and 840 mV respectively in 1M H2SO4 versus silver/silver chloride electrode. The detection limits were found to be 1.5 μM for Pu and 6.5 μM for Np. The investigated method showed good stability, reproducibility, and repeatability.

Quaternary ammonium based task specific ionic liquid for the efficient and selective extraction of neptunium

Abstract: Liquid–liquid extraction of neptunium from aqueous acidic solution using quaternary ammonium based task specific ionic liquid (TSIL) was investigated. The extraction of Np was predominated by the ‘cation exchange’ mechanism via [NpO2·Hpth]+ species for NpO2 2+, while NpO2 + was extracted in ionic liquid as [NpO2·H·Hpth]+. The extraction process was thermodynamically spontaneous while kinetically slower. Na2CO3 as strippant showed quantitative back extraction of neptunium ions from TSIL. TSIL showed excellent radiolytic stability upto 500 kGy gamma exposure. Finally, the TSIL was employed for the processing of simulated high level waste solutions revealing high selectivity of TSIL towards neptunium.

Structures and properties of Np(NO3)nq (n=16, q=-2+3) coordination compound

In the process of nuclear waste disposal, the valuable uranium and plutonium are recycled and separated by dissolving the spent fuel in nitric acid. However, transuranic Np greatly influences the process of separation and recovery. Therefore, it is vital to study the structure and properties of nitrate, which is combined with neptunium ions and nitric acid. Furthermore, there are few researches about nitrate formed by tetravalent neptunium ions. So in this article, by using B3LYP hybrid method of density functional theory, the Gaussian 03 program is used to optimize the geometric construction of the coordination compounds Np(NO3)nq (n=1-6, q=-2-+3) formed by the tetravalent neptunium ions (Np4+) and nitrate ion (NO3-). Under the relativistic effective core potential model, the structure parameters and properties are reported. It is found that NO3- coordinates to Np4+ as a bidentate ligand, and the NpN and NpO bonds are the shortest in Np(NO3)22+, while the binding energy of the Np(NO3)4 is the largest. The infrared spectra of Np(NO3)4 are calculated in the gas and liquid phase. Comparing with the available experimental data, the reliability of the calculation results in this work is confirmed.

The specific sorption of Np(V) on the corundum (α-Al2O3) surface in the presence of trivalent lanthanides Eu(III) and Gd(III): A batch sorption and XAS study

The sorption of pentavalent neptunium, Np(V), on corundum (α-Al2O3) was investigated in the absence and presence of trivalent europium or gadolinium as a competing element under CO2-free conditions. The objective of this study was to investigate how a trivalent metal ion with a higher charge than that of the neptunyl(V) ion would affect the sorption of Np(V) when allowed to adsorb on the mineral surface before the addition of Np(V). Batch sorption experiments conducted as a function of pH (pH-edges) and as a function of Np(V) concentration (isotherms) in the absence and presence of 1×10−5M Eu(III) showed no sign of Eu being able to block Np sorption sites. Surface complexation modelling using the diffuse double layer model was applied to the batch data to obtain surface complexation constants for the formed Np(V) complexes on corundum. To account for potential changes occurring in the coordination environment of the neptunium ion in the presence of a trivalent lanthanide, X-ray absorption spectroscopy (XAS) measurements were carried out on the samples containing only Np(V) and Np(V)+Gd(III). The results reveal the presence of a bidentate Np(V) edge-sharing complex on the corundum surface in the absence of Gd(III), while the coordination environment of Np(V) on the corundum surface could be changed when Gd(III) is added to the sample before the sorption of Np(V).

Structural Environment and Stability of the Complexes Formed Between Calmodulin and Actinyl Ions.

Because of their presence in the nuclear fuel cycle, neptunium and uranium are two actinides of main interest in case of internal contamination. Complexation of U(VI) and Np(V) by the target protein calmodulin (CaM(WT)) was therefore studied herein. Both actinides have two axial oxygen atoms, which, charge aside, makes them very similar structurally wise. This work combines spectroscopy and theoretical density functional theory (DFT) calculations. Structural characterization was performed by extended X-ray absorption fine structure (EXAFS) at the L(III)-edge for each studied actinide. Models for the binding site of the protein were developed and then refined by using DFT to fit the obtained experimental EXAFS data. The effect of hydrolysis was also considered for both actinides (the uranyl experiment was performed at pH 3 and 6, while the neptunyl experiment was conducted at pH 7 and 9). The effect of the pH variation was apparent on the coordination sphere of the uranyl complexes, while the neptunyl complex characteristics remained stable under both studied conditions. The DFT calculations showed that at near physiological pH the complex formed by CaM(WT) with the neptunium ion is more stable than the one formed with uranyl.

Electronic structure of the NpMT 5 (M = Fe, Co, Ni; T = Ga, In) series of neptunium compounds

Evolution of the electronic structure of the NpMGa5 (M = Fe, Co, Ni) series of neptunium compounds, whose crystal structure is similar to that of the known family of Pu115 superconductors, was studied by the LDA + U + SO method. The calculations took into account both the strong electron correlations and the spin‒orbit coupling in the 5f shell of neptunium. For the first time, the electronic structure was calculated for a hypothetical series of compounds in which gallium is replaced with indium. Parameters of the crystal structure of the given series were obtained using the relationship between the parameters of the crystal structure of the earlier-studied compounds PuCoGa5 and PuCoIn5. The analysis of the electronic structure and characteristics of neptunium ions calculated in the framework of the LDA + U + SO method showed that the neptunium ions in NpMIn5 with M = Fe, Co, and Ni should have an electron configuration closer to f4, but a spin and magnetic characteristics close to those in NpMGa5.

Electrochemical preparation and spectroelectrochemical study of neptunium chloride complexes in LiCl–KCl eutectic melts

We prepared neptunium chloride complexes electrochemically and performed spectroelectrochemical measurements of neptunium ions in LiCl–KCl eutectic melts at 450 °C, where the neptunium concentrations studied were in the range of 0.1–1 mM. We observed a highly absorbing f–d transition band of Np3+ at 383 nm, which was used to determine an formal potential of the Np4+|Np3+ redox couple (E°′ = 0.45 V vs. Ag|Ag+) with a solution of concentration as low as ~0.1 mM. This result agreed well with the value (E°′ = 0.42 V vs. Ag|Ag+) determined by cyclic voltammetry with a ~15 mM solution.

Interaction of ozone with variable-valence metal ions in concentrated silicate solutions

Spectrophotometric studies have revealed that ozone oxidizes Cr(III) into Cr(VI), Fe(III) into Fe(VI), Mn(VI) into Mn(VII), and Np(VI) into Np(VII) in the concentrated aqueous silicate solutions. Cr(III) oxidation is accelerated in alkaline-silicate and alkaline solutions as compared to neutral silicate solution. Ferrate and permanganate ions are unstable in Na2SiO3 solutions (0.5–1.3 mol/L of the silicate). Neptunium(VII) ions formed in the course of ozonation are stable in Na2SiO3 solution (1 mol/L) upon drying in air to form solid vitreous mass.

Adsorption Behavior of Neptunium Ions on Pyridine-type Resin in Hydrochloric Acid Solutions

The adsorption behavior of neptunium ions on the synthesized pyridine-type resin (Abbreviated as PYAER) in hydrochloric acid solutions whose concentrations are in the 0.10-11.5 M region has been studied at room temperature. Neptunium has many kinds of valence states of ions; mainly tetravalent, pentavalent, and heptavalent ions. We have controlled the neptunium valence states by using redox reactions and confirmed the valence states in hydrochloric acid solutions using an ultraviolet visible nearinfrared absorption photospectrometry at room temperature before the adsorption experiments described above. After adjusting the valence states of neptunium ions, the adsorption mechanisms on the neptunium ions with PYAER was studied by batch-wise adsorption experiments and the estimated mechanisms were compared with those of heptavalent uranium, trivalent lanthanides, and tetravalent plutonium against PYAER. As a result, it was found that the obtained distribution coefficients (Abbreviated as Kd) of neptunium with PYAER increase with increasing of the concentrations of hydrochloric acid solutions and their order was heptavalent > tetravalent ≒ pentavalent. The Kd values of tetravalent plutonium were found to be between the Kd values of neptunium. On the other hand, it was also revealed that trivalent lanthanides show little adsorption ability for PYAER in the examined hydrochloric acid concentration ranges and PYAER was able to adsorb strongly heptavalent uranium ions with increasing concentrations of hydrochloric acid. In brief, the obtained Kd values increased in order of lanthanides, neptunium (≒ plutonium), and uranium. These results support the positive possibility of the mutual separation of uranium, plutonium containing neptunium, minor actinides (Am, Cm), and lanthanides and were therefore found to be valuable data for our proposed reprocessing process.

Effects of Oxidation States of Np on Polarization Curve of Stainless Steel in Boiling 3M-HNO3

It is well known that some certain metal ions in boiling nitric acid cause corrosion acceleration of stainless steel. Neptunium ion from fission products contained in reprocessing solution is known as corrosion accelerator of the stainless steel. Neptunium has several oxidation states (IV, V and VI) in nitric acid solution (up to 3M-HNO3). The metal ions with the REDOX reaction having near the potential of nitric acid promote corrosion of stainless steel but it is not clear why slight amount of the metal ion accelerates corrosion in boiling nitric acid solution. Simultaneous evaluating the coexisting oxidation state Np(IV), Np(V) and Np(VI) and the electrochemical behaviors on stainless steel in nitric acid solution is required in order to understand the corrosion acceleration mechanism. Specially designed electrochemical test cell integrated with optical cell for spectroscopic analysis was developed for this purpose. From results of electrochemical tests, cathode reaction on stainless steel was activated by Np ions. Np(VI) ion made corrosion potential shift nobler than Np(V) and nobler corrosion potential causes accelerating corrosion of stainless steel in nitric acid solution. From results of spectroscopic analysis for oxidation states of Np ion, Np(V) was easily oxidized to Np(VI) in nitric acid solution and Np(VI) was stable state in boiling 3M-HNO3. It was considered that role of Np ions was mediator to accelerate corrosion due to activating cathode reaction and re-oxidizing cycle in boiling 3M-HNO3.

Spectrophotometric and electrochemical study of neptunium ions in molten NaCl–CsCl eutectic

The chemical oxidation states of NpO2+, Np4+ and Np3+ in NaCl–CsCl eutectic were controlled by using Cl2, O2, H2 and Ar gas mixtures, the redox behavior and electronic absorption properties of their Np ions were studied. The Np4+ was prepared from NpO2Cl by bubbling Cl2 gas into the melt in the presence of carbon rod. Np3+ was quantitatively prepared by bubbling H2–Ar gas mixture. The molar absorptivities of NpO2+, Np4+ and Np3+ were determined in molten NaCl–CsCl eutectic at 923K and hypersensitive transitions of Np4+ and Np3+ ions were assigned. Since the polarizing ability of the cations in the NaCl–CsCl eutectic is lower than that in some other melts, it has been shown that the coordination symmetry of the Np–Cl complex is higher.In the electrochemical measurement of Np4+, the cathodic current for the reduction of Np4+ was found to be controlled by the diffusion of Np4+. The temperature dependence of the diffusion coefficient between 823 and 923K was formulated to be lnDNp4+=-4304/T-6.172. The formal redox potential of the Np4+|Np3+ couple depended on the temperature, this dependence was formulated as ENp4+|Np3+∘′=-1.313+6.210×10-4TV (vs. Cl2|Cl−).

Theoretical studies on level structures and transition properties of neptunium ions

Multiconfiguration Dirac-Fock (MCDF) method was employed to calculate the ionization potentials, ionic radii, excitation energies and oscillator strengths for neptunium ions. In the calculations, main valence correlation effects, Breit interaction and QED effects were taken into account. The good consistency with other available theoretical values demonstrates the validity of the present calculations. These theoretical results therefore can be used to predict some physicochemical properties of Np and its oxides.

Behavior of neptunium ions in organic media

Published data on the effect of organic solvents on the hydrolysis of Np(IV) and redox reactions of Np(IV–VI) are analyzed. In aqueous-organic solutions, Np(IV) ions undergo hydrolysis at higher acidity than in aqueous solutions. With respect to the effect on hydrolysis, the solvents can be ranked in the order methanol > ethanol > dioxane > acetone > acetonitrile. Dimethyl sulfoxide suppresses the hydrolysis. The Np(V) disproportionation in CH3OH and CH3OH + C6H6 solutions in the presence of HCl, HDEHP, or TTA and in a TBP solution was studied. The influence of the solution composition, including the H2O concentration, on the reaction kinetics was examined. The reactions occur faster than in aqueous solutions. The reaction mechanism is the same in all the media: Two solvated Np(V) ions form a complex decomposing upon protonation into Np(IV) and Np(VI). The role of the solvent in the Np(V) reproportionation was examined. In mixed water-ethylene glycol, water-methanol, and water-acetone solvents, with an increase in the fraction of the organic component, the Np(IV) + Np(VI) reaction rate passes through a maximum, which is due to combined effect of two factors: Np(IV) hydrolysis (acceleration) and decrease in [H2O] (deceleration). In TBP solutions, the Np(IV) + Np(VI) reaction decelerates in proportion to [HNO3]−2 and [H2O]. The course of the Np(VI) + H2O2 and Np(IV–VI) + HNO2 reactions in TBP differs from that in aqueous solutions. Deceleration of the Np(VI) reduction and acceleration of the Np(V) oxidation, compared to aqueous solutions, are associated with a decrease in the formal potential of the Np(VI)/(V) couple in going from H2O to TBP. In solutions of KOH in aqueous methanol, Np(VI) rapidly disproportionates to Np(VII) and Np(V). A decrease in the H2O concentration shifts the equilibrium toward Np(VII).

Kinetics of electrochemical oxidation of neptunium ions in nitric acid solutions containing potassium phosphotungstate K10P2W17O61

The kinetics of electrochemical oxidation of Np(IV) and Np(V) ions on a smooth platinum electrode in a 1 M HNO3 solution containing potassium phosphotungstate K10P2W17O61 (KPW) was studied by spectrophotometry. For the redox reactions under consideration, the rate laws were determined, the rate constants were calculated, and the schemes of transformations of Np ions were suggested. The electrochemical oxidation of Np(IV) and Np(V) is described by the first-order rate law with respect to the Np(IV) and Np(V) concentrations, respectively. The rate constants of these reactions are close and comparable with the rate constant of the Np(V) disproportionation.