Np Ions Research Articles

Theoretical perspectives on the reduction of Pu(IV) and Np(VI) by methylhydrazine in HNO3 solution: Implications for Np/Pu separation

Effective adjustment and control of the oxidation state of plutonium (Pu) and neptunium (Np) is an indispensable component of Np/Pu separation in spent nuclear fuel reprocessing. Some hydrazine derivatives including methylhydrazine (CH3N2H3) effectively achieves the reduction of Np(VI) to Np(V) without reducing Pu(IV). Herein, we explored the reduction mechanisms of Pu(IV) and Np(VI) by CH3N2H3 in HNO3 solution using scalar-relativistic density functional theory. We elucidated the difference in the reduction mechanism between Np(VI) and Pu(IV) ions by CH3N2H3. The energy barrier for the reduction of [NpVIO2(H2O)5]2+ and [NpVIO2(NO3)(H2O)3]+ by CH3N2H3 is largely different due to the coordination of nitrate ion. Moreover, the energy barrier of the reduction of [NpVIO2(H2O)5]2+ is apparently lower than that of [PuIV(NO3)2(H2O)7]2+, which is in line with the experimental observations. The results of Mayer bond order and localized molecular orbitals clarify the structural evolution of the reaction pathways. Analysis of the spin density demonstrates that the first Np(VI) and Pu(IV) reduction belongs to the outer-sphere electron transfer and the second Np(VI) and Pu(IV) reduction is the hydrogen transfer. This study explains theoretically why CH3N2H3 reduces Np(VI) but not Pu(IV), and helps to design promising reductants for the Np/Pu separation in spent nuclear fuel reprocessing.

Reaction of Np, Am, and Cm ions with CO2 and O2 in a reaction cell in triple quadrupole inductively coupled plasma mass spectrometry

The ion–molecule reactions of actinides (237Np, 241Am, and 244Cm) in reaction cells of ICP-MS/MS were successfully modeled.

Extraction of Np(IV/VI) from nitric acid medium using three different tripodal diglycolamide ligands in ionic liquid: Batch extraction, spectroscopic and electrochemical investigations

Complexation of Np(IV) and Np(VI) ions was studied in a room temperature ionic liquid (C4mim.NTf2) using three tripodal ligands with an ‘N’ atom, a ‘C’ atom and a benzene ring at the center, which are termed as LI, LII and LIII, respectively . The studies include solvent extraction, visible-near infrared (vis-NIR) spectroscopy, and cyclic voltammetry. The trend of the extraction of Np4+ was LIII > LII > LI, which resembled that reported before in a mixture of molecular diluents but with about 10 times higher ligand concentration. Np(VI) was extracted to a much lower extent than Np(IV). Slope analysis of Np(IV) extraction suggested formation of Np(NO3)4.L species for LII and LIII, while a cationic species of the type [Np(NO3)2.L]2+ was seen for LI. On the other hand, for Np(VI), formation of NpO2(NO3)2.L species was proposed for LI and LIII and [NpO2(NO3).L]+ for LII. All the extracted species conform to a ‘solvation’ type extraction except for Np(IV) with LI and Np(VI) with LII which point to a ‘cation-exchange’ mechanism. The complexation of the metal ions with the tripodal ligands was supported by peak shifts in the vis-NIR as well as the cyclic voltammetric studies.

An1.33T4Al8Si2 (An = Ce, Th, U, Np; T = Ni, Co): Actinide Intermetallics with Disordered Gd1+xFe4Si10-y Structure Type Grown from Metal Flux.

An1.33T4Al8Si2 (An = Ce, Th, U, Np; T = Ni, Co) were synthesized in metal flux reactions carried out in aluminum/gallium melts. In previous work, U1.33T4Al8Si2 (T = Co, Ni) analogues were formed by arc-melting U:T:Si and reacting this mixture in Al/Ga flux. However, in the current work, all compounds were synthesized by using AnO2 reactants, taking advantage of the ability of the aluminum in the flux to act as both solvent and reducing agent. While reactions with T = Co yielded hexagonal Gd1.33Fe4Si10-type quaternary phases for all An, reactions with T = Ni produced these compounds only with An = U and Np. For reactions with An = Ce and Th, the reactions led instead to the formation of AnNi3-xSixAl4-yGay phases, with the tetragonal KCu3S4 structure type. Attempts to synthesize plutonium analogues Pu1.33T4Al8Si2 were also unsuccessful, producing the previously reported PuCoGa5 and Pu2Ni5Si6 instead. Magnetic data collected on the neptunium analogues Np1.33T4Al8Si2 (T = Ni, Co) show antiferromagnetic coupling at low temperatures and indicate a tetravalent state for the Np ions.

Hydration Structure and Hydrolysis of U(IV) and Np(IV) Ions: A Comparative Density Functional Study Using a Modified Continuum Solvation Approach.

We studied the hydration and the first hydrolysis reaction of U(IV) and Np(IV) ions in an aqueous environment, applying a relativistic density functional method together with a recently proposed variant of a continuum solvation model where the solute cavities are constructed with effective atomic radii, based on charge-dependent scaling factors. In this way, one obtains improved solvation energies of charged species. We demonstrate that solute cavities, constructed with scaled atomic radii as described, permit one to calculate hydrolysis constants of acceptable accuracy. As a consequence, one is also able to estimate free hydration energies of U(IV) and Np(IV) in adequate agreement with empirical data. According to the model calculations, U(IV) is coordinated by eight to nine water molecules, while the preferred coordination number of Np(IV) is 8. For the highly charged ions under study, the modified solvation model simultaneously yields improved geometries, hydration energies, and hydrolysis constants.

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.

Interaction of Ozone with Actinides and Lanthanides in Aqueous Solutions

Specific features of the interaction of O3 in aqueous media with f-elements in a wide range of pH values are considered. In neutral solutions, O3 splits-off an O atom, which becomes the beginning of a chain process of O3 decomposition. NO3–, HSO4–, OH– anions initiate the decomposition by forming complexes with O3. The products of the subsequent transformations are OH, HO2, HO3, and H2O2. Phosphate and carbonate ions and HNO3 react with OH radicals and block the chain process. The peroxynitric acid formed from HNO3 serves both as an oxidizing agent and, as a result of hydrolysis, as reducing agent. In solutions of 1 M NaHCO3 or Na2CO3, ozone forms a complex with CO32– and its stability grows. In the alkaline medium, O3 is converted to O3–, but reducing agents are formed in the alkaline solution at a high concentration of O3 in the gas phase. The reactions of O3 with Ce, U, Np, Pu, Am, Bk, Ce, Tb, and Pr ions are analyzed. The reactions occur via a preliminary formation by cations of anion complexes and then complexes with O3. The formation constant of the Pu(IV) complex with O3 in a 4 M HNO3 solution is estimated to be 4.5 × 105 M–1. U(IV) and Pu(III) in an HClO4 solution, Np(IV) and Pu(IV) in an HNO3 solution, Np(IV) and Am(III) in a NaHCO3 solution are oxidized by ozone with transfer of the O. atom, Ce(III) and Bk(III) in an HNO3 solution, U(IV) and Am(III) in a solution of heteropolyanions, Np(IV, V) in a Na2CO3 solution, Np(V,VI), Pu(V, VI), and Am(V,VI) in alkaline media are oxidized with charge transfer to O3 or O3–.

P-Pyridine BODIPY-based fluorescence probe for highly sensitive and selective detection of picric acid.

A novel fluorescence probe p-PBP for PA was synthesized based on a basic N atom as the electronic donor. The probe could detect PA over TNT, CE, PETN, RDX, HMX, NB, NT, DNT, NP, DNP, and common inorganic explosive ions (K+, Ba2+, NH4+, NO3-, ClO3-, and ClO4-), and common ions (Na+, Ca2+, and Mg2+) with high selectivity. The fluorescence quenching was attributed to the photo-induced electron transfer (PET) processes from the excited state of p-PBP to the ground state PA. The detection limit of probe p-PBP for PA was as low as 13.06 nmol/L, which is far lower than the concentration stipulated by the Environmental quality standards for surface water. The response time was less than 30 s. Hence, the fluorescence probe p-PBP was successfully developed to detect the concentration level of PA in real samples, which would provide a novel quantitative analysis method of PA in forensic science.

Determination of the Gravimetric Content of Neptunium in Spent Nuclear Fuel Samples

The reduction of penta- and hexavalent Np ions to the tetravalent state with Fe(II) sulfamate in HNO3 solutions was studied. The dependence of the degree of the Np reduction on the temperature, time, concentrations of HNO3 and Fe(II) sulfamate, and Np(V) to Np(VI) concentration ratio was studied. The influence of the solution flow rate through the chromatographic column on the degree of the Np recovery was examined. A procedure was developed for Np isolation and determination of its gravimetric content in spent nuclear fuel.

Competition in sonochemical degradation of Naphthol Blue Black: Presence of an organic (nonylphenol) and a mineral (bicarbonate ions) matrix

This study is dedicated to high frequency (278 kHz) sonolytic degradation of organic micropollutants in a complex aqueous medium. A model matrix composed of Naphthol Blue Black (an azoic dye, NBB), of Nonylphenol (an endocrine disrupting compound, NP) and of bicarbonate ions was investigated in order to evaluate the influence of NP and/or bicarbonate ions on the kinetics of NBB sonolysis.It was found that the NBB has no significant effect on the NP sonodegradation kinetics whereas the NP considerably decreases the NBB degradation rate. Furthermore it was highlighted the presence of bicarbonate ions is a way to reach a more efficient NBB degradation rate at low micropollutant concentrations, even in the presence of NP. These preferential sonodegradation rates were explained by the zone where the sonoreaction takes place, which was correlated to the physico-chemical properties of the studied molecules.It was also proved the kinetic model of sonolytic degradation of NBB is modified by the presence of bicarbonate ions; the most suitable model in this case is the Serpone's model which assumes the degradation of the compound in the gas/liquid interface and in the solution.

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.

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.

Reaction of ozone with Np(IV) and Pu(IV) oxalates in water

The reaction of the ozone–oxygen mixture with aqueous suspensions of Np(IV) and Pu(IV) oxalates was studied. Both metal cations and oxalate anions are oxidized in the process. The final products are Np(VI) and Pu(VI) hydroxides. The composition of Np(VI) hydroxide was confirmed by X-ray diffraction analysis. Oxidation of Np(IV) oxalate with oxygen leads to the accumulation of Np(V) oxalate and oxalic acid in the solution. At incomplete oxidation of Np(IV) oxalate with ozone in water, Np(V) is also accumulated. Heating considerably accelerates the ozonation. The possible reaction mechanism is briefly discussed. The Np(V) and Np(VI) ions participate in the catalytic cycle of the decomposition of oxalate ions with ozone.

New potentialities of the UNEX process using polyheterocyclic diamides

The possibilities of using 2,2′-bipyridine-6,6′-dicarboxamides in modified versions of the UNEX process were examined. The stability constants of complexes of Eu(III), Am(III), Th(IV), U(VI), and Np(V) ions with certain diamides substituted in the amide moiety or pyridine ring in acetonitrile were determined. The logarithms of the stability constants exceeded 7 for Eu(III) and reached 9 for 5f elements. The ability of the selected diamides in nitrobenzene and trifluoromethyl phenyl sulfone (FS-13) diluents to extract Am(III), Th(IV), U(VI), and Eu(III) was studied.

Three-Dimensional Network of Cation-Cation-Bound Neptunyl(V) Squares: Synthesis and in Situ Raman Spectroscopy Studies.

Cation-cation interactions (CCIs) are an essential feature of actinyl chemistry, particularly neptunyl(V). To better understand the formation mechanisms of CCIs, the crystallization process of Np(V) CCI compounds has been explored during the evaporation of acidic Np(V) stock solutions using X-ray diffraction and both ex situ and in situ Raman spectroscopy. At least four Np solid products have been isolated from evaporation of the same Np(V) acidic solution. In situ evaporation using a continuous wave laser (532 nm) as a local heat source produced similar solid products to ex situ experiments with matching Raman signatures. The formation of these products is highly dependent on the evaporation conditions. Slower evaporation appears to favor the formation of a new neptunyl(V) compound, (NpO2)Cl(H2O)2 (1), over other solid products. The structure of 1 features a three-dimensional network of NpO2(+) cations, where neighboring Np(V) ions are only connected to each other through CCIs in a square arrangement. The O═Np═O stretching region shows similar Raman bands in both the solids and solution suggesting that CCIs between Np(V) cations exist prior to crystallization. These results provide new insight into the formation mechanism of Np(V) CCI compounds from solutions.

Organic reductants of Pu and Np ions in wet technology for spent nuclear fuel reprocessing

Scientific, technical, and patent information on the use of organic compounds as reductants of Pu and Np ions in wet technology for spent nuclear fuel reprocessing is summarized and correlated. Organic derivatives of hydrazine and hydroxylamine, and also oximes and urea derivatives at relatively low acidities and temperatures reduce Np(VI) to Np(V) and Pu(IV) to Pu(III). As the acidity and temperature are increased, Np(V) is slowly reduced to Np(IV). Alcohols, aldehydes, hydrocarbons, and acetic acid were identified among products of oxidation of the organic compounds in nitric acid medium. The above-named organic reductants can be used in different steps of the PUREX process: separation of Pu and Np from U in the first extraction cycle, purification of U to remove Np in the second cycle of uranium purification, and Pu stripping in the step of its final purification. Laboratory experiments performed on countercurrent extraction installations of mixer-settlers and of centrifugal contactors demonstrated the possibility of reaching high separation factors of U from Pu and/or Np and high degrees of plutonium concentration to obtain Pu strips containing up to 150 g L−1 Pu.

Understanding the interactions of neptunium and plutonium ions with graphene oxide: scalar-relativistic DFT investigations.

Due to the vast application potential of graphene oxide (GO)-based materials in nuclear waste processing, it is of pivotal importance to investigate the interaction mechanisms between actinide cations such as Np(V) and Pu(IV, VI) ions and GO. In this work, we have considered four types of GOs modified by hydroxyl, carboxyl, and carbonyl groups at the edge and epoxy group on the surface, respectively. The structures, bonding nature, and binding energies of Np(V) and Pu(IV, VI) complexes with GOs have been investigated systematically using scalar-relativistic density functional theory (DFT). Geometries and harmonic frequencies suggest that Pu(IV) ions coordinate more easily with GOs compared to Np(V) and Pu(VI) ions. NBO and electron density analyses reveal that the coordination bond between Pu(IV) ions and GO possesses more covalency, whereas for Np(V) and Pu(VI) ions electrostatic interaction dominates the An-OG bond. The binding energies in aqueous solution reveal that the adsorption abilities of all GOs for actinide ions follow the order of Pu(IV) > Pu(VI) > Np(V), which is in excellent agreement with experimental observations. It is expected that this study can provide useful information for developing more efficient GO-based materials for radioactive wastewater treatment.

Low-temperature magnetic properties ofNpNi5

We present the result of an extended experimental characterization of the hexagonal intermetallic Haucke compound NpNi$_{5}$. By combining macroscopic and shell-specific techniques, we determine the 5$f$-shell occupation number $n_f$ close to 4 for the Np ions, together with orbital and spin components of the ordered moment in the ferromagnetic phase below T$_C$ = 16 K ($\mu_{S}$ = -1.88~$\mu_{B}$ and $\mu_{L}$ = 3.91~$\mu_{B}$). The apparent coexistence of ordered and disordered phases observed in the M\"{o}ssbauer spectra is explained in terms of slow relaxation between the components of a quasi-triplet ground state. The ratio between the expectation value of the magnetic dipole operator and the spin magnetic moment ($3\langle T_{z}\rangle/ \langle S_{z}\rangle$ = +1.43) is positive and large, suggesting a localized character of the 5$f$ electrons. The angular part of the spin-orbit coupling ($\langle\vec{\ell}\cdot\vec{s}\rangle$ = -5.55) is close to the value of -6.25 calculated for trivalent Np ions in intermediate coupling approximation. The results are discussed against the prediction of first-principle electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction, and of a mean field model taking into account crystal field and exchange interactions.

Theoretical study on K, L, and M X-ray transition energies and rates of neptunium and its ions

The transition energies and electric dipole (E1) transition rates of the K, L, and M lines in neutral Np have been theoretically determined from the MultiConfiguration Dirac—Fock (MCDF) method. In the calculations, the contributions from Breit interaction and quantum electrodynamics (QED) effects (vacuum polarization and self-energy), as well as nuclear finite mass and volume effects, are taken into account. The calculated transition energies and rates are found to be in good agreement with other experimental and theoretical results. The accuracy of the results is estimated and discussed. Furthermore, we calculated the transition energies of the same lines radiating from the decaying transitions of the K-, L-, and M-shell hole states of Np ions with the charge states Np1+ to Np6+ for the first time. We found that for a specific line, the corresponding transition energies relating to all the Np ions are almost the same; it means the outermost electrons have a very small influence on the inner-shell transition processes.

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.