Behavior Of Neptunium Research Articles

Sorption behaviour of neptunium in marine and fresh water bottom sediments in Far East area of Russia (Lake Khanka and Amur Bay)

The concentration and sorption behavior of 237Np on the bottom sediments of water bodies in the Far East region of Russia (Lake Khanka and Peter the Great Bay) were studied for the first time. The 237Np concentrations vary from 1.06 × 10−6 to 4.43 × 10−5 mBq g−1 in the bottom sediments of Lake Khanka and from 1.05 × 10−4 to 2.52 × 10−3 mBq g−1 for Amur Bay. The experiment on the adsorption of Np on marine and lake sediment showed that it is sorbed through complexation with silicates (albite, leucite). The Np sorption isotherm on marine sediments is described by the Langmuir equation; the distribution coefficients (Kd) of Np vary from 57 to 588 mL g−1. For lake sediments, the isotherm is described by the Henry equation; the Kd value reaches 935 mL g−1.

Impact of a Biological Chelator, Lanmodulin, on Minor Actinide Aqueous Speciation and Transport in the Environment.

Minor actinides are major contributors to the long-term radiotoxicity of nuclear fuels and other radioactive wastes. In this context, understanding their interactions with natural chelators and minerals is key to evaluating their transport behavior in the environment. The lanmodulin family of metalloproteins is produced by ubiquitous bacteria and Methylorubrum extorquens lanmodulin (LanM) was recently identified as one of nature's most selective chelators for trivalent f-elements. Herein, we investigated the behavior of neptunium, americium, and curium in the presence of LanM, carbonate ions, and common minerals (calcite, montmorillonite, quartz, and kaolinite). We show that LanM's aqueous complexes with Am(III) and Cm(III) remain stable in carbonate-bicarbonate solutions. Furthermore, the sorption of Am(III) to these minerals is strongly impacted by LanM, while Np(V) sorption is not. With calcite, even a submicromolar concentration of LanM leads to a significant reduction in the Am(III) distribution coefficient (Kd, from >104 to ∼102 mL/g at pH 8.5), rendering it even more mobile than Np(V). Thus, LanM-type chelators can potentially increase the mobility of trivalent actinides and lanthanide fission products under environmentally relevant conditions. Monitoring biological chelators, including metalloproteins, and their biogenerators should therefore be considered during the evaluation of radioactive waste repository sites and the risk assessment of contaminated sites.

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 carbonate on the migration behavior of neptunium in compacted bentonite

Effect of carbonate on the migration behavior of neptunium in compacted bentonite

Distribution Behavior of Neptunium by Extraction with N,N-dialkylamides (DEHDMPA and DEHBA) in Mixer-Settler Extractors

ABSTRACTThe extraction properties of N,N-di(2-ethylhexyl)-2,2-dimethylpropanamide (DEHDMPA) and N,N-di(2-ethylhexyl)butanamide (DEHBA) for Np(V) and Np(VI) were studied by a batch method using various nitrate ion concentrations. The distribution ratios of Np(VI) obtained with DEHDMPA and DEHBA exceeded unity when the nitrate ion concentration was > 3 mol/dm3, while DEHDMPA and DEHBA barely extracted Np(V). A continuous counter-current experiment using mixer-settler extractors was performed to evaluate the behavior of Np in a process comprising two cycles using DEHDMPA and DEHBA as extractants. The feed was nitric acid containing U, Pu, Np, and several fission products. The results indicated that part of Np(V) changed its valence state to Np(IV) or Np(VI) after the 1st experimental cycle. The recoveries of Np in the streams of U fraction and U-Pu fraction were 63.7% and 29.1%, respectively.

Chemical behavior of neptunium in the presence of technetium in nitric acid media

Abstract The chemical behavior of Np is influenced by many factors, such as U(IV), hydrazine, Pu and Tc, which coexist in U/Pu separation stage of PUREX process. Firstly, the reduction of Np(V) to Np(IV) by Tc(IV) was studied in nitric acid media, and the rate equation could be expressed as − d[Np(V)]/dt = k[Np(V)][Tc(IV)]0.8[HNO3]1.2, k = 28.5 ± 0.9 (L/mol)2/min at 25 ℃, the activation energy was E a = 70.0 kJ/mol. Thereafter, the chemical behavior of Np was investigated in aqueous nitric acid solution containing Tc(VII), U(IV) and N2H4. Np(V) was reduced to Np(IV) quickly, Tc(VII) was reduced to Tc(IV) and U(IV) was oxidized to U(VI) at the same time. Thereafter Np(IV) was entirely reoxidized to Np(V) when U(IV) and hydrazine were completely consumed. The fast reduction of Np(V) was mainly ascribed to Tc(IV). In addition, Tc(VII) and nitric acid had little influence on the fast oxidation of Np(IV).

Unusual redox stability of neptunium in the ionic liquid [Hbet][Tf(2)N

The behavior of neptunium in the ionic liquid betaine bistriflimide, [Hbet][Tf2N], has been studied spectroscopically at room temperature and 60 °C for the first time. An unprecedented complex redox chemistry is observed, with up to three oxidation states (iv, v and vi) and up to six Np species existing simultaneously. Both redox reactions and coordination of betaine are observed for Np(iv), (v) and (vi). Elevating the temperature accelerates the coordination of Np(v) with betaine and reduction reactions slow down.

Simulation of the neptunium behavior during the first solvent extraction cycle in the PUREX process

In order to co-recycle neptunium, uranium and plutonium using the PUREX process, numerical simulations were used to investigate the behaviors of neptunium in the first solvent extraction cycle. Counter-current extraction experiments using a miniature mixer-settler were performed to verify the simulation. Calculated results based on experiments show that the concentrations of nitric and nitrous acid, saturation of uranium, flow ratio and residence time have important effects on the pathway of neptunium, and accumulation of neptunium may appear with the presence of uranium and plutonium.

Counter-Current Extraction of Neptunium from Simulated Pressurized Heavy Water Reactor High Level Waste Using N,N,N',N'-Tetraoctyl Diglycolamide

The extraction behavior of neptunium was investigated from Pressurized Heavy Water Reactor Simulated High Level Waste (PHWR-SHLW) using 0.05 M N,N,N',N'-tetraoctyl diglycolamide (TODGA) + 5% iso-decanol dissolved in n-dodecane as the extractant. Batch extraction data revealed that the extraction of Np(IV) was a magnitude of order higher than that of the Np(VI). The stoichiometry of the extracted species was investigated by slope analysis and was found to be 2:1 (Ligand : Metal) for Np(IV) and 1:1 for Np(VI). The distribution data suggested that the neptunium in SHLW was predominantly converted into Np(VI) and its extraction decreased. Counter-current extraction studies employing 12 stages of mixer settler suggested that ∼97% extraction of Np was possible from PHWR-SHLW at organic-to-aqueous phase ratio (O/A) of 1. However, stripping was efficient and > 99.9% of Np could be back extracted in 8 stages from the loaded organic phase.

Physical Characterization and Reactivity of the Uranyl Peroxide [UO2(η2-O2)(H2O)2]·2H2O: Implications for Storage of Spent Nuclear Fuels

The unusual uranyl peroxide studtite, [UO(2)(η(2)-O(2))(H(2)O)(2)]·2H(2)O, is a phase alteration product of spent nuclear fuel and has been characterized by solid-state cyclic voltammetry. The voltammogram exhibits two reduction waves that have been assigned to the U(VI/V) redox couple at -0.74 V and to the U(V/IV) redox couple at -1.10 V. This potential shows some dependence upon the identity of the cation of the supporting electrolyte, where cations with larger ionic radii exhibit more cathodic reduction potentials. Raman spectroelectrochemistry indicated that exhaustive reduction at either potential result in a product that does not contain peroxide linkers and is likely to be UO(2). On the basis of the reduction potentials, the unusual behavior of neptunium in the presence of studtite can be rationalized. Furthermore, the oxidation of other species relevant to the long-term storage of nuclear fuel, namely, iodine and iodide, has been explored. The phase altered product should therefore be considered as electrochemically noninnocent. Radiotracer studies with (241)Am show that it does not interact with studtite so mobility will not be retarded in repositories. Finally, a large difference in band gap energies between studtite and its dehydrated congener metastudtite has been determined from the electronic absorption spectra.

Comparison of Extraction Behavior of Neptunium from Nitric Acid Medium Employing Tri-n-Butyl Phosphate and N,N-dihexyl Octanamide as Extractants

Extraction behavior of neptunium has been compared for tri-n-butyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) extractants as a function of nitric acid concentration (0.5 − 6 M HNO3), uranium loading (50 and 300 g/L relevant to Pu rich fast reactor and Pressurized Heavy Water Reactor, PHWR spent fuels, respectively), and in the presence of oxidizing and reducing agents. These studies suggest the possibility of co-recovery of U(VI), Pu(IV) and Np(IV) from spent fuel dissolver solutions (of Pu rich fuels) employing DHOA as extractant.

Redox Behavior of Neptunium(V) in Tributyl Phosphate and N,N-Dihexyl Octanamide Extractants Dissolved in n-Dodecane

Spectrophotometric investigations have been carried out on the disproportionation of Np(V) to form Np(IV) and Np(VI) in 1.1 mol⋅L−1 solutions of tributyl phosphate (TBP) and in N,N-dihexyl octanamide (DHOA) in n-dodecane medium. The Np(V) was found to coordinate with Np(IV) in 1.1 mol⋅L−1 TBP solution in n-dodecane to form a mixed valence “cation–cation” complex by bonding through an axial oxo group on Np(V). By contrast, this interaction was less prominent in the case of 1.1 mol⋅L−1 DHOA solutions. The effect of 1-octanol, added as phase modifier, on the disproportionation behavior of Np(V) was also investigated. An attempt was made to calculate the disproportionation/reduction rate constants for Np(V) under the conditions of these studies. Absorbance measurements on the Np stripped from organic phases revealed the occurrence of Np(V) in the aqueous phase.

Reduction and removal of neptunium from synthetic high level liquid waste by TRPO process

Valency control of neptunium is an important issue in the partitioning of high level liquid waste (HLLW) from power-reactor spent fuel treatment. The redox behavior of neptunium in HLLW is quite different from that in nitric acid because of the effect of the large amount of ions in HLLW. In order to remove neptunium from HLLW, we studied the reduction of neptunium in synthetic HLLW (SHLLW) to maintain its valency at IV so that it can be extracted by TRPO extractant in the well developed Chinese TRPO process. Five different reductants were tested and the reduction behavior was investigated. The influence of some active elements in SHLLW was studied. The mechanism that the reductants react with neptunium through Fe element was supposed and proved by experiments. The reduction rate of Np(V) was highly enhanced by Fe element. Finally, a hybrid reductant was suggested and good reduction efficiency was obtained.

Neptunium(V) sorption on kaolinite

Abstract The sorption behavior of neptunium(V) onto the clay mineral kaolinite was studied in batch experiments under different experimental conditions: [Np(V)]=7 × 10−12–8 × 10−6 M, solid-to-liquid ratio 2–20 g L−1, I=0.1 and 0.01 M NaClO4, pH=6–10, ambient air and Ar atmosphere. The short-lived isotope 239Np (T 1/2=2.36 d) was used instead of 237Np (T 1/2=2.14 × 106 a) to study the sorption behavior of Np(V) at environmentally-relevant concentrations, i.e., 7 × 1012 M Np. In addition, 239Np(V) served as tracer to measure sorption isotherms over six orders of magnitude in Np concentration (4.8 × 10−12–1.0 × 10−4 M). The results show that Np(V) sorption on kaolinite is strongly influenced by pH, CO2, and ionic strength. The sorption of 8 × 10−6 M Np(V) at pH 9.0, and ionic strength of 0.1 M NaClO4 was proportional to the solid-to-liquid ratio of kaolinite in the range of 2–10 g L−1. In the absence of CO2, the Np(V) uptake increased continuously with increasing pH value up to 97% at pH 10. Under ambient CO2, the sorption of Np decreased above pH 8 up to zero at pH 10. An increase of Np(V) concentration from 7 × 10−12 to 8 × 10−6 M resulted in a shift of the sorption pH edge by up to one pH unit to higher pH values. The ionic strength influenced the Np(V) sorption onto kaolinite only in the presence of ambient CO2. Under Ar atmosphere the sorption of Np(V) was independent from ionic strength, indicating the formation of inner-sphere complexes of Np(V) with kaolinite. Time-dependent batch experiments at pH 9.0 under ambient CO2 showed that the sorption of Np(V) on kaolinite is fast and fully reversible over six orders in Np(V) concentration.

Study on coordination characteristics of neptunium and uranium ions in calcium nitrate hydrate melt by Raman spectrometry and UV/Vis/NIR spectrometry

Extraction behavior of neptunium (Np) by tri-n-butyl phosphate from calcium nitrate hydrate melt was investigated. Distribution ratio of Np was found to increase with the decrease of water content. Adding nitric acid into the system resulted in an increase of the distribution ratio. In order to understand the extraction trends, Np species in the hydrate melt were analyzed by Raman spectrometry and UV/Vis/NIR spectrometry. Major fraction was assigned to be NpO22+ of Np(VI) and small fraction to be NpO2+ of Np(V). A shift of the v1 symmetric vibrational frequency of NpO22+ in nitrate media was found in Raman spectra. This suggests a coordination circumstance change of NpO22+.

Reduction behavior of neptunium(V) at a gold or platinum electrode during controlled potential electrolysis and procedures for electrochemical preparations of neptunium(IV) and (III)

Abstract Unique current–time relations were observed during the controlled potential electrolysis for the reduction of NpO 2 + to Np4+ or Np3+ at a gold or platinum electrode in perchloric acid solution. The reduction process of NpO 2 + was elucidated based on controlled potential electrolysis and voltammetry as follows: Though NpO 2 + is not reduced directly at a gold electrode, it is reduced by chemical reaction with Np3+ which is produced by the electrode reduction of Np4+. Here, Np4+ is initially produced through the disproportionation of NpO 2 + , and generated progressively through the reduction of NpO 2 + . The reduction of NpO 2 + at a platinum electrode is initiated by the electrocatalytic reduction to Np4+ with the hydrogen atom which is produced by the electrode reduction of H+ and adsorbed on the platinum electrode surface. The Np4+ produced functions as an electron transfer mediator to reduce NpO 2 + similar to the reaction at a gold electrode. The above described catalytic reduction processes of NpO 2 + were elucidated with the aid of digital simulation of bulk electrolyses. Methods useful for the preparation of Np4+ and Np3+ by bulk electrolysis were proposed based on the experimental results.

Neptunium(V) sorption onto gibbsite

The sorption behavior of neptunium(V) on gibbsite (γ-Al(OH)3) was studied by batch experiments in the presence/absence of ambient CO2 as a function of solid-to-liquid ratio, pH, ionic strength, and Np(V) concentration. The sorption of 8×10−6 M Np(V) at ionic strength of 0.1 M NaClO4 was proportional to the solid-to-liquid ratio of gibbsite with a Kd value of ∼45±6 mL/g at pH 9.0. The sorption behavior of Np(V) on gibbsite was strongly affected by ambient CO2 and pH in the pH range of 6.0–10.0. In the presence of CO2 at ionic strengths of 0.01 and 0.1 M NaClO4, the maximum Np(V) uptake was at pH 8.5 for 8×10−6 and 7×10−12 M Np(V). In the absence of CO2 at the same ionic strengths, the Np(V) uptake increased with increasing pH up to ∼95% at pH 10.0. The sorption edges of Np(V) shifted by approximately one unit toward higher pH while increasing the concentration from 7×10−12 to 8×10-−6 M Np(V) and the amount of Np(V) sorbed at pH 8.5 decreased by ca. 40% under ambient air conditions. Ionic strength had no effect on the sorption of Np(V) onto gibbsite whether CO2 was present or not, indicating the formation of inner-sphere surface complexes between Np(V) and γ-Al(OH)3.

Electrochemical and Complexation Behavior of Neptunium in Aqueous Perchlorate and Nitrate Solutions

Electrochemical and complexation properties of neptunium (Np) are investigated in aqueous perchlorate and nitrate solutions by means of cyclic voltammetry, bulk electrolysis, UV-visible absorption, and Np L(III)-edge X-ray absorption spectroscopies. The redox reactions of Np(III)/Np(IV) and Np(V)/Np(VI) couples are reversible or quasi-reversible, while the electrochemical reaction between Np(III/IV) and Np(V/VI) is irreversible because they undergo structural rearrangement from spherical coordinating ions (Np(3+) and Np(4+)) to transdioxoneptunyl ions (NpO2(n+), n = 1 for Np(V) and 2 for Np(VI)). The redox reaction of the Np(V)/Np(VI) couple involves no structural rearrangement on their equatorial planes in acidic perchlorate and nitrate solutions. A detailed analysis on extended X-ray absorption fine structure (EXAFS) spectra suggests that Np(IV) forms a decaaquo complex of [Np(H2O)10](4+) in 1.0 M HClO4, while Np(V) and Np(VI) exist dominantly as pentaaquoneptunyl complexes, [NpO2(H2O)5](n+) (n = 1 for Np(V) and 2 for Np(VI)). A systematic change is observed on the Fourier transforms of the EXAFS spectra for all of the Np oxidation states as the nitrate concentration is increased in the sample, revealing that the hydrate water molecules are replaced by bidentate-coordinating nitrate ions on the primary coordination sphere of Np.

Solvent Extraction Behavior of Neptunium (IV) Ions between Nitric Acid and Diluted 30% Tri‐butyl Phosphate in the Presence of Simple Hydroxamic Acids

Formo‐ and aceto‐hydroxamic acids are very effective reagents for stripping tetravalent actinide ions such as Np(IV) and Pu(IV) ions from a tri‐butyl phosphate phase into nitric acid. Distribution data for Np(IV) in the presence of these hydroxamate ions have now been accumulated and trends established. Stability constants for aceto‐hydroxamate complexes of Np(IV) and Np(V) ions have also been determined in a perchlorate medium, and these reaffirm the affinity of hydroxamate ligands for actinide (IV) ions over actinyl (V,VI) ions.

Study of Np speciation in citrate medium

In the framework of the French Environmental Nuclear Toxicology program, additional experiments related to the decorporation of actinides are planned. Decorporation is the removal or release from target organs (bones, liver, kidney …), tissues or cells of radioactive material previously incorporated in them, using chelating agents or other administrated pharmaceutical agents. The contradictory data on the neptunium complexation behaviour within blood and its transfer to target organs, as well as the inefficiency of therapeutic treatments, led us to study the complexation of this element with biological constituents. Within this purpose, the in vitro behaviour of Np(IV) and Np(V) in simple media simulating biological fluids was studied. This study was more specifically focused on the behaviour of neptunium with citrate ion, which is an essential component in a number of metalloenzyme active sites. In order to determine the speciation of this system, spectrophotometry was more particularly used. Concerning the complexation phenomenon, the existence of several complexes of Np(V) with various acido-basic forms of the citrate anion was observed; regarding Np(IV), complexes with Cit3- have been observed. From the quantitative study of these equilibria, the values of the absolute constants for the complexation of Np(IV) and Np(V) with citrate were determined. Concerning the stability of neptunium towards oxydo-reduction, it was confirmed that Np(VI) was very quickly reduced to Np(V) by the citrate anions, whereas Np(IV) was stable. In the case of Np(V), it was observed that, depending on the pH and the citrate concentration, Np(V) was unstable and was reduced to Np(IV). The E–pH diagrams, constructed using the stability constants determined in this study, showed that this instability was due to the Np(V) disproportionation.