A comprehensive survey of reference materials for their use in quality assurance for the determination of Np-237 in environmental samples

Neptunium-237 (237Np) is one of the most hazardous radionuclides of public concern due to its radiological toxicity, long half-life and high environmental mobility. Standard reference materials (SRMs) with well characterized 237Np activity concentrations are valuable for method development and validation of 237Np determination in environmental samples. In this study, a comprehensive literature survey of 237Np activity concentrations in 25 SRMs was carried out covering the various matrices of sediment, soil, seawater, atmospheric particles, and biota. After data screening, arithmetic mean of the activity concentrations of 237Np in literatures were calculated as statistical values (SVs). Then, accuracies of different instrument analyses and acid digestion methods were evaluated based on the SVs. Finally, by optimizing the sector field ICP-MS analytical method, 237Np activity concentration in JSAC-0471 soil SRM was measured. The application of SVs provides 237Np activity concentration information in more SRMs. Besides SRMs certified by technically validated procedures, SRMs with SVs and the measurement of JASC-0471 provide more options for suitable selection of SRMs for 237Np determination.

Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes

The study of the redox chemistry of mid-actinides (U-Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non-aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=Pt Bu(pyrr)2 ]- ; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium (1-M, 2-M, M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U5+/4+ , U6+/5+ , and to an unprecedented, well-behaved Np5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.

Assessment of soil-soil solution distribution coefficients of global fallout 237Np and 239Pu in Japanese upland soils

Neptunium-237 and ²³⁹Pu are important radionuclides in the safety assessment related to geological disposal of radioactive waste because of the possibility of long-term exposure to humans. Mobilities of these radionuclides in the environment are of particular importance for their radiation dose evaluation; therefore, in this study, we have made the assessment of the soil-soil solution distribution coefficient (K_d, L/kg) using global fallout ²³⁷Np and ²³⁹Pu in Japanese upland soils. The K_d values were determined by extracting these radionuclides from 24 soil samples using a laboratory batch method. The desorption K_d values of ²³⁷Np ranged from 3.3 × 10² to 1.0 × 10⁴ L/kg, and their geometric mean (GM) and arithmetic mean (AM) were 1.7 × 10³ L/kg and 2.6 × 10³ L/kg, respectively. The desorption K_d values of ²³⁹Pu were found to vary from 9.4 × 10³ to 7.1 × 10⁴ L/kg, and their GM and AM were 3.3 × 10⁴ L/kg and 4.0 × 10⁴ L/kg, respectively. In Japanese upland soils, the K_d value of ²³⁹Pu was one order of magnitude higher than that of ²³⁷Np.

Comparative uptake, translocation, and plant mediated transport of Tc-99, Cs-133, Np-237, and U-238 in Savannah River Site soil columns for the grass species Andropogon virginicus

This study examines the ability of the grass species Andropogon virginicus to alter the subsurface transport and redistribution of a suite of radionuclides (⁹⁹Tc, ¹³³Cs (stable analog for ¹³⁵Cs and ¹³⁷Cs), ²³⁷Np, ²³⁸U) with varying chemical behaviors in a Savannah River Site soil via the use of vegetated and unvegetated soil columns. After an acclimation period, a small volume of solution containing all radionuclides was introduced into the columns via Rhizon© pore water sampling tubes. Plants were grown for an additional 4 weeks before shoots were harvested, and columns were prepared for sampling. Plant presence led to decreased radionuclide release from the columns, mainly due to radionuclide specific combinations of system hydrology differences resulting from plant transpiration as well as plant uptake. For the most mobile radionuclides, ⁹⁹Tc followed by ²³⁷Np, plant presence resulted in significantly different soil concentration profiles between vegetated and unvegetated columns, including notable upward migration for ²³⁷Np in columns with plants. Additionally, plant uptake of ⁹⁹Tc was the greatest of all the radionuclides, with plant tissues containing an average of 44 % of the ⁹⁹Tc, while plant uptake only accounted for <2 % of ²³⁷Np and <0.5 % of ¹³³Cs and ²³⁸U in the system. Although overall plant uptake of ¹³³Cs and ²³⁸U were similar, the majority of ¹³³Cs taken up by plants was associated with ¹³³Cs already available in the aqueous phase while ²³⁸U uptake was mainly associated with the solid phase, meaning that plant activity resulted in a fraction of the native ²³⁸U being mobilized and thus, made available for plant uptake. Overall, this study quantified the influence of several plant-mediated physical and biogeochemical factors that have significant influence on radionuclide mobility and transport in this complex system which can be further utilized in future system or site-specific environmental transport and risk assessment models.

Soil-soil solution distribution coefficients of global fallout 239Pu and 237Np in Japanese paddy soils

²³⁹Pu and ²³⁷Np are long-lived radionuclides that emit alpha rays, and once released into the environment, they are present there for a long period of time. Therefore,²³⁹Pu and ²³⁷Np are important radionuclides in the safety assessment related to geological disposal of radioactive waste because of the possibility of long-term exposure to humans. Mobilities of these radionuclides in the environment are of particular interest; therefore, in this study, we have made the first-time determination of the soil-soil solution distribution coefficient (K_d, L/kg) using global fallout ²³⁹Pu and ²³⁷Np in soil. The K_d values were determined by extracting these radionuclides from 23 soil samples using a laboratory batch method. The desorption K_d values of ²³⁹Pu were found to vary from 3.2 × 10³ to 1.4 × 10⁵ L/kg, and their geometric mean (GM) and arithmetic mean (AM) were 2.3 × 10⁴ L/kg and 3.2 × 10⁴ L/kg, respectively. The desorption K_d values of ²³⁷Np ranged from 8.9 × 10² to 2.1 × 10⁴ L/kg, and their GM and AM were 4.1 × 10³ L/kg and 6.4 × 10³ L/kg, respectively. A comparison between the obtained K_d values of ²³⁹Pu and ²³⁷Np indicated that the former were about 6-fold higher than the latter in Japanese paddy field soils. Pearson's correlation analysis suggested that the main factors contributing to the sorption and desorption of Pu isotopes in Japanese paddy soils were related to the amounts of Fe and Mn oxides, while the sorption and desorption of ²³⁷Np in them would be controlled by an ion-exchange reaction, and/or complex-formation with organic matter, especially in Fluvisols.

Spectroscopic characterisation and thermodynamics of the complexation of Np(V) with sulfate up to 200 °C

In the present work the complexation of Np(V) with sulfate in aqueous solution is studied in a temperature range up to 200 °C by absorption spectroscopy. For this purpose, a new spectroscopic setup is implemented and tested for its suitability for Vis/NIR absorption spectroscopy at elevated temperatures. The complexation of Np(V) with sulfate is studied as a function of the total ligand concentration at various temperatures (T = 25-200 °C) and ionic strengths (I_m(NaClO₄) = 1.0-4.0 mol kg^-1 NaClO₄). The exclusive formation of NpO₂(SO₄)^- up to 200 °C is confirmed by peak deconvolution and slope analyses. The thermodynamic stability constants log β⁰₁(T) are obtained from linear regressions according to the specific ion interaction theory (SIT). A systematic increase of the log β⁰₁(T) is observed with increasing temperature, resulting in a linear correlation of log β⁰₁(T) with T^-1. The magnitude of the increase is 1.9 logarithmic units at 200 °C in comparison to log β⁰₁(25 °C) = 1.05 ± 0.16. Thus, the standard reaction enthalpy and entropy (Δ_rH⁰_m, Δ_rS⁰_m) are determined with the integrated Van't Hoff equation revealing Δ_rH⁰_m = 31.0 ± 1.0 kJ mol^-1 and Δ_rS⁰_m = 123 ± 9 J mol^-1 K^-1. In addition, the stoichiometric sum of the specific binary ion-ion interaction coefficient (Δε₀₁(T)) is determined up to 200 °C showing an insignificant temperature dependence. Thus, a temperature-independent ε(Na⁺, NpO₂(SO₄)^-) = 0.07 ± 0.11 is calculated for the temperature range up to 200 °C. Comparison of the present results with literature data confirms the excellent applicability of the new high-temperature absorption spectroscopic setup for complexation studies up to 200 °C.

First study of 237Np in Chinese soils: Source, distribution and mobility in comparison with plutonium isotopes

In this study, the distribution and migration of ²³⁷Np and ^239+240Pu in soils in the vicinity (<5 km) of Qinshan and Tianwan Nuclear Power Plants in China were studied, which is the first specific study of global fallout ²³⁷Np in Chinese soils. The ²³⁷Np and ^239+240Pu concentrations in surface soils showed large spatial inhomogeneity. A remarkable ^239+240Pu concentration (4.783 mBq/g) was observed in a surface soil near Qinshan NPP and stands for the ever reported highest value in the Chinese soils. The inventories of ^239+240Pu in two Qinshan and Tianwan soil cores were estimated to be 128.8 Bq/m² and 121.0 Bq/m², respectively; while the ²³⁷Np inventories were 0.039 Bq/m² and 0.035 Bq/m² at these sites, respectively. The ²⁴⁰Pu/²³⁹Pu atomic ratios in these soils indicated that the global fallout is the main source of Pu in these regions. However, the non-isotopic ²³⁷Np/²³⁹Pu atomic ratio in environmental soil is not a sensitive indicator for source identification. Furthermore, we conducted pilot study on the migration behaviors of ²³⁷Np and ^239+240Pu in soil core at Qinshan site with the Convection-Dispersion Equation (CDE) model. The obtained apparent dispersion coefficients of ²³⁷Np (2.82 ± 2.06 cm²/y) was 5 times higher than that of ^239+240Pu (0.57 ± 0.16 cm²/y), proving that ²³⁷Np has stronger migration ability than Pu isotopes in the Qinshan soil. Finally, we predicted that with the increase of migration time, both ²³⁷Np and ^239+240Pu concentration in the soil will gradually become more evenly distributed among different soil layers due to the dominant dispersion effects.

Electrochemical precipitation of neptunium with a micro electrochemical quartz crystal microbalance

Microliter volumes are used in electrochemical detection and preconcentration of radionuclides to reduce the dose received by researchers and equipment. Unfortunately, there is a lack of analysis of radionuclides with coupled electrochemical techniques and microliter volume reactors. The goals of this work are 1) to develop a miniaturized micro-electrochemical quartz crystal microbalance (µeQCM) reactor for use in small volume (50-200 µL) electrogravimetric experiments and 2) to use this reactor to characterize the preconcentration of neptunium on carbon electrodes via electroprecipitation. We successfully deposited neptunium in the new µeQCM reactor and verified its operation. We found that preconcentration of neptunium on carbon coated electrodes was possible by chronoamperometry at -1.6 VAg/AgCl. The mass shift of the resulting precipitate was indicative of the amount of neptunium on the electrode, although the correlation between the mass increase and activity of the preconcentrated material was not linear. Neptunium precipitate reduced electron transfer to the solution as evidenced by the increase in charge transfer resistance compared to bare electrodes.

Kinetics of neptunium sorption and desorption in the presence of aluminum (hydr)oxide minerals: Evidence for multi-step desorption at low pH

Kinetics analyses of sorption and desorption provide important insight into reaction mechanisms occurring at the mineral-water interface. They are also needed to determine when equilibrium is achieved, identify intermediate chemical species, and inform models describing neptunium mobility. Neptunium sorption to and desorption from four different aluminum (hydr)oxides - bayerite (α-Al(OH)₃), gibbsite (γ-Al(OH)₃), corundum (α-Al₂O₃), and γ-alumina (γ-Al₂O₃) - were investigated as a function of mineral concentration (5 - 170 m² L^-1), neptunium concentration (10^-9 - 10^-7 M), and pH (5.5 - 10.5). Neptunium sorption was characterized by a two-step reaction with an initial fast sorption step occurring within minutes followed by a slower equilibrium process, which was attributed to initial sorption of neptunium to a small number of strong sorption sites followed by sorption of neptunium to a larger number of weak sorption sites. The kinetics data were modeled using the linear and non-linear forms of the pseudo-first and pseudo-second order rate equations and the goodness of fit parameters were compared. Non-linear pseudo-second order rate constants described neptunium sorption to aluminum (hydr)oxides most accurately and were used to determine the reaction orders with respect to mineral concentration and [H⁺]. Neptunium desorption experiments demonstrated that the desorption mechanism changed as a function of pH and that the forward and reverse reactions were not equivalent. At pH ≥ 7.5, desorption reached steady-state within an hour and was accurately described by the non-linear pseudo-second order rate equations. A desorption plateau was observed at pH 5.5 that could not be described by either pseudo-first or -second order kinetics, suggesting the possibility of a multi-step desorption reaction. The comparatively slow desorption kinetics observed here suggests that sorbed neptunium could be slowly released back into the aqueous phase and act as a continuous source of contamination to the environment.

Determination of plutonium isotopes in environmental samples by extraction chromatography with triisooctylamine - polyethylene resin

Method for determination of plutonium isotopes in various environmental samples is presented. The separation and purification of plutonium is attained by extraction chromatography with triisooctylamine - polyethylene resin in nitric and hydrochloric acid media. Plutonium is measured by alpha-particle spectrometry after electrodeposition on a stainless steel disk. The analytical quality was checked by analyzing reference materials with different matrices from IAEA (Soil-6, IAEA-375, IAEA-384, IAEA-414) and NPL (AL-2009, AL-2010, AL-2011, AL-2013, AB-2014). The major advantages of the method are the low cost of the analysis, high radiochemical yields and high decontamination factors from the matrix elements, natural and man-made radionuclides.

Separation of neptunium (IV) from actinides by solid phase extraction using a resin containing Aliquat 336

An extraction chromatographic resin material containing Aliquat 336 as the liquid anion exchanger extractant and Chromosorb W as the solid support was prepared and tested for the uptake of UO₂²⁺, Np⁴⁺, Pu⁴⁺, and Pu³⁺ from nitric acid feed solutions. The resin beads were characterized by thermogravimetry/differential thermogravimetry (TG/DTG) and scanning electron microscopy (SEM) surface morphology analysis. The uptake trend for the metal ions from 3 M HNO₃ was found to be Pu⁴⁺ >> Np⁴⁺ >> UO₂²⁺ > Pu³⁺ which clearly followed the trend of their ionic potentials. In view of the significant difference in the uptake of Np⁴⁺ with respect to those of UO₂²⁺ and Pu³⁺, a separation scheme was developed for the selective separation of Np from feeds containing U, Np and Pu in nitric acid. The purity of the product was verified by alpha spectrometry.

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.

The influence of citrate and oxalate on 99TcVII, Cs, NpV and UVI sorption to a Savannah River Site soil

Batch sorption experiments were conducted with 0.5-50 ppb ⁹⁹Tc, ¹³³Cs, ²³⁷Np and U in the presence and absence of citrate and/or oxalate in a 25 g/L Savannah River Site (SRS) soil suspension. Citrate and oxalate were the ligands of choice due to their relevancy to plant exudates, the nuclides were selected for their wide range of biogeochemical behavior, and the soil from SRS was selected as a model Department of Energy (DOE) site soil. Batch samples were continually mixed on a rotary shaker and maintained at a pH of approximately 5. Analysis via ICP-MS indicated that sorption of ²³⁷Np increased with ligand concentration compared to baseline studies, as did sorption of ⁹⁹Tc although to a lesser extent. The increased sorption of ²³⁷Np is proposed to be due to a combination of factors that are dependent on the ligand(s) present in the specific system including, ligand dissolution of the soil by citrate and formation of tertiary soil-oxalate-Np complexes. The increased ⁹⁹Tc sorption is attributed to the dissolution of the soil by the ligands, leading to an increase in the number of available sorption sites for ⁹⁹Tc. Uranium sorption decreased and dissolution of native uranium was also observed with increasing ligand concentration, thought to be a result of the formation of strong U-ligand complexes remaining in the aqueous phase. The majority of these effects were observed at the highest ligand concentrations of 50 mg_C/L. No notable changes were observed for the ¹³³Cs system which is ascribed to the minimal interaction of Cs⁺ with these organic ligands.

Enzymatic activity of the CaM-PDE1 system upon addition of actinyl ions

The threat of a dirty bomb which could cause internal contamination has been of major concern for the past decades. Because of their high chemical toxicity and their presence in the nuclear fuel cycle, uranium and neptunium are two actinides of high interest. Calmodulin (CaM) which is a ubiquitous protein present in all eukaryotic cells and is involved in calcium-dependent signaling pathways has a known affinity for uranyl and neptunyl ions. The impact of the complexation of these actinides on the physiological response of the protein remains, however, largely unknown. An isothermal titration calorimetry (ITC) was developed to monitor in vitro the enzymatic activity of the phosphodiesterase enzyme which is known to be activated by CaM and calcium. This approach showed that addition of actinyl ions (AnO₂ⁿ⁺), uranyl (UO₂²⁺) and neptunyl (NpO₂⁺), resulted in a decrease of the enzymatic activity, due to the formation of CaM-actinide complexes, which inhibit the enzyme and alter its interaction with the substrate by direct interaction. Results from dynamic light scattering rationalized this result by showing that the CaM-actinyl complexes adopted a specific conformation different from that of the CaM-Ca²⁺ complex. The effect of actinides could be reversed using a hydroxypyridonate actinide decorporation agent (5-LIO(Me-3,2-HOPO)) in the experimental medium demonstrating its capacity to efficiently bind the actinides and restore the calcium-dependent enzyme activation.

Concurrent determination of 237Np and Pu isotopes using ICP-MS: analysis of NIST environmental matrix standard reference materials 4357, 1646a, and 2702

An optimized method was developed to analyze environmental soil and sediment samples for (237)Np, (239)Pu, and (240)Pu by ICP-MS using a (242)Pu isotope dilution standard. The high yield, short time frame required for analysis, and the commercial availability of the (242)Pu tracer are significant advantages of the method. Control experiments designed to assess method uncertainty, including variation in inter-element fractionation that occurs during the purification protocol, suggest that the overall precision for measurements of (237)Np is typically on the order of ± 5%. Measurements of the (237)Np concentration in a Peruvian Soil blank (NIST SRM 4355) spiked with a known concentration of (237)Np tracer confirmed the accuracy of the method, agreeing well with the expected value. The method has been used to determine neptunium and plutonium concentrations in several environmental matrix standard reference materials available from NIST: SRM 4357 (Radioactivity Standard), SRM 1646a (Estuarine Sediment) and SRM 2702 (Inorganics in Marine Sediment).

The production of Neptunium-236g

Radiochemical analysis of (237)Np is important in a number of fields, such as nuclear forensics, environmental analysis and measurements throughout the nuclear fuel cycle. However analysis is complicated by the lack of a stable isotope of neptunium. Although various tracers have been used, including (235)Np, (239)Np and even (236)Pu, none are entirely satisfactory. However, (236g)Np would be a better candidate for a neptunium yield tracer, as its long half-life means that it is useable as both a radiometric and mass spectrometric measurements. This radionuclide is notoriously difficult to prepare, and limited in scope. In this paper, we examine the options for the production of (236g)Np, based on work carried out at NPL since 2011. However, this work was primarily aimed at the production of (236)Pu, and not (236g)Np and therefore the rate of production are based on the levels of (236)Pu generated in the irradiation of (i) (238)U with protons, (ii) (235)U with deuterons, (iii) (236)U with protons and (iv) (236)U with deuterons. The derivation of a well-defined cross section is complicated by the relevant paucity of information on the variation of the (236m)Np:(236g)Np production ratio with incident particle energy. Furthermore, information on the purity of (236g)Np so produced is similarly sparse. Accordingly, the existing data is assessed and a plan for future work is presented.

An overview of results obtained in intercomparison exercises for determination of actinides

In recent years the Jožef Stefan Institute participated in numerous intercomparison exercises for determination of natural and man-made radionuclides. The reported values were mostly in good agreement with the resulting reference values. This paper describes an analysis of the measurement results obtained in intercomparison exercises for determination of actinides in the period from 2009 to 2011, organised by the National Physical Laboratory and the Bundesamt für Strahlenschutz. The review covers neptunium, plutonium and americium radioisotopes over this period.