Mössbauer spectroscopy of237Np in solid solutions of actinide dioxides

Emission Mössbauer (nuclear gamma-resonance) spectra of²³⁷Np resulting from the alpha-decay of²⁴¹Am in solid solutions AmO₂–ThO₂ and AmO₂–UO₂ prepared by precipitation from LiF–NaF eutectic melts under fluorine-oxygen exchange were first investigated in the temperature range from 77 to 296 K. The resonant absorbers are NpO₂ and NpAl₂. The possibility is stated of managing the form of the emission Mössbauer spectra of²³⁷Np in dioxides by the directed change of the matrix stoichiometry or by its irradiation by visible light. At temperatures from 77 to 230 K the authors measured for the first time the NGR absorption spectra of²³⁷Np in solid solution NpO₂–ThO₂ with various concentrations of neptunium dioxide. An increase of the absorption line width (approximately by a factor of 4) with a decrease in the NpO₂ contents (to 10 mol.%) was explained by the influence of fast electron exchange between Np⁴⁺ and Np⁵⁺ states observed in the NpO₂–ThO₂ solid solution at low concentrations of neptunium dioxide.     

Thermal stability and degradation kinetic study of white and colored cotton fibers by thermogravimetric analysis

Complexation of neptunium(V) with fluoride in aqueous solutions at elevated temperatures was studied by spectrophotometry and microcalorimetry. Two successive complexes, NpO₂F(aq) and NpO₂F₂⁻, were identified by spectrophotometry in the temperature range of 10–70°C. Thermodynamic parameters, including the equilibrium constants and enthalpy of complexation between Np(V) and fluoride at 10–70°C were determined. Results show that the complexation of Np(V) with fluoride is endothermic and that the complexation is enhanced by the increase in temperature — a two-fold increase in the stability constants of NpO₂F(aq) and more than five-fold increase in the stability constants of NpO₂F₂⁻ as the temperature is increased from 10 to 70°C.     

Use of multiphoton ionization of molecules for the determination of neptunium traces in solution

The possibility of detecting trace amounts of neptunium atoms and molecular compounds by laser resonance ionization spectroscopy has been investigated in acetylene-air and acetylene-nitrous oxide flames. Experimentally evaluated atomization coefficients of neptunium in the process of thermal dissociation in flames (10^–4% for C₂H₂-air and 10^–2% for C₂H₂–N₂O) do not allow to use atomic ionization signal for determination of neptunium microamounts in solutions. High yield multiphoton ionization of NpO molecule has been realized in fuel rich C₂H₂–N₂O flame. This pehnomenon has been used for determination of trace quantities of Np in solution. The detection limit was 2·10^–9 g/ml.     

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.     

Preconcentration of neptunium by supported liquid membranes for luminescent analysis of environmental samples

The possibilities of liquid membrane preconcentration of neptunium from environmental samples of different nature have been studied. The use the solid-supported liquid membrane containing a trioctylmethylammonium nitrate carrier allows to achieve preconcentration factors of 10²–5×10². The teflon solid support does not interact with the luminescent matrix (CaF₂, PbMoO₄) during calcination at 900 °C, so it makes practical to measure the neptunium content by luminescence without reextraction to aqueous solution. As a result, the detection limit of neptunium is lowed down to 10^–13 g ml^–1 and 5×10^–13 g g^–1 for pure solutions and soils, respectively.     

Determination of 237 Np in marine sediment and seawater

Procedures for determination of neptunium in marine sediment and seawatersamples are described. Iron hydroxide Fe(OH)₂ –Fe(OH)₃ is used for preliminary pre-concentration of neptunium. Secondly,neptunium Np⁴⁺ and Pu³⁺ are separated by tri-isooctylamine-(TIOA)extraction in 8–10M HCl by redox with SO₃ ^2–-Fe³⁺ Neptunium Np⁴⁺ and uranium U⁶⁺ areseparated by back extraction the Np⁴⁺ with 2M HCl. Finally, theneptunium is purified from the uranium and thorium by anion exchange in 8MHNO₃ and 12M HCl. The stripping of 6M HCl + NH₂ OH HClfurther separates the neptunium Np³⁺ and uranium. Reduction bySO 3_2– –Fe³⁺ appeared to be an efficientway to obtain Np⁴⁺ The decontamination factors of the procedureare 4.0. 10⁴ for ²³² Th, 5.6 . 10⁴ for uraniumand 1.6 . 10⁴ for plutonium.     

Pentakis(carbamide)dioxoneptunium(V) Nitrate [NpO2{OC(NH2)2}5](NO3): Synthesis,Crystal Structure,and Some Properties

X-ray diffraction and spectroscopic data for a new Np(V) compound, namely, [NpO₂{OC(NH₂)₂}₅](NO₃) (I) are presented. Crystals are monoclinic, space group P2₁, a = 11.142(2) Å, b = 7.6379(9) Å, c = 11.143(2) Å, = 108.9(1)°, Z = 2, V = 897.1(2) ų. The neptunium atom has a typical pentagonal-bipyramidal environment with five oxygen atoms of the carbamide molecules in the equatorial plane. The nitrate ion is in the outer sphere. Carbamide is a strong molecular ligand with respect to Np(V) and hence, cation–cation bonds of the NpO₂ ⁺ ions are not realized in structure I. The wave length of the f–f transition in the electronic absorption spectra of crystalline complex I and Np(V) in a saturated carbamide solution is virtually the same and is equal to 991 nm.     

A pulse radiolysis study of the reactivity of neptunoyl ions relative to inorganic free radicals

Conclusions 1. Pulse radiolysis was used to find the rate constants for the reactions of OH, HSO₄, NO₃, and Cl₂ radicals with neptunoyl ions.Change in the NO₃ and H⁺ ion concentrations do not affect the term k[NO₃ + NpO₂ ⁺], while k[Cl₂ ^– + NpO₂ ⁺] increases with increasing chloride concentration due to the formation of neptunoyl ion chlorocomplexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 456–458, February, 1986.     

The unusual stability of [NpO2Cl4]2−: Synthesis and characterisation of [NpO2(DPPMO2)2Cl]2[NpO2Cl4] and [Ph3PNH2]2[NpO2Cl4]

The [NpO₂(DPPMO₂)₂Cl][NpO₂Cl₄] complex (where DPPMO₂ = bis(diphenylphosphino)methanedioxide) contains the linear neptunyl group, {NpO₂}²⁺, with two bidentate P=O donor ligands. Coordinating anion Cl^? fills the fifth equatorial coordination site yielding a complex of general formula [NpO₂(DPPMO₂)₂X]₂[Y] (1) (where X = Cl^? and Y = [NpO₂Cl₄]^2?. Reaction between our newly prepared neptunium starting material [NpO₂Cl₂(thf)]_n and phosphinimine ligand produced crystals of [Ph₃PNH₂]₂[NpO₂Cl₄] (2). Compounds 1 and 2 have been structurally characterised.     

Sorption and desorption of neptunium(V) on calcareous soil and its solid components: A comparative study

The sorption and desorption isotherms of untreated calcareous soil and three treated soils to remove CaCO₃, organic matter (OM) and both CaCO₃ and OM were determined and analyzed with the Freundlich equation at pH 7.8, moderate concentrations of NpO₂ ⁺ (~10^-5mol/l), in the presence of 0.01 mol/l CaCl₂ and under ambient aerobic conditions. The relative contribution of CaCO₃ and OM to the neptunium(V) sorption on calcareous soil and the sorption/desorption hysteresis is discussed. The effects of adding fulvic acid (FA) and carbonate in to the solution on the sorption of neptunium(V) on the soils were also studied. The sorption and desorption characteristics of NpO₂ ⁺, Zn²⁺, Sr²⁺ and Cs⁺ on the soils are compared.     

Comprehensive Thermodynamic Model Applicable to Highly Acidic to Basic Conditions for Isosaccharinate Reactions with Ca(II) and Np(IV)

Isosaccharinate (ISA^–), a degradation product of cellulose codisposed in low-level nuclear wastes, is expected to be one of the dominant complexing ligands for radionuclides, especially tetravalent actinides. This paper presents a comprehensive thermodynamic model for isosaccharinate reactions with Ca(II) and Np(IV). The model is valid for a wide range of pH values (2–14), ISA^– concentrations (ranging up to 0.1 m), and ionic strengths (ranging up to 6.54 m), and is based on (1) NMR investigations of HISA(aq) (-D-isosaccharinic acid) and ISL(aq) [dehydration product of HISA(aq)], and the solubility of Ca(ISA)₂(c) as a function of pH and concentrations of Ca and ISA^–; (2) NpO₂(am) solubility in a wide range of pH values (2–14) and total ISA concentrations of 0.0016 and 0.008 m and at fixed pH values of approximately 5 and 12 with total ISA concentrations ranging from 0.0001 to 0.1 m; and (3) solvent extraction of Np-ISA solutions, containing fixed NaClO₄ concentrations ranging from 0.103 to 6.54 m and at fixed pC _H+ values ranging from 1.5 to 1.9, with dibenzoylmethane. Pitzer's ion-interaction approach was used to interpret the data. The different aqueous species required to explain these data included HISA(aq), ISL(aq), Ca(ISA)⁺, Np(OH)₃(ISA)(aq), Np(OH)₃(ISA)₂ ^–, Np(OH)₄(ISA)^–, and Np(OH)₄(ISA)₂ ^2–. The values of equilibrium constants for reactions involving these species and determined from these data provided close agreements between the observed and predicted concentrations in all of the systems investigated in this study and those reported previously.     

Adsorption characteristics of metal-EDTA complexes onto hydrous oxides

The adsorption characteristics of a variety of metal-EDTA complexes onto hydrous oxides, principally aluminum oxide (γ-Al₂O₃), were examined in aqueous solution. Adsorption of these complexes increased with increasing proton concentration due to the formation of surface complexes between EDTA and the surface hydroxo groups, specifically the AlOH₂⁺ surface groups. The pH-dependent adsorptive behavior and the magnitude of adsorption of the “free” EDTA species were similar to those of the metal complexes. The results also showed that the adsorption of “free” EDTA was exothermic, while the adsorption of Ni(II)-EDTA complexes was endothermic in the lower pH region (3.5) and exothermic at higher pH values (6.0). This implied that the surface preferred the NiHEDTA⁻¹ species rather than the NiEDTA⁻² species. Specific adsorption of the metal complexes was evidenced by the charge reversal exhibited by the γ-Al₂O₃ particles at the highest surface loadings. A quantitative model was formulated based on the pH-dependent speciation of the oxide surface, speciation of the metal complexes in solution, and ζ potential measurements. This model proved valid over a wide range of pH (3–10) and for both high (>50% coverage) and low (<10% coverage) surface loadings.     

Methyl ethyl ketone extraction of Tc species

Methyl ethyl ketone extraction of technetium species from aqueous solutions of neutron irradiated ammonium molybdate crystals was studied; the two species extracted were separated by high voltage paper electrophoresis. The one was the^99mTcO ₄ ^– ion and the other, a^99mTc non-charged immobile species, probably TcO₂.aq, which concentrated at the point of application on the electrophoresis paper strip.     

Étude des réactions d'oxydo—réduction du neptunium dans le mélange Rbcl—CsCl (25–75% mol)

The oxidation—reduction reactions o f neptunium in molten RbCl—CsCl.Standard potentials of the systems NpO₂(VI)—NpO₂(V) and Np(IV)—Np(III) and the equilibrium constant of the following disproportionation reaction 2NP⁴⁺ + 2H₂O + 2Cl^- ? NpO⁺₂ + Np³⁺+ 4HCI have been determined in a (Rb 0,25; Cs 0,75 )C1 melt in the temperature range 660—750°C by absorption spectrophotometry. The results are com- pared with those obtained previously in (Li 0,7; K 0,3)C1, (Li, K)C1 eutectic and (Li 0,55; CsO,45)Cl.     

Electrochemical vapor generation of selenium species after online photolysis and reduction by UV-irradiation under nano TiO<Subscript>2</Subscript> photocatalysis and its application to selenium speciation by HPLC coupled with atomic fluorescence spectrometry

An online UV photolysis and UV/TiO₂ photocatalysis reduction device (UV–UV/TiO₂ PCRD) and an electrochemical vapor generation (ECVG) cell have been used for the first time as an interface between high-performance liquid chromatography (HPLC) and atomic fluorescence spectrometry (AFS) for selenium speciation. The newly designed ECVG cell of approximately 115 L dead volume consists of a carbon fiber cathode and a platinum loop anode; the atomic hydrogen generated on the cathode was used to reduce selenium to vapor species for AFS determination. The noise was greatly reduced compared with that obtained by use of the UV–UV/TiO₂ PCRD–KBH₄–acid interface. The detection limits obtained for seleno-DL-cystine (SeCys), selenite (Se^IV), seleno-DL-methionine (SeMet), and selenate (Se^VI) were 2.1, 2.9, 4.3, and 3.5 ng mL^–1, respectively. The proposed method was successfully applied to the speciation of selenium in water-soluble extracts of garlic shoots cultured with different selenium species. The results obtained suggested that UV–UV/TiO₂ PCRD–ECVG should be an effective interface between HPLC and AFS for the speciation of elements amenable to vapor generation, and is superior to methods involving KBH₄.     

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.     

Extraction procedures for chemical speciation of arsenic in atmospheric total suspended particles

An arsenic chemical speciation study was performed in 2000, using air filters on which total suspended particles (TSP) were collected, from the city of Huelva, a medium size city with huge industrial influence in SW Spain. Different procedures for extraction of the arsenic species were performed using water, NH₂OH.HCl, and H₃PO₄ solutions, with either microwave or ultrasonic radiation. The best optimised extraction methods were use of 100 mmol L^–1 NH₂OH.HCl and 10 mmol L^–1 H₃PO₄ and microwave radiation for 4 min. High-performance liquid chromatography coupled with hydride generation and atomic fluorescence spectrometry (HPLC–HG–AFS) was employed for determination of the arsenic species. The results from 12 TSP air filters collected on a monthly basis showed extraction was quantitative (94% with NH₂OH.HCl and 86% H₃PO₄). Only inorganic arsenic species (arsenite and arsenate) were detected. The mean arsenite concentration was 1.2±0.3 ng m^–3 (minimum 0.3 ng m^–3, maximum 1.8 ng m^–3). The mean arsenate concentration was 10.4±1.8 ng m^–3, with greater monthly variations than arsenite (minimum 2.1 ng m^–3, maximum 30.6 ng m^–3). The high level of arsenic species in the TSP samples can be related to a copper smelter located in the region.     

Étude quantitative d'équilibres chimiques en solution dans les sels fondus par spectrophotométrie d'absorption: Application au neptunium

The following reactions: NpO₂⁺+4 HCl ? Np⁴⁺ + 2 H₂O + $\text{1}\text{2}$ Cl₂ + 3Cl^- NpO₂⁺+$\text{1}\text{2}$ Cl₂ ? NpO₂²⁺ +Cl^- 2HCl+O^2- ? H₂O +Cl^- have been examined quantitatively. The reactions were studied in fused LiCl-KCl sparged with gas mixtures of definite compositions. The concentrations of the diffrent neptunium species were measured by absorption spectrophotometry. The values of respective equilibrium constants are: K = (9.3±0.4)· 10^-6 atm(^{-$\text{1}\text{2}$}); K₁ = (2.3±0.1)·10^-2 atm(^{-$\text{1}\text{2}$}); k = 10^3.8 1 mol^-1 atm^-1 The standard potential of the system NpO₂²⁺ / NpO₂⁺ was determinedto be E⁰ = 0.220 V (vs.standard chlorine electrode).     

Solubility and Raman Spectroscopic Study of As(III) Speciation in Organic Compound-Water Solutions. A Hydration Approach for Aqueous Arsenic in Complex Solutions

Solubilities of arsenolite (As₂O₃, cub.) were measured from 22 to 90°C in water–acetone, water–acetic acid, and water—formic acid solutions of compositions ranging from the pure organic compound to pure water. Raman spectra were obtained at ambient temperature on As-containing water–acetic acid and water–acetone solutions. Results show that arsenic solvation by these organic compounds is negligible and hydroxide species dominate As speciation over a wide range of water activity (a_{H 2 O}> 0.01). The solubility data were analyzed using an approach based on stoichiometric hydration reactions. Results show that As₂O₃ solubility can be described as a function of water activity, independently of the nature of the organic compound, by involving two neutral As hydroxide complexes: As(OH)₃ and As(OH)₃·4H₂O. Stability constants derived for these species indicate that hydration weakens with increasing temperature. Calculations using these constants show that at low temperatures the tetrahydrate As(OH)₃·4H₂O is dominant in water-rich solutions; by contrast, in high-temperature crustal fluids, As(OH)₃ becomes the major As species. The proposed hydration model can be used to analyze solubility of arsenic-bearing minerals and arsenic transport in complex H₂O–CO₂—electrolyte solutions encountered in natural and industrial environments.     

Analytical methods for the speciation of selenium compounds: a review

Selenium, like sulphur, exists in the environment in several oxidation states and as a variety of inorganic and organic compounds. Dissolved inorganic selenium can be found in natural waters as selenide Se (–II), as colloidal elemental selenium Se (0), as selenite anions HSeO ₃ ^– and SeO ₃ ^2– i.e. Se (+IV) and as the selenate anion (SeO ₄ ^2– ) i.e. Se (+VI). Organic forms of selenium that may be found in organisms, air or in the aqueous environment, are volatile (methylselenides) or non volatile (trimethylselenonium ion, selenoamino acids and their derivatives). Knowledge of the different chemical forms and their environmental and biomedical distribution is important because of the dependence of bioavailability and toxicity on speciation. This paper reviews the different analytical methods used for the speciation of selenium compounds, with special attention to inorganic selenium and organoselenium species.