Thermodynamics of neptunium(V) complexes with phosphate at elevated temperatures

The complexation of Np(V) with phosphate at elevated temperatures was studied by a synergistic extraction method. A mixed buffer solution of TRIS and MES was used to maintain an appropriate pH value during the distribution experiments. The distribution ratio of Np(V) between the organic and aqueous phases was found to decrease as the concentrations of phosphate were increased. Stability constants of the 1:1 and 1:2 Np(V)-HPO₄ ²⁻ complexes, dominant in the aqueous phase under the experimental conditions, were calculated from the effect of [HPO₄ ²⁻] on the distribution ratio. The thermodynamic parameters including enthalpy and entropy of complexation between Np(V) and HPO₄ ²⁻ at 25 °C–55 °C were calculated by the temperature coefficient method.     

Distribution and identification of Plutonium(IV) species in tri-n-butyl phosphate/HNO3 extraction system containing acetohydroxamic acid

There was a significant research progress achieved with the aim to modify conventional PUREX process by stripping of plutonium from the tri-n-butyl phosphate (TBP) extraction product in the form of non-extractable complexes upon addition of back-hold complexation agents. The present paper reports effects of such salt-free complexant, acetohydroxamic acid (HAHA), on distribution ratio of Pu(IV) under wide concentration of nitric acid and additional nitrate. General formula of plutonium species present in the organic phase can be described as Pu(OH)_x(AHA)_y(NO3)_4−x−y·2TBP·wHNO₃.     

Kinetics of reductive stripping of Pu(IV) in the tributylphosphate-kerosene/nitric acid-water system using dihydroxyurea

The kinetics of the reductive stripping of plutonium(IV) by dihydroxyurea (DHU) in 30% TBP/kerosene-HNO₃ system was studied with a constant interfacial area cell. The stripping rate of plutonium(IV) increases with the increase of the stirring speed of two phases and the interfacial area. The activation energy of this process is 28.4 kJ/mol. Under the given experimental conditions, the mass transfer of Pu is not controlled by redox reaction, but controlled by molecular diffusion from the organic phase to organic film layer and from the aqueous film layer to aqueous phase. The rate equation of reductive stripping (process is controlled by diffusion) was obtained as: r ₀ = k′[Pu(IV)]₀[DHU]_a ^0.16[HNO₃]_a ^−0.34. The rate constant k′ is (5.0±0.4)·10⁻² (mol/L)^0.18·min⁻¹ at 18.0°C.     

Environmental Mobility of Pu(IV) in the Presence of Ethylenediaminetetraacetic Acid: Myth or Reality?

Ethylenediaminetetracetic acid (EDTA), which was co-disposed with Pu at several US Department of Energy sites, has been reported to enhance the solubility and transport of Pu. It is generally assumed that this enhanced transport of Pu in geologic environments is a result of complexation of Pu(IV) with EDTA. However, the fundamental basis for this assumption has never been fully explored. Whether EDTA can mobilize Pu(IV) in geologic environments is dependent on many factors, chief among them are not only the complexation constants of Pu with EDTA and dominant oxidation state and the nature of Pu solids, but also (1) the complexation constants of environmentally important metal ions (e.g., Fe, Al, Ca, Mg) that compete with Pu for EDTA and (2) EDTA interactions with the geomedia (e.g., adsorption, biodegradation) that reduce effective EDTA concentrations available for complexation. Extensive studies over a large range of pH values (1 to 14) and EDTA concentrations (0.0001 to 0.01 mol⋅L⁻¹) as a function of time were conducted on the solubility of 2-line ferrihydrite (Fe(OH)₃(s)), PuO₂(am) in the presence of different concentrations of Ca ions, and mixtures of PuO₂(am) and Fe(OH)₃(s). The solubility data were interpreted using Pitzer’s ion-interaction approach to determine/validate the solubility product of Fe(OH)₃(s), the complexation constants of Pu(IV)-EDTA and Fe(III)-EDTA, and to determine the effect of EDTA in solubilizing Pu(IV) from PuO₂(am) in the presence of Fe(III) compounds and aqueous Ca concentrations. Predictions based on these extensive fundamental data show that environmental mobility of Pu as a result of Pu(IV)-EDTA complexation as reported/implied in the literature is a myth rather than the reality. The data also show that in geologic environments where Pu(III) and Pu(V) are stable, the EDTA complexes of these oxidation states may play an important role in Pu mobility.     

Migration of239Pu in soluble and insoluble forms in soil

A field experiment study was performed at the rural site of South-East Lithuania. The main tasks of the study included an evaluation of the peculiarities of partition of²³⁹Pu in soluble (²³⁹Pu(NO₃)₄,²³⁹PuCl₃) and insoluble (²³⁹PuO₂) forms in soddy and forest soil horizons. The results of durable experiments (418 and 326 days) have shown that from 44.1% to 92.2% of²³⁹Pu of investigated chemical forms were accumulated in the top (0–5 cm) soil layer. Some share (5.7–39.2%) of plutonium from studied chemical forms was found in the 5–20 cm layers of studied soil samples (columns). Obtained distribution of plutonium in soil layers may be attributed to the consideration that the migration rate to the soil depth for plutonium is 0.1–1.0 cm·y⁻¹ but for some part of plutonium 10 times higher migration rate is characteristic as well. Plutonium transfer factor (TF) to the grassland plants was calculated, the values ranged from 10⁻² to 10⁻¹.     

Sorption of Pu in various oxidation states onto multiwalled carbon nanotubes

The sorption of Pu(IV), polymeric Pu(IV), Pu(V) and Pu(VI) from the 0.1 M NaClO₄ solution onto multiwalled carbon nanotubes was investigated. The kinetic study of the sorption process have shown that the polymeric Pu(IV) has the highest sorption rate, while decrease of sorption rate for plutonium aqua-ions in the order Pu(VI) > Pu(IV) > Pu(V) was found. Strong dependence of sorption kinetics of ionic plutonium species on pH was shown, in contrast to polymeric species, that were shown to quantitatively sorb (99%) in the wide pH range (pH 2–10). Two different sorption mechanisms for ionic and polymeric plutonium species were proposed: on the bases of sorption isotherms chemisorptions of plutonium aqua-ions onto carbon nanotubes and through intermolecular interaction for the polymeric plutonium species was defined. Distribution coefficients of plutonium in various oxidation states were found to increase with pH, showing the highest values for polymeric plutonium sorption (K _d  = 2.4 × 10⁵ mL g⁻¹ at pH = 6).     

Stability constants of Np(V) complexes with fluoride and sulfate at variable temperatures

Summary A solvent extraction method was used to determine the stability constants of Np(V) complexes with fluoride and sulfate in 1.0M NaClO₄ from 25 to 60 °C. The distribution ratio of Np(V) between the organic and aqueous phases was found to decrease as the concentrations of fluoride and sulfate were increased. Stability constants of the 1 : 1 Np(V)-fluoride complexes and the 1 : 1 Np(V)-sulfate and 1 : 2 Np(V)-sulfate complexes, dominant in the aqueous phase under the experimental conditions, were calculated from the effect of [F^-] and [SO₄^2-] on the distribution ratio. The enthalpy and entropy of complexation were calculated from the stability constants at different temperatures by using the Van't Hoff equation.     

Cesium, strontium, europium(III) and plutonium(IV) complexes with humic acid in solution and on montmorillonite surface

The effect of Aldrich humic acid (HA) on the mobility of¹³⁷Cs,⁸⁵Sr,¹⁵²Eu and²³⁹Pu radionuclides was studied in Ca-montmorillonite suspensions. Verified 2-sites-2-species (2s2s) models correspond to an intensive interaction of all elements with humificated surface, what is in a remarkable contrast with the weak complexation of cesium and even strontium in solutions — the neutral ligand interaction constants β (l/mol) are log β<−9.9 and 7.56±0.21 for Cs and Sr, respectively. The result for europium complexation in solution, log β=12.49±0.18 is in a good agreement with literature data. For plutonium(IV) not only a high proton competitive constant in solution was obtained, log β β=(−0.67±0.32)+3pH, but also a strong chemisorption, which at high concentrations of humic acid (above 0.05 g/l) indicates the formation of bridge humate complexes of plutonium on the humificated surface. Logarithms of heterogeneous interaction constants ( ₂₄ l/g) of the elements with surface humic acid are 4.47±0.23, 4.39±0.08, and 6.40±0.33 for Cs, Sr, and Eu(III), respectively, and the logarithm of the proton competitive constant ( ₂₄, l/g) for Pu(IV) −3.80±0.72. Distribution coefficients of humic acid and metal humates between 0.01 g HA/l solution and montmorillonite were derived as logK _d(AH)=−1.04±0.11, logK _d(EuA)=1.56±0.11 and logK _d(PuA)=2.25±0.04, while the values for Cs and Sr were obtained with very high uncertainty. Speciation of the elements on montmorillonite surface is illustrated as a function of equilibrium concentration of humic acid in solution and of pH.     

Extraction of actinides by quaternary amines from hydrochloric acid medium

The extraction of Np(IV), Pu(IV) and U(VI) from aqueous hydrochloric acid into quaternary amines has been studied. The dependence of the distribution coefficient on amine concentration suggests that the actinide ions extracted are NpCl ₆ ²⁻ PuCl ₆ ²⁻ and UO₂Cl ₄ ²⁻ . This is further supported by the absorption spectra of the amine extracts of these actinide ions. Based on the extraction data obtained, a simple method for the separation of typical metal ions such as Cs, lanthanides and Zr from U(VI) and Pu(IV) is suggested.     

Redox speciation of plutonium

Knowledge of the oxidation state distribution of plutonium in natural waters is necessary in modeling its behavior in environmental systems. The redox speciation of plutonium is complicated by such effects as hydrolysis, complexation, disproportionation, solubility, and redox interchange reactions. The insolubility of Pu(OH)₄ is often the limiting factor of the net solubility of plutonium in oxic natural waters where Pu(V)O ₂ ⁺ is the most stable oxidation state. Perturbations to the oxidation state speciation due to the complexation chemistry of the different oxidation states of plutonium and to the insolubility of plutonium(IV) in neutral aquatic systems are discussed. The merits and limitations of some chemical separation techniques used to study redox speciation of plutonium are presented, and recommendations made for obtaining reliable oxidation state distribution data.     

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.     

Recovery of plutonium from phosphate containing aqueous analytical waste solutions using macroporous anion exchange resin

Distribution ratios of Pu(IV) between 7.5M HNO₃+0.75M H₃PO₄+0.3M H₂SO₄ media and a macroporous anion-exchange resin Amberlyst A-26 (MP) increased from 40 to 250 when 1M aluminium nitrate was added to the aqueous medium. When 1M ferric nitrate was used in place of aluminium nitrate the distribution ratio further increased to 850. The 10% Pu(IV) breakthrough capacities with a 5 ml bed resin column, using synthetic feed solutions containing 1M aluminium nitrate, were 1.4 g l^–1, 3.2 g l^–1 at flow rates of 30 ml per hour and 10 ml per hour, respectively. The corresponding 10% Pu(IV) breakthrough capacities in the presence of 1M ferric nitrate were 8.5 g l^–1 and 12.8 g l^–1. More than 97% of plutonium could be recovered from actual analytical phosphate waste solutions.     

Plutonium(IV) mononitrate and dinitrate complex formation in acid solutions as a function of ionic strength

Titrations of Pu(IV) with HNO₃ in a series of aqueous HClO₄ solutions ranging in ionic strength from 2 to 19 molal were followed using visible and near-infrared absorption spectrophotometry. The Pu 5f-5f spectra in the visible and near IR range change with complex formation. At each ionic strength, a series of spectra were obtained by varying nitrate concentration. Each series was deconvoluted into spectra of Pu⁴⁺ (aq), Pu(NO₃)³⁻ and Pu(NO₃)₂ ²⁺ complexes, and simultaneously their formation constants were determined. When corrected for the incomplete dissociation of nitric acid, the ionic strength dependence of each formation constant can be described by two parameters, β⁰ and Δε using the formulae of specific ion interaction theory.     

Some remarks on the extraction of Pu(IV) and Th with tridodecylamine from nitric acid soil leach solutions and the possible existence of a univalent anionic Th-nitrate complex

The extraction of Pu(IV) and Th with tridodecylamine—xylene mixtures from about 6M nitric acid soil leach solutions was studied as a function of the chemical composition of the aqueous phase (iron and calcium concentration, acidity) and the amine concentration in the extractant. No correlation was found between the partition coefficients of Pu(IV) and Th and the composition parameters mentioned above at any of the amine concentrations examined. The slope, in a bilogarithmic plot, of the partition coefficients versus the amine concentrations was found to be close to 2 for Pu(IV) as well as Th in pure 6.5M nitric acid solution, thus indicating the presence of the complexes Pu(NO₃) ₆ ²⁻ and Th(NO₃) ₆ ²⁻ in the extract. When the pure nitric acid solution was replaced by soil leach solutions of similar molarity in HNO₃, the slope remained 2 for Pu(IV), but changed to 1.5 for Th. A possible reason for this slope yielded by Th may be the coexistence of the complexes Th(NO₃) ₆ ²⁻ and Th(NO₃) ₅ ⁻ in the extraction phase. Presented at the 4th SAC Conference on Analytical Chemistry, Birmingham 1977.     

Radiation chemistry of actinides

The paper presents data on the development of a mathematical model describing the radiation-chemical behaviour of plutonium in 3–6 mol/l HNO₃ with uranium present and absent. Using experimental data on the kinetics of Pu(VI) reduction and Pu(IV) oxidation, and comparing them with the calculated values, we succeeded in finding the previously unknown rate constants of many reactions between plutonium ions and HNO₃ radiolysis products, between plutonium ions and between plutonium ions and uranium ions. The mechanism of the radiation-chemical processes has been established.     

Studies on the recovery of Pu(IV) from hydrochloric acid-oxalic acid solutions using an anion exchange method

Sorption of Pu(IV) from hydrochloric acid-oxalic acid solutions has been investigated using different anion exchangers, viz., Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) for the recovery of plutonium from plutonium oxalate solutions. Distribution ratios of Pu(IV) for its sorption on these anion exchangers have been determined. The sorption of Pu(IV) from hydrochloric acid solutions decreases drastically in the presence of oxalic acid. However, addition of aluminium chloride enhances the sorption of plutonium in the presence of oxalic acid, indicating the feasibility of recovery of plutonium. Pu(IV) breakthrough capacities have been determined with a 10 ml resin bed of each of these anion exchangers at a flow rate of 60 ml per hour using a solution of Pu(IV) with the composition: 6M HCl+0.05M HNO₃+0.1M H₂C₂O₄+0.5M AlCl₃+100 mg.l^–1 Pu(IV). The 10% Pu(IV) breakthrough capacities for Dowex-1X4, Amberlite XE-270 (MP) and Amberlyst A-26 (MP) are 15.0, 8.9 and 6.2 g of Pu(IV) l^–1 of resin respectively.     

Improved sample preparation method for environmental plutonium analysis by ICP-SFMS and alpha-spectrometry

A rapid and simple sample preparation method for plutonium determination in environmental samples by inductively coupled plasma sector field mass spectrometry (ICP-SFMS) and alpha-spectrometry is described. The developed procedure involves a selective CaF₂ co-precipitation for preconcentration followed by extraction chromatographic separation. The proposed method effectively eliminates the possible interferences in mass spectrometric analysis and also removes interfering radionuclides that may disturb alpha-spectrometric measurement. For ²³⁹Pu, ²⁴⁰Pu and ²⁴¹Pu limits of detection of 9.0 fg·g⁻¹ (0.021 mBq), 1.7 fg·g⁻¹ (0.014 mBq) and 3.1 fg·g⁻¹ (11.9 mBq) were achieved by ICP-SFMS, respectively, and 0.02 mBq by alpha-spectrometry. Results of certified reference materials agreed well with the recommended values.     

PuPO<Subscript>4</Subscript>(cr,hyd.) Solubility Product and Pu<Superscript>3+</Superscript> Complexes with Phosphate and Ethylenediaminetetraacetic Acid

To determine the solubility product of PuPO₄(cr, hyd.) and the complexation constants of Pu(III) with phosphate and EDTA, the solubility of PuPO₄(cr, hyd.) was investigated as a function of: (1) time and pH (varied from 1.0 to 12.0), and at a fixed 0.00032 mol⋅L⁻¹ phosphate concentration; (2) NaH₂PO₄ concentrations varying from 0.0001 mol⋅L⁻¹ to 1.0 mol⋅L⁻¹ and at a fixed pH of 2.5; (3) time and pH (varied from 1.3 to 13.0) at fixed concentrations of 0.00032 mol⋅L⁻¹ phosphate and 0.0004 mol⋅L⁻¹ or 0.002 mol⋅L⁻¹ Na₂H₂EDTA; and (4) Na₂H₂EDTA concentrations varying from 0.00005 mol⋅L⁻¹ to 0.0256 mol⋅L⁻¹ at a fixed 0.00032 mol⋅L⁻¹ phosphate concentration and at pH values of approximately 3.5, 10.6, and 12.6. A combination of solvent extraction and spectrophotometric techniques confirmed that the use of hydroquinone and Na₂S₂O₄ helped maintain the Pu as Pu(III). The solubility data were interpreted using the Pitzer and SIT models, and both provided similar values for the solubility product of PuPO₄(cr, hyd.) and for the formation constant of PuEDTA⁻. The log ₁₀ of the solubility product of PuPO₄(cr, hyd.) [PuPO₄(cr, hyd.) \rightleftarrows\rightleftarrows Pu³⁺+PO₄^3-\mathrm{Pu}^{3+}+\mathrm{PO}_{4}^{3-}] was determined to be −(24.42±0.38). Pitzer modeling showed that phosphate interactions with Pu³⁺ were extremely weak and did not require any phosphate complexes [e.g., PuPO₄(aq), PuH₂PO₄²⁺\mathrm{PuH}_{2}\mathrm{PO}_{4}^{2+}, Pu(H₂PO₄)₂⁺\mathrm{Pu(H}_{2}\mathrm{PO}_{4})_{2}^{+}, Pu(H₂PO₄)₃(aq), and Pu(H₂PO₄)₄^-\mathrm{Pu(H}_{2}\mathrm{PO}_{4})_{4}^{-}] as proposed in existing literature, to explain the experimental solubility data. SIT modeling, however, required the inclusion of PuH₂PO₄²⁺\mathrm{PuH}_{2}\mathrm{PO}_{4}^{2+} to explain the data in high NaH₂PO₄ concentrations; this illustrates the differences one can expect when using these two different chemical models to interpret the data. Of the Pu(III)-EDTA species, only PuEDTA⁻ was needed to interpret the experimental data over a large range of pH values (1.3–12.9) and EDTA concentrations (0.00005–0.256 mol⋅L⁻¹). Calculations based on density functional theory support the existence of PuEDTA⁻ (with prospective stoichiometry as Pu(OH₂)₃EDTA⁻) as the chemically and structurally stable species. The log ₁₀ value of the complexation constant for the formation of PuEDTA⁻ [ Pu³⁺+EDTA^4-\rightleftarrows PuEDTA^-\mathrm{Pu}^{3+}+\mathrm{EDTA}^{4-}\rightleftarrows \mathrm{PuEDTA}^{-}] determined in this study is −20.15±0.59. The data also showed that PuHEDTA(aq), Pu(EDTA)₄^5-\mathrm{Pu(EDTA)}_{4}^{5-}, Pu(EDTA)(HEDTA)⁴⁻, Pu(EDTA)(H₂EDTA)³⁻, and Pu(EDTA)(H₃EDTA)²⁻, although reported in the literature, have no region of dominance in the experimental range of variables investigated in this study.     

Estimate of the stability constants of trivalent actinide and lanthanide complexes with O-donor ligands in aqueous solutions

The stability constants of the 1: 1 complexes of trivalent actinide and lanthanide cations with O-donor ligands (OH⁻, CO₃²⁻; carboxylate anions: acetate, propionate, isobutyrate, benzoate) formed in aqueous solutions have been approximately calculated by integrating the ligand density distribution function. The contributions of the covalent interaction to the formation of coordination bonds of these cations with O-donor ligands have been estimated.     

Interaction of Molybdenum(VI) with Methyliminodiacetic Acid at Different Ionic Strengths by Using Parabolic,Extended Debye-Hückel and Specific Ion Interaction Models

The main aim of this research is to study the complexation of molybdenum(VI) with methyliminodiacetic acid in NaClO₄ aqueous solutions at pH = 6.00 and ionic strengths (0.1<I/mol⋅dm⁻³<1.0) at 25 °C by using potentiometric and UV spectrophotometric measurements in order to obtain thermodynamic stability constants at I=0 mol⋅dm⁻³. A comparison with previous literature data was made for the stability constants, though few data were available. The stability constants data have been analyzed and interpreted by using extended Debye-Hückel theory, specific ion interaction theory and parabolic model. Finally it might be concluded that parabolic model applies better for this complexation reaction.