AKUFVE STUDIES ON EXTRACTION BEHAVIOUR OF NEPTUNIUM FROM SIMULATED HLW SOLUTIONS BY 30% TBP

ABSTRACT

The extraction behaviour of neptunium from 3 M nitric acid as well as simulated pressurised heavy water reactor high level radioactive waste (PHWR-HLW) solution by 30% TBP/dodecane was studied using AKUFVE. Np(IV)/Np(V) was oxidised to Np(VI) using oxidising agents, such as K₂Cr₂O₇, VO₂ ⁺ and NaNO₂. Stripping of neptunium from the loaded TBP phase was studied using reducing agents like hydrogen peroxide, ascorbic acid, hydroxyl-amine hydrochloride and hydrazine sulphate. Results of these extraction and stripping studies have been discussed in this paper.     

Organically templated Np(IV) coordination polymer with in situ formed oxalate anion (ImidazoleH)[Np(C2O4)(CH3SO3)3(H2O)2]

The reaction of Np(V) methanesulfonate solution with imidazole led to the in situ formation of oxalic acid and the reduction of Np(V) to Np(IV). As a result, the novel organically templated organic polymer, (ImidazoleH)[Np(C₂O₄)(CH₃SO₃)₃(H₂O)₂], was formed. The structure consists of infinite chains of [Np(C₂O₄)(CH₃SO₃)₃(H₂O)₂]⁻ and imidazolium cations. The crystal structure is confirmed by IR and UV–vis spectroscopic data. This complex is the first example of structurally characterized 1:1 An(IV) oxalate.     

Solubilities of actinides in a domestic groundwater and a bentonite porewater calculated by using PHREEQC

This study presents the solubilities and speciations of actinides, calculated by the PHREEQC (V.2) code in a granitic groundwater and a Ca-bentonite porewater under a reducing condition. The respective solubilities for the amorphous U, Am, Th, Np and Pu compounds in the groundwater were 2.2 × 10⁻⁵, 1.2 × 10⁻⁷, 3.1 × 10⁻⁹, 3.4 × 10⁻⁹ and 6.3 × 10⁻¹¹ mole/L, and these values are comparable to the results calculated by the MUGREM and EQ3/6 codes. The major dissolved species for U, Am, Th, Np and Pu were UO₂(OH)₃⁻, Am(OH)₂⁺, Th(OH)₄(aq), Np(OH)₃CO₃⁻ and Pu(OH)₃CO₃⁻, respectively. However, carbonate complex ions were anticipated as the major species in the porewater except for thorium due to an increase of the carbonate concentration and a decrease of the pH.     

Extraction of Actinide(III,IV, V,VI) Ions and TcO4 − by N,N,N′,N′‐Tetraisobutyl‐3‐Oxa‐Glutaramide

Abstract

The extraction behavior of U(VI), Np(V), Pu(IV), Am(III), and TcO₄ ^? with N,N,N′,N′‐tetraisobutyl‐3‐oxa‐glutaramide (TiBOGA) were investigated. An organic phase of 0.2 mol/L TiBOGA in 40/60% (V/V) 1‐octanol/kerosene showed good extractability for actinides (III, IV, V VI) and TcO₄ ^? from aqueous solutions of HNO₃ (0.1 to 4 mol/L). At 25°C, the distribution ratio of the actinide ions (D _An) generally increased as the concentration of HNO₃ in the aqueous phase was increased from 0.1 to 4 mol/L, while the D _Tc at first increased, then decreased, with a maximum of 3.0 at 2 mol/L HNO₃. Based on the slope analysis of the dependence of D _M (M=An or Tc) on the concentrations of reagents, the formula of extracted complexes were assumed to be UO₂L₂(NO₃)₂, NpO₂L₂(NO₃), PuL(NO₃)₄, AmL₃(NO₃)₃, and HL₂(TcO₄) where L=TiBOGA. The enthalpy and entropy of the corresponding extraction reactions, Δ_r H and Δ_r S, were calculated from the dependence of D on temperature in the range of 15–55°C. For U(VI), Np(V), Am(III) and TcO₄ ^?, the extraction reactions are enthalpy driven and disfavored by entropy (Δ_r H<0 and Δ_r S<0). In contrast, the extraction reaction of Pu(IV) is entropy driven and disfavored by enthalpy (Δ_r H>0 and Δ_r S>0). A test run with 0.2 mol/L TiBOGA in 40/60% 1‐octanol/kerosene was performed to separate actinides and TcO₄ ^? from a simulated acidic high‐level liquid waste (HLLW), using tracer amounts of ²³⁸U(VI), ²³⁷Np(V), ²³⁹Pu(IV), ²⁴¹Am(III) and ⁹⁹TcO₄ ^?. The distribution ratios of U(VI), Np(V), Pu(IV), Am(III) and TcO₄ ^? were 12.4, 3.9, 87, >1000 and 1.5, respectively, confirming that TiBOGA is a promising extractant for the separation of all actinides and TcO₄ ^? from acidic HLLW. It is noteworthy that the extractability of TiBOGA for Np(V) from acidic HLLW (D _Np(V)=3.9) is much higher than that of many other extractants that have been studied for the separation of actinides from HLLW.     

Electrode reaction of the Np3+/Np couple at liquid Cd and Bi electrodes in LiCl–KCl eutectic melts

The electrode reactions of the Np³⁺/Np couple at liquid Cd and Bi electrodes were investigated by cyclic voltammetry at 723, 773 and 823 K in the LiCl–KCl eutectic melt. It was found that the diffusion of Np³⁺ in the salt phase was the rate-determining step in the cathodic reaction when the concentration of NpCl₃ was less than about 1 wt % and the liquid Cd or Bi phase was not saturated with Np. The redox potentials of the Np³⁺/Np couple at the liquid Cd electrode at 723, 773 and 823 K were observed to be more positive than those at the Mo electrode by 0.158, 0.140 and 0.126 V, respectively. The potential shift results from a lowering of the activity of Np in the Cd phase according to the formation of the NpCd₁₁ alloy at 723 K and NpCd₆ at 773 and 823 K. The redox potentials of the Np³⁺/Np couple at the liquid Bi electrode at 723, 773 and 823 K were more positive than those at the Mo electrode by 0.427, 0.419 and 0.410 V, respectively, which is attributable to a lowering of the activity of Np in the Bi phase due to the formation of NpBi₂.     

Efficient microwave hydrothermal synthesis of nanocrystalline orthorhombic LiMnO2 cathodes for lithium batteries

Rod-like orthorhombic LiMnO₂ nanocrystals were successfully synthesized using temperature-controlled microwave hydrothermal route (TCMH) in a short time (30 min) at a temperature as low as 160 °C. o-LiMnO₂ obtained by two different methods was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemistry test. SEM revealed that the product obtained in case of TCMH was rod-like with a diameter of 40 nm, while the nanoparticles with 200 nm diameter were obtained by traditional hydrothermal route (TH). The dramatic formation of o-LiMnO₂ in the microwave hydrothermal field influenced the morphology and crystal structure of the final products. The formation and preferred growth orientation mechanism of o-LiMnO₂ in the microwave irradiation process was discussed. Electrochemistry performance exhibited that the as-synthesized o-LiMnO₂ nanorods reached the maximum discharge capacity of 194.2 mAh g⁻¹ at 0.1 C rate after several cycles between 2.2 and 4.4 V vs. Li⁺/Li at room temperature, which was higher than the electrochemical performance of o-LiMnO₂ obtained by TH. The experimental results showed that the TCMH method provided an effective way for preparing o-LiMnO₂ cathode material in lithium-ion batteries.     

Das redox-standardpotential von Co3+/Co2+

The standard potenial of the process Co³⁺+e=Co²⁺ has been calculated on the basis of the calorimetric determination of the thermal effect of the reaction Fe²⁺+Co³⁺=Fe³⁺+Co²⁺. The standard potential has also been estimated by extrapolating the cathode and anode tafel curves to intersection. The data available concerning the value of E^o_{Co³⁺/Co²⁺} have been thoroughly analysed, and the average value E^o from all independent methods of estimating the standard potential an acidic medium has been shown to be + 1·45. It has been found from the data on potentials of cobalt oxides in an alkali medium that the standard potential Co³⁺/Co²⁺ must be within the range 1·40–1·53 V. This value agrees with the value 1·45 V obtained in acidic media.     

Extraction of Neptunium (IV) Ions into 30% Tri‐Butyl Phosphate from Nitric Acid

The distribution of Np(IV) between 0.08–4.5 M HNO_3(aq,eqm) and ～30% tributyl phosphate has been modelled, accounting for the formation of 1:1 and 1:2 nitrate complexes and Np(IV) hydrolysis in the aqueous phase and the extraction of Np(NO₃)₄(TBP)₂ into TBP. The potential formation and extraction of NpOH(NO₃)₃(TBP)₂ and Np(NO₃)₄(TBP)₂.HNO₃ species, including spectroscopic evidence, and oxidations of Np(IV) to Np(V) and Np(VI) in the solvent phase have also been considered. The model highlights some key gaps in the available thermodynamic data.     

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

The 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.     

Behaviors of plutonium and neptunium in nitric acid solutions containing hydrazine and technetium ions

The valence behaviors of plutonium and neptunium in the interaction of Pu(IV) and Np(V) with hydrazine and tetravalent uranium in technetium(VII)-containing aqueous nitric acid are reported. At [HNO₃] = 1 mol/l and Pu(IV) and Tc(VII) concentrations of ～0.1 and 0.01–0.2 mol/l, respectively, Pu(IV) is reduced to Pu(III) and is then entirely reoxidized to Pu(IV). Neptunium(V) in 1–3 M HNO₃ undergoes reduction to Np(IV) and then turns back into Np(V). The resulting solution usually contains a mixture of Np(IV) and Np(V). The reduction of Pu(IV) to Pu(III) and the reduction of Np(V) to Np(IV) are accompanied by hydrazine decomposition and by the reduction of most of the Tc(VII) to its lower valence forms. The conversions of Pu(III) into Pu(IV) and of Np(IV) into Np(V) are accompanied by the oxidation of these forms of technetium to Tc(VII). The introduction of diethylenetriaminepentaacetic acid into the reaction system makes Pu(III) more stable against reoxidation into Pu(IV) by reducing the hydrazine decomposition rate, enhances the conversion of Np(V) into Np(IV), and hampers Np(IV) oxidation to Np(V).     

Coordination Polymers of 1,9-Bis(8-Hydroxyquinolin-5-yl)-2,5,8-Trioxanonane

ABSTRACT

Coordination polymers of a novel bis(oxine) bidentate ligand, namely 1,9-bis(8-hydroxyquinolin-5-yl)-2,5,8-trioxanonane (BHQTN) (H₂L) have been prepared with the metal ions Zn⁺², Cu⁺², Ni⁺², Co⁺², and Mn⁺². The novel bis(bidentate) ligand BHQTN was synthesized by condensation of 5-chloromethyl-8-hydroxyquinoline hydrochloride with diethylene glycol in the presence of a base catalyst. All of these coordination polymers and the parent ligand were characterized by elemental analyses and IR spectral studies. The diffuse reflectance spectral studies and magnetic susceptibilities of all of the coordination polymers have also been performed. Thermogravimetric parameters such as T_o, T₁₀, T_max., IPDT, and the activation energy of the thermo-degradation process were calculated.     

EXTRACTION OF NEPTUNIUM AND PLUTONIUM NITRATES WITH n-OCTYL(PHENYL)-N,N-DIISOBUTYL-CARBAMOYLMETHYLPHOSPHINE OXIDE∗

Distribution equilibria of Np and Pu in various valence states and stripping of Np(V) were studied in a system involving 0.20 M n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide + 1.2 M tributyl phosphate in dodecane as a solvent. Np(V) is weakly extractable at <2 M nitric acid but more effectively extractable at >3 M nitric acid, due to the disproportionation of Np(V) to Np(IV) and Np(VI). Addition of nitrous acid to solutions containing Np(V) increases the over-all extractabllity of Np, due to its partial oxidation to Np(VI). This is also the case In the presence of oxalic acid, both at room temperature and at 40^°C. Both Np(IV) and Np(VI) are highly extractable, but the reduction of Np(V,VI) to Np(IV) is slow even with a reductant as strong as sodium formaldehyde sulfoxylate. Pu(IV) is highly extractable and its reduction to Pu(III) with sulfoxylate Is incomplete in the two-phase system if the aqueous phase contains >0.5 M nitric acid. Extracted Np(VI) can be stripped by reduction to Np(V). The stripping rate is, however, slow with nitrous acid as the reductant at low nitric acid concentration. The reduction of Np(VI) by hydrogen peroxide is fast, but is followed by further reduction to Np(IV). Sulfurous acid reduces Np(VI) rapidly and, if no iron is present, only to Np(V), but Fe(II) induces further reduction to Np(IV).     

Electrode reaction of the Np3+/Np couple in LiCl–KCl eutectic melts

The electrochemical behaviour of the Np³⁺/Np couple in the LiCl–KCl eutectic salt was investigated by electromotive force measurements, cyclic voltammetry and chronopotentiometry in the temperature region between 723 and 823 K. The standard redox potential of the Np³⁺/Np couple vs Ag/AgCl (1.00 wt %) was measured and given by the equation, E _Np ³⁺ _/Np ^° = –2.0298 + 0.000706 T, where E is in V and T in K. The electrode reaction of the Np³⁺/Np couple was almost reversible under the conditions studied. The diffusion coefficient of Np³⁺, D _Np ³⁺, in the LiCl–KCl eutectic melts between 723 and 823 K was represented by the equation, D _Np ³⁺ = 2.22 × 10^–6 – 6.88 × 10^–9 T + 5.60 × 10^–12 T ² cm² s^–1. The adsorption and desorption peaks of Np at the Mo working electrode caused by underpotential deposition were also observed in the cyclic voltammograms, and the work function of Np was evaluated as 3.04 eV by peak analysis of the cyclic voltammograms.     

Electrochemical studies on [M(diars)<Subscript>2</Subscript>X<Subscript>2</Subscript>]<Superscript>+</Superscript> where [X = Cl,Br; M = Os,Re, Ru,Rh; diars = <Emphasis Type="Italic">o</Emphasis>-phenylenebis (dimethylarsine)] at bare and Nafion modified electrodes

The voltammetric behaviour of the M(III/II) redox couple of the complexes of the general formula [M(diars)₂X₂]⁺ where M = Os, Re, Ru, Rh and diars = o-phenylenebis-(dimethylarsine) has been investigated in aqueous and aqueous/ethanol solvents at bare and Nafion modified electrodes. The formal reduction potential falls within the range of 0.097–0.0920 V and increases with the dielectric constant of the solvent. In acertonitrile, both the oxidized and reduced forms of the complex showed no adsorption at the surface of the glassy carbon electrode as shown by cyclic voltammetry. The electrochemistry is consistent with a quasireversible one electron transfer between the di and tri cationic forms of the complexes. A current-enhancement of ∼2 was observed at Nafion-modified glassy carbon over the bare electrode. Well defined voltammograms were obtained at Nafion-modified glassy carbon with a detectable signal down to 10⁻⁹ M range by differential pulse voltammetry. Electrochemical detection of dopamine in presence of [Os(4-mediars)₂Cl₂]⁺ at Nafion modified electrode indicates an increase in peak current. This shows that this complex acts as biochemical-sensor for dopamine.     

Neptunium Extraction and Stability in the GANEX Solvent: 0.2 M TODGA/0.5 M DMDOHEMA/Kerosene

Batch distribution studies show that above ~1 M HNO₃ Np(IV) and Np(VI) are well extracted into a solvent of 0.2 M TODGA/0.5 M DMDOHEMA in kerosene that has been formulated for the extraction of transuranic actinides in the GANEX (grouped actinide extraction) process. Np(IV) and Np(VI) are quite stable in the solvent phase, although Np(VI) is slowly reduced to Np(IV) on standing. Stripping of Np(IV,VI) ions from the GANEX solvent has been shown to be quite efficient using acetohydroxamic acid providing aqueous HNO₃ concentrations are below ~0.3 M. In contrast, Np(V) shows much lower distribution ratios and is unstable in the GANEX solvent phase with quite rapid formation of Np(IV) observed. Closer analysis shows this to be due to Np(V) disproportionation, which is enhanced at higher organic phase acidities. Disproportionation of Np(V) is also shown to occur in separate TODGA and DMDOHEMA kerosene solutions. These observations thus enable conditions for neptunium extraction to be optimized during the design of a GANEX flowsheet.     

Removal of vapor-phase elemental mercury by N-doped CuCoO4 loaded on activated carbon

The adsorption ability for vapor-phase Hg° on commercially available granular activated carbon (AC) loaded with CuCoO₄ (AC–C), CuCoO₄ + NH₄Cl (AC–Cl), and CuCoO₄ + NH₄Br (AC–Br) were investigated in an attempt to produce more economic and effective sorbents for the control of Hg^o emission from combustion processes. According to the X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) and mass balance analysis on mercury, we found that N-doped AC–C had bigger S_BET and the nitrogen doping greatly improved CuCoO₄ Hg^o oxidation ability. It was considered that nitrogen atoms in doped CuCoO₄ polycrystalline and anion Cl⁻/Br^- activated by AC and N-doped CuCoO₄ were responsible for the significant enhancement in Hg^o oxidation ability of AC–Cl and AC–Br. We explored the Hg^o oxidation ability of the three kinds of materials under various loading values and adsorption temperatures. We found that AC–Cl and AC–Br were more sensitive to loading value and had much higher Hg^o oxidation ability than AC–C over a wide range temperature. The longevities of AC–C, AC–Cl and AC–Br were all less than the corresponding Al₂O₃ carrier material due to CuCoO₄ decomposition effect on AC, which destroyed the porous structure of AC. The effect of 0.31 vol.% SO₂ on the Hg^o oxidation ability of AC–C, AC–Cl and AC–Br was insignificant which indicated that N-doping did not adversely affect Co³⁺ and Cu²⁺-octahedral structure.     

TEMPERATURE EFFECT ON THE EXTRACTION OF Np(V) BY n-(OCTYLPHENYL)-N,N- DIISOBUTYLCARBAMOYLMETHYLPHOSPHINE OXIDE

ABSTRACT

The temperature effect on the solvent extraction of Np(V) by n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) from nitric acid solutions was studied. A slight decline of distribution ratio was observed at increasing temperature in the CMPO and CMPO-TBP systems, It was found that HNO₃ is extracted by CMPO with the following extraction stoichiometry:

Thermodynamic parameters of extraction of Np from 0.5M and 4.0M HNO₃ were determined. At ≤2M HNO₃, addition of TBP suppresses the distribution ratio of Np. However, at 4M HNO₃, relative to the CMPO alone, a mixture of 0.2M CMPO and 1.4M TBP shows a slight increase in distribution ratio of Np.     

Redox Reactions of Thallium Clusters in Aqueous Solution

The redox chemistry of thallium clusters in aqueous solution has been investigated by pulse radiolysis and γ-radiolysis. Using Ru(bpy)₃²⁺ as a redox indicator, it is shown that the redox potential of the Tl₄^2+/+ couple is more positive than −1.27 V NHE, and that the Tl⁺/Tl₂⁺ couple's redox potential is more negative than −1.27 V NHE. Ru(bpy)₃⁺ reduces Tl₄²⁺ with a second-order rate constant of 6±2 × 10⁸ M⁻¹s⁻¹. Metallic particles of thallium are predicted to have a surface plasmon absorption band at 205 nm in water. The reduced Tl content in a sol can be measured by air-free titration with CCl₄ or methylviologen.     

Characterization of nickel-bearing ferrites obtained as by-products of hydrochemical wastewater purification processes

The efficiency of the ‘ferrite process’ for the purification of wastewater heavily contaminated with nickel is evaluated, and the solid residues formed are characterised. The efficiency of the purification process is always above 99.9% for Fe²⁺/Ni²⁺ ratios greater than 3. The tested Fe²⁺/Ni²⁺ molar ratios (15/1, 7/1 and 3/1) yielded three different nickel ferrites. Inductively-coupled plasma atomic emission spectroscopy (ICP-AES), potentiometric titration, X-ray fluorescence (XRF), X-ray diffractometry (XRD) and differential scanning calorimetry (DSC) yielded Ni_xFe_1−x^IIFe₂^IIIO₄ (x=0.18, 0.40 and 0.65, respectively) as the most probable stoichiometry, and inverse spinel as the most probable structure. Heating at 600 °C causes the transformation of the solids into a mixture of NiFe₂O₄, α-Fe₂O₃ and NiO. Electrochemical analysis of the solid nickel ferrites was performed using carbon paste electrodes (CPEs) in HClO₄ and HCl media. In each case, the first cyclic voltammogram showed the participation of solid species in the electrochemical transformation process, since the shape of the redox peaks could be related to the structure and stoichiometry of the ferrites. In second and successive scans, the voltammograms indicated the redox couples Fe_ads³⁺+1e⁻⇔Fe_ads²⁺ (0.525 V vs. Ag/AgCl) and Ni_ads²⁺+2e⁻⇔Ni(s) (−0.470 V) in HClO₄, and FeCl_2,ads⁺+1e⁻⇔FeCl_ads⁺+Cl⁻ (0.475 V) and NiCl_x,ads^(x−2)−+2e⁻⇔Ni(s)+xCl⁻ (−0.550 V) in HCl.