Reactions Of Am Research Articles

Study of Am electrodeposition from AmCl3-LiCl-KCl Molten Salt

In this work, the electrodeposition reaction of Am in AmCl3-LiCl-KCl molten salt was studied using voltammetry and chronoamperometry. Electrodeposition of Am was shown to proceed via a two − step reaction scheme, involving the one-electron transfer reduction of Am3+ to Am2+, followed by the two-electron transfer reduction of intermediate Am2+ to Am0. A low Coulombic efficiency of Am deposition was measured despite the Am deposition occurring within the thermodynamic stability window of the supporting LiCl-KCl electrolyte. We hypothesize that the low efficiency is due to out-diffusion of the intermediate Am2+ species away from the electrode surface. Experimental observations provide evidence of a kinetically − limited electrodeposition reaction, which allows for the loss of intermediate Am2+ via diffusion leading to Am deposition inefficiency.

Design criteria for tetradentate phenanthroline-derived heterocyclic ligands to separate Am(III) from Eu(III)

To design novel phenanthroline-derived soft ligands for selectively separating minor actinides from lanthanides, four tetradentate phenanthroline-derived heterocyclic ligands (BTPhen, BPyPhen, BPzPhen, and BBizPhen) were constructed and their complexation behaviors with Am(III) and Eu(III) were systematically investigated by density functional theory (DFT) coupled with relativistic small-core pseudopotential. In all the 1:1-type species, the metal ion is in the center of the cavity and coordinates with two nitrogen atoms (N1 and N1') of the phenanthroline skeleton and the other two nitrogen atoms (N2 and N2') of the auxiliary groups. The bond lengths of Am-N are comparable to or even shorter than those of Eu-N bonds because the ionic radii of Am(III) are larger than those of Eu(III). Additionally, the negative Delta Delta G (Am/Eu) value for the reaction of [M(H2O)(4)(NO3)(3)] + L -> ML(NO3)(3) + 4H(2)O indicates that the complexation reaction of Am(III) is more energetically favorable than that of Eu(III); this can be considered as an important design criterion to screen phenanthroline-derived ligands for MA(III) extraction. According to this criterion, the selectivity of tetradentate phenanthroline-derived ligands for separating Am(III) over Eu(III) follows the order of BTPhen > BBizPhen > BPyPhen > BPzPhen.

A density functional theory study of complex species and reactions of Am(III)/Eu(III) with nitrate anions

Complex behaviours of americium(III) and europium(III) with nitrate anions have been studied using density functional theory and small-core quasi-relativistic effective core potential. The structural parameters of a series of hydrated and nitrate complexes of Am(III) and Em(III) are probed and compared to available EXAFS studies. The thermodynamic stability of these species is calculated by simulating related complexation reactions both in the gas phase and aqueous solution. For taking into account the solvent effect, both the solvation model density (SMD) implicit solvent model and the conductor-like screening model (COSMO) are adopted for comparison. From our calculations, the octa–aqua and non-aqua species of Am(III) and Eu(III), [M(H2O)8,9]3+ (M = Am, Eu), form square antiprism and trigonal prism structures, respectively. In the presence of rich-enough nitrate ions, at most three nitrate anions can chelate to Am(III)/Eu(III) ions in a bidentate fashion in the first coordination shell, owing to the strong Coulomb interactions. From the changes of the Gibbs free energy for complexation reactions, , the bidentate complexes are more favourable than the monodentate ones with the same coordination numbers. The comparison of ionic exchange reactions further suggests a stronger coordination ability and harder acidity of Eu(III) than Am(III). The SMD and COSMO predict the same trend in solvent effect, whereas the latter seems to give obviously lower changes of the Gibbs free energy.

Solid phase extraction of trivalent actinides and lanthanides using a novel CMPO-RTIL based chromatographic resin

Abstract A novel extraction chromatographic resin was prepared by impregnating CMPO-RTIL (C8mim+·NTf2 −) on Chromosrob-W for the removal of trivalent actinides from aqueous solution and its behaviour was compared with previously reported CMPO-n-dodecane resin. Under identical condition, the K d value of Am(III) by CMPO-RTIL resin (K d at 3 M HNO3 = 5805 ± 525) was one order magnitude higher than that obtained with CMPO-n-dodecane resin (K d at 3 M HNO3 = 610 ± 55). The sorption reaction of Am(III) followed a first-order rate law with both the resins. The metal sorption capacities for Eu(III) at 3 M HNO3 were found to be 18.95 ± 1.13 mg and 20.52 ± 1.25 mg/g of CMPO-RTIL and CMPO-n-dodecane resins, respectively. The analysis of the sorption data by different models supported the hypothesis that the sorption of Eu(III) on the resins was actually limited by the saturation of the CMPO complexing sites on the resin. The free energy for the sorption of Eu(III) obtained from D–R isotherm was 17.67 ± 1.63 kJ/mol and 16.51 ± 2.05 kJ/Mol for CMPO-RTIL and CMPO-n-dodecane resins, respectively. The sorption free energy is an indication of chemisorption monolayer mechanism. The monolayer sorption mechanism was also confirmed by the theoretical approach calculated based on the sorption kinetics.

Specific features of reactions of Am(V) with reductants in weakly acidic solutions

In EDTA solutions with pH ∼5 at 25°C, Am(V) in a concentration of 5 × 10−4−3 × 10−3 M slowly transforms into Am(III). The Am(V) reduction and Am(III) accumulation follow the zero-order rate law. In the range 60–80°C, the reaction is faster. In some cases, an induction period is observed, disappearing in acetate buffer solutions. In the range pH 3–7, the rate somewhat increases with pH. In an acetate buffer solution, an increase in [EDTA] accelerates the process. The activation energy is 47 kJ mol−1. Zero reaction order with respect to [Am(V)] is observed in solutions of ascorbic and tartaric acids, of Li2SO3 (pH > 3), and of hydrazine. The process starts with the reaction of Am(V) with the reductant. The Am(III) ion formed in the reaction is in the excited state, *Am(III). On collision of *Am(III) with Am(V), the excitation is transferred to Am(V), and it reacts with the reductant: *Am(V) + reductant → Am(IV) + R1 and then Am(IV) + reductant → *Am(III) + R1, Am(V) + R1 → Am(IV) + R2. A branched chain reaction arises. The decay of radicals in side reactions keeps the system in the steady state; therefore, zero reaction order is observed.

Extraction of Am(III) at the Interface of Organic-Aqueous Two-Layer Flow in a Microchannel

Extraction of Am(III) was performed at the interface of organic-aqueous two-layer flow in a micro-channel having an asymmetric cross section. A solution of 3 mol/dm3 nitric acid containing 243Am(III) and octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphineoxide diluted with n-dodecane were introduced into the microchannel as the aqueous phase and organic phase, respectively. The two phases formed a stable two-layer flow with an interface parallel to the sidewall of the microchannel, and they were separated from each other at the divergence point of the microchannel. The extraction reaction of Am(III) proceeded at the interface of the two phases, and reached the equilibrium state while the two phases passed through the microchannel.

Oxidation of Am(III) to Am(IV) with ozone in solutions of heteropolyanions

Oxidation of Am(III) in the presence of K10P2W17O61 and K8SiW11O39 (L) at 20dGC was studied by spectrophotometry. Am(III) is oxidized with ozone to Am(IV) only in the pH range 3.5–1.0. In the case of formation of a complex with Am: L = 1: 1, the reaction is approximately first-order with respect to the metal, and its rate decreases with a decrease in pH from 3.5 to 1.0. Am(IV) in solution in the presence of ozone is slowly reduced with products of water α-ray radiolysis, predominantly with ?2?2. The rate constant of the reaction of Am(IV) with ?2?2 was estimated.

Mechanism of Am(III) oxidation with ozone in carbonate solutions

The reaction of ozone with Am(III) in bicarbonate and carbonate solutions was studied by spectrophotometry. On adding ozone-saturated water to a 2 × 10−4 M Am(III) solution in 1 M NaHCO3, about 1/3 of Am remains in the trivalent state and 2/3 is converted to Am(V), with no accumulation of Am(IV). The reaction of Am(III) with ozone involves replacement of H2O molecules in the coordination sphere of Am(III) by O3 molecule, followed by elimination of the O2 molecule. The third O atom remains bonded with Am, converting it to the pentavalent state.

Cyclic Voltammetry Behavior of Americium at a Liquid Cadmium Electrode in LiCl-KCl Eutectic Melts

The electrode reactions of americium at a liquid cadmium electrode were investigated by cyclic voltammetry of AmCl3-(LiCl-KCl)eut. at both 723 and 773 K in comparison with those at a molybdenum electrode. The redox peaks assigned to Am(III)/Am(0) (in Cd) were observed with the liquid Cd electrode, while the redox reactions of Am(III)/Am(II) and Am(II)/Am(0) were observed with the Mo electrode. The formal standard potential of Am(III)/Am(0) obtained with the liquid Cd electrode is more positive than those calculated for the Mo electrode at both 723 and 773 K. The potential shifts were attributed to the lowering of the activity of Am by the formation of the intermetallic compound at the interface between Cd and the molten salt. The Gibbs free energies of formation of the Am-Cd intermetallic compound, which could be AmCd6, are estimated to be -119 and -113 kJ/mol at 723 and 773 K, respectively.

Synthesis and Characterization of f-Element Iodate Architectures with Variable Dimensionality, α- and β-Am(IO3)3

Two americium(III) iodates, beta-Am(IO3)3 (I) and alpha-Am(IO3)3 (II), have been prepared from the aqueous reactions of Am(III) with KIO(4) at 180 degrees C and have been characterized by single-crystal X-ray diffraction, diffuse reflectance, and Raman spectroscopy. The alpha-form is consistent with the known structure type I of anhydrous lanthanide iodates. It consists of a three-dimensional network of pyramidal iodate groups bridging [AmO8] polyhedra where each of the americium ions are coordinated to eight iodate ligands. The beta-form reveals a novel architecture that is unknown within the f-element iodate series. beta-Am(IO3)3 exhibits a two-dimensional layered structure with nine-coordinate Am(III) atoms. Three crystallographically unique pyramidal iodate anions link the Am atoms into corrugated sheets that interact with one another through intermolecular IO3-...IO3- interactions forming dimeric I2O10 units. One of these anions utilizes all three O atoms to simultaneously bridge three Am atoms. The other two iodate ligands bridge only two Am atoms and have one terminal O atom. In contrast to alpha-Am(IO3)3, where the [IO3] ligands are solely corner-sharing with [AmO8] polyhedra, a complex arrangement of corner- and edge-sharing mu2- and mu3-[IO3] pyramids can be found in beta-Am(IO3)3. Crystallographic data: I, monoclinic, space group P2(1)/n, a = 8.871(3) A, b = 5.933(2) A, c = 15.315(4) A, beta = 96.948(4) degrees , V = 800.1(4) A(3), Z = 4; II, monoclinic, space group P2(1)/c, a = 7.243(2) A, b = 8.538(3) A, c = 13.513(5) A, beta = 100.123(6) degrees , V = 822.7(5) A(3), Z = 4.

Reaction of Am(VI) with Na4XeO6 in Alkaline Solutions in the Presence of Ozone

The reaction of Am(VI) with perxenate ions XeO 6 4− in 1 M NaOH solutions was studied. A solid compound [Am(VI) : Na4XeO6 = 1 : 1] is formed in reaction of 1 mM Am(VI) with solid Na4XeO6; its ozonation in a thin layer yields an Am(VII) compound.

Synthesis, Structure, and Spectroscopic Properties of Am(IO3)3 and the Photoluminescence Behavior of Cm(IO3)3

We have prepared Am(IO(3))(3) as a part of our continuing investigations into the chemistry of the 4f- and 5f-elements' iodates. Single crystals were obtained from the reaction of Am(3+) and H(5)IO(6) under mild hydrothermal conditions. Crystallographic data on an eight-day-old crystal are (21 degrees C, Mo Kalpha, lambda = 0.71073 Angstroms): monoclinic, space group P2(1)/c, a = 7.2300(5) Angstroms, b = 8.5511(6) Angstroms, c = 13.5361(10) Angstroms, beta = 100.035(1) degrees, V = 824.06(18), Z = 4. The structure consists of Am(3+) cations bound by iodate anions to form [Am(IO(3))(8)] units, where the local coordination environment around the americium centers is a distorted dodecahedron. There are three crystallographically unique iodate anions within the structure that bridge in both bidentate and tridentate fashions to form the overall three-dimensional structure. Repeated collection of X-ray diffraction data with time for a crystal of (243)Am(IO(3))(3) revealed an anisotropic expansion of the unit cell, presumably from self-irradiation damage, to generate values of a = 7.2159(7) Angstroms, b = 8.5847(8) Angstroms, c = 13.5715(13) Angstroms, beta = 99.492(4) degrees, V = 829.18(23) after approximately five months. The Am(IO(3))(3) crystals have also been characterized by Raman spectroscopy and the spectral results compared to those for Cm(IO(3))(3). Three strong Raman bands were observed for both compounds and correspond to the I-O symmetric stretching of the three crystallographically distinct iodate anions. The Raman profile suggests a lack of interionic vibrational coupling of the I-O stretching, while intraionic coupling provides symmetric and asymmetric components that correspond to each iodate site. Photoluminescence data for both Am(IO(3))(3) and Cm(IO(3))(3) are reported here for the first time. Assignments for the electronic levels of the actinide cations were based on these photoluminescence measurements and indicate the presence of vibronic coupling between electronic transitions and IO(3)(-) vibrational modes in both compounds.

Redox Behavior of Am(IV) and Methods of Its Preparation in Alkaline Media

A scheme was suggested for Am(OH)4 isolation by treatment of Am(OH)3 suspension in 0.1–1.0 M NaOH with ozone (3.5–5 vol %)-oxygen mixture (4–5 l h−1 flow rate) at 20°C for 40 min, followed by ultrasonic treatment of the resulting Am(VI) (44 kHz, 1 W cm−3) for 45 min. The separated precipitate of Am(III) hydroxy peroxide was treated with 1–2 mlof 7–10 M NaOH to form Am(OH)4. Mixing suspensions of equivalent amounts of Am(III) and Am(V) hydroxides in NaOH also gives Am(IV) hydroxide in a >98% yield. The reproportionation Am(III) + Am(V) = 2Am(IV) in 1 M NaOH starts on heating above 70°C, whereas at NaOH concentration higher than 7 M it is completed even at room temperature. The reaction of Am(III) with Am(VI) in alkaline solutions, Am(VI) + 2Am(III) → 3Am(IV), occurs during mixing the reactants. The equilibrium reaction of Am(OH)3 with [Fe(CN)6]3− in alkaline solutions was studied. It was shown that increasing the alkali concentration to 2 M NaOH promotes formation of Am(OH)4. At further increase in the alkali concentration, Am(V) is formed.

FTICR-MS study of the gas-phase thermochemistry of americium oxides

Gas-phase ion chemistry experiments with americium using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) were performed for the first time. Reactions of Am + and AmO + with the oxidants N 2O, C 2H 4O (ethylene oxide), H 2O, O 2, CO 2, and NO have been studied. Am + formed AmO + with all the reagents except NO, while AmO + only reacted with C 2H 4O to form AmO 2 + and other products. These results allowed us to estimate the previously unknown Am +O and OAm +O bond dissociation energies. The ionization energies (IE) of AmO and AmO 2 could also be determined by two different types of experiments: charge-transfer “bracketing” with AmO 2 +, yielding IE(AmO 2)=7.23±0.15 eV, and AmO + reactivity with dienes, following a model developed by Helmut Schwarz and coworkers, leading to IE(AmO)=5.9±0.2 eV. With this last method, we were also able to resolve the disagreement between the two literature values for the ionization energy of PuO.

Reactions of Am(III) and Eu(III) with potassium ferricyanide in nitric acid solutions

Summary The reactions between Am or Eu present in 0.1 M nitric acid solutions, alone or both together, with 0.25 M K3Fe(CN)6, were studied at room temperature. When Am was the only trivalent metal ion in solution, precipitation of AmFe(CN)6 occurs and the residual Am concentration is about equal to 0.95 mM. However, when Am initial concentration is less than the above specified value for AmFe(CN)6 solubility, Am residual concentration measured is lower that its concentration in the initial solutions. The solubility of EuFe(CN)6 was found to be equal to 30 mM. However, when precipitate formation occurs, the Eu residual concentration after phase separation is about in average 8 mM. When Am and Eu were simultaneously present at the same concentration in solution, the solubility of Am differs little from that measured with Am alone. For initial Am concentrations below 1 mM, the solubility of Am is higher than that observed in the absence of Eu. For initial concentrations of about 8 mM, the Am solubility is lower (about 0.7 mM) than that observed for Am alone. When the initial Eu concentration is constant at 29 mM, the Am precipitation efficiency is much higher than observed in the absence of Eu, for Am concentrations between 30 and 0.5 mM. The residual Am concentratixon in solution thus drops considerably in the presence of Eu (29 mM), and is about 0.04 mM for initial Am concentrations below 4 mM.

Precipitation of nanoscale icosahedral quasicrystalline phase in Hf–Cu amorphous alloy promoted by the addition of Ni

Hf 70Cu 30 and Hf 70Cu 20Ni 10 amorphous alloys were prepared by a single roller melt-spinning method and the crystallization processes were studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). For the Hf 70Cu 30 alloy, the stable Hf 2Cu crystalline phase precipitates directly from the amorphous matrix. For the Hf 70Cu 20Ni 10 alloy, the crystallization proceeds through the reactions of Am→I-phase→Hf 2(Cu, Ni), where Am- and I-phase represent the amorphous and icosahedral quasicrystalline phases, respectively. The present result demonstrates that the addition of Ni to the Hf–Cu alloy is effective in promoting the precipitation of the I-phase. To explain the above experimental results, the existence of icosahedron-like atomic clusters in the amorphous state of Hf 70Cu 20Ni 10 alloy is suggested.

Crystallization processes from supercooled liquid of Cu 40Ti 30Ni 15Zr 10Sn 5 and Zr 60Ni 25Al 15 amorphous alloys

This paper is intended to investigate the effect of additional solute elements on the glass-forming ability by studying the crystallization processes of Cu 40Ti 30Ni 15Zr 10Sn 5 and Zr 60Ni 25Al 15 amorphous alloys in the supercooled liquid region. The crystallization of Cu 40Ti 30Ni 15Zr 10Sn 5 amorphous alloy takes place through the reactions of Am→CuTi+Cu 10Zr 7→Cu 10Zr 7+Cu 3Ti+CuTi 2+Cu 2Ti. Enrichment of Sn in the Cu 10Zr 7 phase was revealed by compositional analysis. On the other hand, the crystallization of Zr 60Ni 25Al 15 amorphous alloy takes place through the reaction of Am→Zr 5Ni 4Al+Zr 6NiAl 2. The common feature in the crystallization processes of the two alloys is the necessity of significant redistribution of the solute elements (Sn or Al), which is believed to play an important role in the stability of the supercooled liquid against crystallization.

Dynamic crystallization process in a supercooled liquid region of Cu 40Ti 30Ni 15Zr 10Sn 5 amorphous alloy

Recently, a new bulk amorphous system Cu 40Ti 30Ni 15Zr 10Sn 5 has been reported. The glass transition temperature T g is 735 K and the crystallization temperatures are 780 K ( T x1 ) and 816 K ( T x2 ), respectively. The phase transition of the Cu 40Ti 30Ni 15Zr 10Sn 5 amorphous alloy annealed at 735 ( T g), 758 ( T g+ T x1 )/2, 780 ( T x1 ), 900 and 1000 K, respectively, was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC) and high-resolution analytical electron microscopy (HRATEM). The crystallization of the alloy proceeds by the process Am (amorphous state) →CuTi+Cu 10Zr 7→Cu 3Ti+Cu 2Ti+CuTi 2+Cu 10Zr 7. Two exothermic peaks are observed in the DSC curve of the as-quenched sample, corresponding to the reactions of Am→CuTi+Cu 10Zr 7 and CuTi+Cu 10Zr 7→Cu 3Ti+CuTi 2+Cu 2Ti+Cu 10Zr 7, respectively. During the first-stage of crystallization, a significant redistribution of Zr and Sn was recognized by a composition analysis with energy dispersive X-ray spectroscopy (EDS), implying that the phase transition is controlled mainly by the rearrangement of the solute elements Zr and Sn. The necessity of this long-range rearrangement of Zr and Sn in the first-stage of crystallization seems to be one of the reasons for the high stability of the supercooled liquid in the present alloy system.

Americium Valence Control Experiments in the Dissolved Solution of Irradiated Nuclear Fuel

A method based on valence control and solvent extraction is presented for separating minor actinides (Am, Np) from nuclear fuel reprocessing solution. With this method, Am is adjusted from trivalence to hexavalence by adding 1.6M (NH4)2S2O8 and 0.01 M AgNO3 to the solution of irradiated nuclear fuel. In the present study, the resulting valence of Am was determined by spectrophotometry. The oxidizing reaction of Am (III) to Am(VI) is exposed to competition by the oxidation to Pu(VI) of the Pu(IV) co-present in the solution, which results in lowering of the yield of Am(VI) to 90%±4% in a solution containing 15 g/l of Pu. Removal beforehand of Pu(IV) from the solution will permit Am(III) to be oxidized quantitatively. Neptunium also is oxidized to hexavalence under the same conditions that yield Am (VI). The resulting hexavalent actinides are both extractable with TBP (tri-n-butyl phosphate). This means that both the minor actinides americium and neptunium can be recovered from raffinate solution generated from the co-decontamination step in spent fuel reprocessing.

Americium Valence Control Experiments in the Dissolved Solution of Irradiated Nuclear Fuel.

A method based on valence control and solvent extraction is presented for separating minor actinides (Am, Np) from nuclear fuel reprocessing solution. With this method, Am is adjusted from trivalence to hexavalence by adding 1.6M (NH4)2S2O8 and 0.01 M AgNO3 to the solution of irradiated nuclear fuel. In the present study, the resulting valence of Am was determined by spectrophotometry. The oxidizing reaction of Am (III) to Am(VI) is exposed to competition by the oxidation to Pu(VI) of the Pu(IV) co-present in the solution, which results in lowering of the yield of Am(VI) to 90%±4% in a solution containing 15 g/l of Pu. Removal beforehand of Pu(IV) from the solution will permit Am(III) to be oxidized quantitatively. Neptunium also is oxidized to hexavalence under the same conditions that yield Am (VI). The resulting hexavalent actinides are both extractable with TBP (tri-n-butyl phosphate). This means that both the minor actinides americium and neptunium can be recovered from raffinate solution generated from the co-decontamination step in spent fuel reprocessing.