Oxidation Of Am Research Articles

Computational Comparative Study of the Binding of Americium with N-Donor Ligands.

The accessibility of multiple valence states of americium (Am) inspired redox-based protocols aimed at efficient separation of trivalent Am (Am3+) from trivalent lanthanides (Ln3+) alternative to the traditional liquid-liquid extraction. This requires an extensive understanding of the coordination chemistry of Am in its various accessible valence states in the aqueous phase. In this work, by means of DFT calculations, the coordination of AmIII-VI with five typical N-donor ligands, i.e., terpyridine (tpy), bispyrazinylpyridine (dpp), bistriazinylpyridine (BTP), bistriazinyl bipyridine (BTBP), and bistrazinyl phenanthroline (BTPhen), was studied in terms of energy and topological analysis. The results show that the exchange of aqua ligands of hydrated ions by N-donor ligands is an entropy-driven process and enthalpically unfavorable. Topological analysis suggests a distinct mechanism of BTP to modulate the redox potential of Am(III) in that BTP can assist the relay of the leaving electron of AmIII, while the other N-donor ligands can detain the oxidation of Am by offering their electron instead. This comparative study enriches our understanding of the coordination chemistry of high-valent Am with N-donor ligands and recommends the ligand design toward the modulation of redox potentials of hydrated Am(III) ions.

The Speciation of Americium Cations in Neat Water Implicated from DFT Studies.

The recent observed manipulatable redox potential of trivalent americium ion in the aqueous phase by modifying an electrode offers an alternative to accomplish the separation. In order to understand extensively the speciation of Am, which is the prerequisite to understanding the mechanism of the oxidation of Am, we conducted a density functional study to identify the potential species of Am in its tri-, tetra-, and pentavalent states in aqueous phase. Based on the speciation analysis, the calculations implicate a stepwise mechanism for the oxidation of hydrated Am(III), which predominantly exists in its hydrated monatomic cationic form (Am3+(aq)). The two sequential one-electron oxidation processes first produce AmO2+(aq), which may establish an equilibrium with Am4+(aq), and the AmO2+(aq) may then evolve to the dioxo americyl(V) ion. These results suggest the copresence of Am4+(aq) and AmO2+(aq), which builds a bridge for the conversion of americium ion from a monatomic ion to dioxo americyl(V).

Ultra-Efficient Americium/Lanthanide Separation through Oxidation State Control.

Lanthanide/actinide separation is a worldwide challenge for atomic energy and nuclear waste treatment. Separation of americium (Am), a critical actinide element in the nuclear fuel cycle, from lanthanides (Ln) is highly desirable for minimizing the long-term radiotoxicity of nuclear waste, yet it is extremely challenging given the chemical similarity between trivalent Am(III) and Ln(III). Selective oxidation of Am(III) to a higher oxidation state (OS) could facilitate this separation, but so far, it is far from satisfactory for practical application as a result of the unstable nature of Am in a high OS. Herein, we find a novel strategy to generate stable pentavalent Am (Am(V)) through coordination of Am(III) with a diglycolamide ligand and oxidation with Bi(V) species in the presence of an organic solvent. This strategy leads to efficient stabilization of Am(V) and an extraordinarily high separation factor (>104) of Am from Ln through one single contact in solvent extraction, thereby opening a new avenue to study the high-OS chemistry of Am and fulfill the crucial task of Ln/Am separation in the nuclear fuel cycle. The synergistic coordination and oxidation process is found to occur in the organic solvent, and the mechanism has been well elucidated by quantum-theoretical modeling.

Separation of minor actinides from highly acidic solutions using diglycolamide modified mesoporous silica synthesized via a novel “ring-opening click” reaction

Solid-phase extraction represents a promising approach for partitioning of trivalent minor actinides (MAs) from high-level liquid waste, which is crucial for the decontamination of radioactive waste. However, highly acidic effluents poseaformidablechallengefor extraction materials. Herein, the diglycolamide ligands functionalized mesoporous silica robust matrixs were prepared, resulting in rapid and efficient separation of MAs from strong nitric acid media. This paper reports the first modification of SBA-15-type mesoporous silica through a “ring-opening click” reaction under mild conditions. Five modified silica materials with monolayer diglycolamide ligands containing different terminal groups were obtained using low temperature (0–25 °C) procedures. The rigid and open pore structure of SBA-15 is retained under such mild conditions, allowing facile diffusion and open access to the monolayer ligands. The functionalized product SBA-15-DEDGA exhibited a strong affinity for trivalent MAs in highly acidic solutions and could separate trace amounts of radioactive Am(III) from 1 to 8 M nitric acid solutions with high efficiency (> 98%) within 1 min; the largest distribution coefficient exceeded 5 × 104 mL/g. Solid-state nuclear magnetic resonance spectroscopy combined with X-ray photoelectron spectroscopy of Eu(III) revealed that the binding mechanism involves direct coordination of Eu(III) to a tridentate oxygen-binding site. Additionally, selective separation of americium from curium was achieved with separation factors up to 191 ± 20 through oxidation of Am(III) by sodium persulfate. The superlative and efficient sorption performance of these materials demonstrates their utility in the immobilization of trivalent actinides and the separation of americium from curium.

Separation of Americium and Curium in Nitric Acid Solutions via Oxidation of Am(III) by Bismuthate and Perxenate Ions

Studies have been carried out in order to develop new ways to separate Am and Cm in nitrate solutions. It was shown that Am(VI) produced by oxidation of Am(III) by sodium bismuthate in 0.1 and 3.0 M HNO3 solutions is extracted with 30% tri-n-butyl phosphate solution in Isopar M diluent. Using this extractive agent in a mixture with a synergic additive of 0.1 M of trioctylphosphine oxide with 0.1 M HClO4 makes it possible to extract into the organic phase up to 90% of the starting amount of Am with not more than 3% of Cm contained in solution. It was found that, upon introduction of Na4XeO6·8H2O into a 0.1 M HNO3 solution containing Am(III) and NaBiO3, the solution becomes alkaline (рН ~ 10) and Am(III) is oxidized to Am(IV) to give a stable complex of composition Am(IV)·XeO6. As a result, Am remains in solution. Sodium bismuthate present in solution is hydrolyzed to give the solid phase Bi2O5 by coprecipitation of hydrolyzed Cm(III). Thus, the redox separation of Am(IV) from Cm(III) in the solution formed as a result of the interaction Am(III) with a 0.1 M HNO3 solution with sodium bismuthate and perxenate it contains is a simpler and more effective way, compared with the developed extractive method.

Effective separation of Am(III) and Cm(III) using a DGA resin via the selective oxidation of Am(III) to Am(V)

A new method was developed to effectively separate Am(III) from Cm(III). Am(III) was selectively oxidized to Am(V) using a mixture of Na2S2O8, Ag(I), and NaOCl in 0.01 M HNO3. Cm(III) was selectively retained on a DGA resin, while Am(V) had no retention. A separation factor of 110 ± 20 was usually obtained from a single separation. The new separation method was applied to determine Cm isotopes 244, 245 and 246 by accelerator mass spectrometry (AMS) in spent nuclear fuel samples.

Electrochemical oxidation of trivalent americium using a dipyrazinylpyridine modified ITO electrode.

We present here the electrochemical oxidation of Am(iii) to AmVO2+ and AmVIO22+ in pH 1 nitric acid using a mesoporous tin-doped indium oxide electrode modified with a covalently attached dipyrazinylpyridine ligand. The applied potential affects the distribution of Am oxidation products. At potential 1.8 V, only Am(v) is observed, while increasing the potential to as much as 2.0 V, results in oxidation of Am(iii) to Am(v) and subsequent oxidation of Am(v) to Am(vi). At applied potentials >2.0 V, Am(iii) is oxidized to Am(v), while Am(vi) is reduced to Am(v). The latter reduction reaction is likely due to the increased rate of hydrogen peroxide formation from the 2-electron oxidation of water at the electrode at these high potentials. The development of future ligand modified electrodes for actinide oxidations must consider how they facilitate Am oxidations while disfavoring unwanted or competing reactions.

Organic and Aqueous Redox Speciation of Cu(III) Periodate Oxidized Transuranium Actinides

A hexavalent group actinide separation process could streamline used nuclear fuel recycling and waste management. The limiting factor to such a process compatible with current fuel dissolution practices is obtaining and maintaining hexavalent Am, AmO22+, in molar nitric acid because of the high reduction potential of the Am(VI)/Am(III) couple (1.68 V vs SCE). Two strong oxidants, sodium bismuthate and Cu3+ periodate, have demonstrated quantitative oxidation of Am under molar acid conditions, and better than 50% recovery by diamyl amylphosphonate (DAAP) is possible under these same conditions. This work considers the use of Cu3+ periodate to oxidize Np(V) to Np(VI) and Pu(IV) to Pu(VI) and to recover these elements by extraction with DAAP. A metal:oxidant ratio of 1:1.2 and 1:3 was necessary to quantitatively oxidize Np(V) and Pu(IV), respectively, to the hexavalent state. Extraction of hexavalent Np, Pu, and Am by 1 M DAAP in n-dodecane was measured using ultraviolet–visible (PuO22+, AmO22+) and near-infr...

Experimental and Theoretical Studies of Actinide and Lanthanide Ion Transport Across Supported Liquid Membranes

In this work, we propose a new transport mechanism for metal ions relevant for used nuclear fuel separation processes by a supported liquid membrane (SLM). Two SLM extraction systems were investigated where the membrane was impregnated with either di-(2-ethylhexyl)phosphoric acid (HDEHP) or tributyl phosphate (TBP). A HDEHP impregnated membrane was used to extract neodymium (III), representative of a typical trivalent lanthanide. Cerium, which was oxidized by sodium bismuthate from trivalent to tetravalent state, was extracted by TBP. Oxidized cerium was used as a surrogate for oxidized americium to investigate the kinetics and possibility of americium and curium separation by membrane extraction. Both extraction systems were operated at varying nitric acid concentrations, and changes in the kinetics and extraction efficiency of metal ions were investigated. The proposed transport mechanism that was chosen for our studies was modified from the previous works by Danesi et al.[1,2] and Cussler et al.[3] The mechanism was selected due to the ability to accommodate and describe transport phenomena across a SLM when formation of extractant nano-channels in the membrane may exist. We were able to obtain acceptable fit of the models to our overall data trends although chemical and physical conditions must be well established and purity and homogeneity of the membrane are critical. A reverse transport of metal ions was observed when leaving the system for longer times which agrees with our model. The membrane was investigated for degradation and shown to be stable after contact with up to 7 M nitric acid for over 2000 minutes. Finally, we examined the possibility of partitioning americium from curium using a SLM impregnated by TBP. Separation of americium from curium was observed although not to a degree that was expected based on the Ce(IV) transport. Incomplete oxidation of Am(III) to Am(V) and reduction of Am(VI) on the membrane surface are possible causes for this observed discrepancy. Our model was, however, able to accurately predict Cm(III) transport through the membrane.

Americium(III) oxidation by copper(III) periodate in nitric acid solution as compared with the action of Bi(V) compounds of sodium, lithium, and potassium

Abstract The oxidative action of a Cu(III) periodate compound toward Am(III) in nitric acid was studied. The extent of oxidation of Am(III) to Am(VI) was investigated using a constant initial Cu(III)-to-Am(III) molar ratio of 10:1 and varying nitric acid concentrations from 0.25 to 3.5 mol/L. From 0.25 to 3 mol/L HNO3, more than 98% of the Am(III) was oxidized to Am(VI); however, at 3.5 mol/L HNO3, the conversion to Am(VI) was only 80%. Increasing the Cu(III)-to-Am(III) molar ratio to 20:1 in 3.5 mol/L HNO3 resulted in 98% conversion to Am(VI). For comparison, oxidation of Am(III) with NaBiO3 was studied at 3.5 mol/L HNO3 and the same stoichiometric excess of Bi(V) oxidant over Am(III) (stoichiometric ratio of 3.33:1). With NaBiO3, the extent of Am(III) conversion to Am(VI) was only 19%, while with the Cu(III) compound this value was found to be about 4 times higher under otherwise identical conditions. Similar results were obtained with other Bi(V) salts. These results show that the Cu(III) periodate compound is a superior oxidant to NaBiO3, yielding rapid conversion to Am(VI) in a homogeneous acidic solution, and is, therefore, an excellent candidate for further development of Am separation systems.

Characterizing Diamylamylphosphonate (DAAP) as an Americium Ligand for Nuclear Fuel-Cycle Applications

Successful deployment of the currently-envisioned advanced nuclear fuel cycle requires the development of a partitioning scheme to separate Am from the lanthanides. The Am/lanthanide separation is challenging since all the metals are normally trivalent and have similar ionic radii. Oxidation of Am to higher oxidation states is one option to achieve such a separation. Hexavalent Am has now been routinely prepared in our laboratory in strongly acidic solution using sodium bismuthate as the oxidant, and then extracted into diamylamylphosphonate/dodecane solution. Here, we have characterized this phosphonate-containing solvent with regard to the extraction of Am, the lanthanides, Cm, other fission product, and/or inert constituents expected in dissolved nuclear fuel. Additionally, the effects of irradiation on dispersion numbers and the phosphonate concentration were investigated.

Separation of Americium from Curium by Oxidation and Ion Exchange

Nuclear energy has the potential to be a clean alternative to fossil fuels, but in order for it to play a major role in the US, many questions about the back end of the fuel cycle must be addressed. One of these questions is the difficult separation of americium from curium. Here, we report the oxidation of Am in two systems, perchloric acid and nitric acid and the affect of changing the acid has on the oxidation. K(d) values were observed and a direct separation factor was calculated and was seen to be as high as 20 for four metal(IV) pillared phosphate phosphonate inorganic organic hybrid ion exchange materials. These ion exchangers are characterized by very low selectivity for cations with low charge but extremely high uptake of ions of high charge.

Use of ozone for dissolving high-level plutonium dioxide in nitric acid in the presence of Am(V,VI) ions

Plutonium dioxide recovered in the course of reprocessing of SNF from WWER reactors (so-called high-level PuO2) was subjected to dissolution in 0.6–3.0 M HNO3 in the presence of Am(III) ions under ozonation with an ozone-oxygen mixture containing 30–180 mg l−1 O3. Measurements of the rate of the PuO2 dissolution in 3 M HNO3 in the temperature interval from 30 to 80°C showed that, with an increase in the ozone concentration in the ozone-oxygen mixture from 30 to 180 mg l−1, the dissolution rate increases by a factor of 4–5. The acceleration of the PuO2 dissolution is attributed to the formation of Am(V,VI) by homogeneous oxidation of Am(III) ions with ozone dissolved in HNO3. The Am dioxocations formed act as PuO2 oxidants and are continuously regenerated by the oxidation of Am(III) with ozone. This assumption is confirmed by an additional increase in the dissolution rate, observed on introducing Am(III) into the initial electrolyte for the PuO2 dissolution.

Redox-Based Separation of Americium from Lanthanides in Sulfate Media

In the development of advanced nuclear fuel cycles, a primary motivation is the reduction of the long-term radiotoxicity of the wastes. From a few decades to several thousand years after removal of the fuel from the reactor, americium (Am) dominates the radiotoxicity of used fuel, thus its transmutation represents an attractive option for improved management of the residues. However, every viable scenario for transmutation of Am demands some degree of separation of Am from fission product lanthanides. Partitioning of Am from curium further simplifies the transmutation process. The mutual separation of these elements is very challenging due to their similar chemistry. The focus of this work is on the utilization of the upper oxidation states of americium (Am(V/VI)) to facilitate a more efficient separation of Am from fission product lanthanides and curium. A minimum 90% efficient separation of americium from the lanthanides and curium has been achieved in the laboratory by selectively oxidizing trivalent Am to the hexavalent state using Na2S2O8. At equilibrium, lanthanides are precipitated as sodium lanthanide sulfate double salts, while oxidized Am species remain predominantly in the supernatant phase. Trivalent curium is readily coprecipitated with the NaLn(SO4)2 solid. Parallel studies of oxidized uranium, neptunium, and plutonium solutions support the conclusion that Am(VI) has sufficient stability to allow the separation to be completed in a reasonable time frame. A particular advantage of this approach is the absence of readily oxidized species that can quickly reverse the oxidation of Am and thus negate the separation. A variety of chemical and physical characterization techniques have been applied to profile the performance of the system.

Solid-liquid separation of oxidized americium from fission product lanthanides

The separation of americium from the lanthanides and curium is a requirement if transmutation of americium is to be performed in advanced nuclear fuel cycles. Oxidation of Am3+ to AmO2+ or AmO22+ may allow separation of Am from Ln and Cm in one step, since the lanthanides and curium do not have higher oxidation states as accessible. Two possible solid-liquid separation methods have been developed to address this difficult separation. Under acidic conditions using oxone or persulfate, the oxidation and retention of tracer Am in the aqueous phase has been observed with a separation factor of 11 ± 1. Most of these studies have been conducted using 237NpO2(NO3), 233UO2(NO3)2, 238Pu(NO3)4 and 241Am(NO3)3 at radiotracer concentrations. Lanthanides precipitate as the sodium or potassium europium double sulfate salt. Under basic conditions, ozone oxidation of Am(CO3)OH(s) solubilizes Am from a lanthanide carbonate hydroxide solid phase to the aqueous phase as the AmO2(CO3)34-or AmO2(CO3)35- species. For the ozone oxidation of the americium tracer a separation factor of 1.6 ± 0.8 and 47 ± 2 for the oxidation/separation in Na2CO3 and NaHCO3 respectively.

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.

Complexation of amine free radicals during the radiation-induced chain oxidation reaction of N,N,N′,N′-tetramethyl-4,4′-diaminodiphenylmethane

Complexation of free radicals derived from the amine in the chain redox reaction in γ-irradiated solid PVC films doped with N,N,N′-N′-tetramethyl-4,4′-diaminodiphenylmethane (Am) and CBr4 was studied by means of the ESR and luminescence techniques. The effect of concentration of the additives Am and CBr4 on the quenching of fluorescence of Am· radicals was examined. The data were rationalized in terms of the Stern-Volmer formalism for static quenching, which results from the formation of nonfluorescing ground-state charge-transfer complexes between the radical Am·, as well as its dimeric form Am 2 · , and CBr4. The association constants of the complexes were determined. The mechanism of radiation-induced chain oxidation of Am involving these complexes is discussed.

Electrochemical Oxidation of Am(III) and Am(V) Ions in HNO3 Solutions Containing Potassium Phosphotungstate K10P2W17O61

The kinetics of electrochemical oxidation of Am(III) and Am(V) on Pt electrode in solutions of concentrated nitric acid containing potassium phosphotungstate was studied spectrophotometrically. The processes occurring in the system follow the rate law for reversible reaction. The direct reaction is first-order with respect to the concentration of Am(III) ions, and the reverse process is a zero-order reaction.

Participation of the charge-transfer complexes in the radiation-induced chain oxidation of aromatic amines in polymer films

The chain redox reaction in γ-irradiated solid films of poly(vinyl chloride) (PVC), containing N,N,N′,N′-tetramethyl-4,4′-diaminodifenylmethane (Am) and CBr4, was studied. The formation of charge-transfer complexes between Am and CBr4 and also between Am radicals and CBr4, the participation of these complexes in the chain oxidation of Am were investigated by ESR, spectrophotometry and luminescence methods. The spectral-luminescent properties of Am radicals both isolated and interacting with the PVC matrix were discussed. It was discovered the quenching of luminescence of Am radicals by molecules of CBr4. The effects of annealing temperature and concentration of CBr4 on the quenching of luminescence of Am radicals were observed. Stern–Folmer’s equation was used for the description of the static quenching of Am radical fluorescence. It was shown that this quenching could be due to the formation of ground-state charge-transfer complexes between Am radicals and CBr4. The kinetic features and mechanism of the radiation-induced chain oxidation of Am with the participation of the charge-transfer complexes were investigated.

An experimental investigation of accumulation and transmutation behavior of americium in the MOX fuel irradiated in a fast reactor

Americium isotopes generated in the MOX fuel irradiated in the experimental fast reactor JOYO were analyzed by applying a sophisticated radiochemical technique. Americium was isolated from the irradiated MOX fuel by a combined method of anion-exchange chromatography and oxidation of Am. The isotopic ratios of americium and its content were determined by thermal ionization mass spectroscopy and α-spectrometry, respectively. The americium isotopic ratio was similar for all the specimens, but was significantly different from that of PWR-MOX. On the basis of present analytical results, the accumulation and transmutation behavior of americium nuclides in a fast reactor is discussed from the viewpoints of neutron spectrum dependence and the isomeric ratio of the 241Am capture reaction. The estimated isomeric ratio is about 87%, which is close to the latest evaluated value. A rapid estimation method of Am content by using the 240Pu to 239Pu ratio was adopted and proved to be valid for the spent fuel irradiated in the fast reactor.