Extraction of americium(VI) by a neutral phosphonate ligand

Recently the use of the more unusual hexavalent oxidation state of americium has been receiving increased attention for the purpose of developing an efficient Am/Cm or Am/lanthanide separation system. We have already demonstrated the feasibility of performing this separation with 30% TBP in dodecane, and are now looking at different extractants to increase Am(VI) distribution ratios. Following on from this the extraction of bismuth oxidized americium from nitric acid solutions by dibutyl butyl phosphonate has been studied. The results of this study indicate that increasing the basicity of the extractant molecule has significantly improved the extraction efficiency.     

Determination of americium-243 through its daughter product neptunium-239

The extraction of Np(IV), Zr, Nb, Cs, Ce(III) and Am(III) from nitric acid solutions containing oxalate and phosphate ions by solutions of 1-phenyl-3-methyl-4-benzoylpyrazolon-5 (PMBP) and tri-n-butyl phosphate (TBP) in benzene has been investigated. A solution 0.1M in respect to PMBP and 0.25M in respect to TBP was found to extract 99% of neptunium from aqueous solutions 1M in respect to H₃PO₄ and 0.5M in respect to HNO₃. Under these conditions, the extraction of the other investigated elements does not exceed 0.1%. Based on this finding, a procedure was developed to determine²⁴³Am through its daughter product²³⁹Np in solutions containing large quantities of curium and its fission products. The sensitivity of the procedure is 1·10⁻⁷ mg of²⁴³Am in the sample. The²⁴³Am content is obtained by calculation from measurements of the γ-activity of the extracted²³⁹Np. The purification ratio of²³⁹Np is∼10⁵ from Zr, Nb and Ru, ∼10⁸ from Ce and Cm and >10¹² from Cs.     

Separation of neptunium, plutonium, americium and curium from uranium with di-(2-ethylhexyl)-phosphoric acid (HDEHP) for radiometric and ICP-MS analysis

The possibility of using di-(2-ethylhexyl)-phosphoric acid (HDEHP) in solvent extraction for the separation of neptunium, plutonium, americium and curium from large amounts of uranium was studied. Neptunium, plutonium, americium and curium (as well as uranium) were extracted from HNO₃, whereafter americium and curium were back-extracted with 5M HNO₃. Thereafter was neptunium back-extracted in 1M HNO₃ containing hydroxylamine hydronitrate. Finally, plutonium was back-extracted in 3M HCl containing Ti(III). The method separates²³⁸Pu from²⁴¹Am for α-spectroscopy. For ICP-MS analysis, the interferences from²³⁸U are eliminated: tailing from²³⁸U, for analysis of²³⁷Np, and the interference of²³⁸UH⁺ for analysis of²³⁹Pu. The method has been used for the analysis of actinides in samples from a spent nuclear fuel leaching and radionuclide transport experiment.     

Spectrochemical analysis of curium and americium samples

Spectrochemical procedures have been developed to determine impurities in americium and curium samples. The simultaneous separation of many impurity elements from the base material (americium and curium) is carried out with extraction and extraction-chromatographic methods using di(2-ethyl hexyl phosphoric acid (D2EHPA).

It is shown that part of the elements (alkalis, alkaline earths, silicon, tungsten, tantalum and other elements) are separated with extraction or sorption of americium and curium; the other part (rare earths, titanium, zirconium, niobium, molybdenum) with the Talspeak process.

Two fractions in the extraction chromatography and three fractions in the extraction separation of americium and curium, containing impurities, are analyzed separately by a.c. or d.c. arc spectrography. To increase the sensitivity of the spectrographic analysis and accelerate the burn-up of impurities from the crater of the carbon electrode bismuth fluoride and sodium chloride were used as chemically active substances. The extraction of impurities from weighed quantities of americium and curium samples of 5–10 mg permits the lower limit of determined impurity concentrations to be extended to 1 × 10⁻⁴–5 × 10⁻³% m/m.     

Kinetics of americium and europium extraction by tert-butylthiacalix[4]arene from alkaline media

Kinetics of ¹⁵²Eu and ²⁴¹Am extraction by nitrogen-bearing alkyl amino phenol oligomer YaRB and tert-butylthiacalix[4]arene TCA from carbonate-alkaline media was studied. Both extractants efficiently extract americium and europium in pH interval 12–14. The maximum of americium extraction is located at the lower values of pH, compared with europium. YaRB extracts americium and europium faster than TCA, and at the same time, americium is extracted faster than europium by both extractants. In general, thiacalixarene TCA is regarded as more efficient extractant than alkyl amino phenol oligomer YaRB.

     

Sequential determination of Am, Cm, Pu, Np and U by extraction chromatography

Environmental contamination by artificial radionuclides and the evaluation of their sources require precise isotopic analysis and accurate determination of actinide elements above all plutonium and americium. These can be achieved by alpha spectrometry or by inductively coupled plasma mass spectrometry (ICP-MS) after chemical separation. In the present work, a simple, rapid method has been developed for the sequential separation of actinide elements from aqueous solutions and their determination by alpha spectrometry. Extraction chromatography was applied to the separation of ²⁴¹Am, ²⁴⁴Cm, ^{239 + 240,238}Pu, ²³⁷Np and ^238,235,234U using microporous polyethylene supporting tri-n-octylamine as the stationary phase and hydrochloric acid with and without reducing agents as the mobile phase. Actinide in 9 M HCl solution is introduced into the anion exchange column; Pu (IV), Np (IV) and U(VI) are retained on the column while Am (III) and Cm passed through. Pu is eluted first, reductively, after which, Np and then U are eluted. The method can be applied to all aqueous solutions which do not contain strong complexing or precipitation agents for the elements considered.     

Rapid column extraction method for actinides and strontium in fish and other animal tissue samples

The analysis of actinides and radiostrontium in animal tissue samples is very important for environmental monitoring. There is a need to measure actinide isotopes and strontium with very low detection limits in animal tissue samples, including fish, deer, hogs, beef and shellfish. A new, rapid separation method has been developed that allows the measurement of plutonium, neptunium, uranium, americium, curium and strontium isotopes in large animal tissue samples (100–200 g) with high chemical recoveries and effective removal of matrix interferences. This method uses stacked TEVA Resin®, TRU Resin® and DGA Resin® cartridges from Eichrom Technologies (Darien, IL, USA) that allows the rapid separation of plutonium (Pu), neptunium (Np), uranium (U), americium (Am), and curium (Cm) using a single multi-stage column combined with alphaspectrometry. Strontium is collected on Sr Resin® from Eichrom Technologies (Darien, IL, USA). After acid digestion and furnace heating of the animal tissue samples, the actinides and ^89/90Sr are separated using column extraction chromatography. This method has been shown to be effective over a wide range of animal tissue matrices. Vacuum box cartridge technology with rapid flow rates is used to minimize sample preparation time.     

Radiation Chemistry of Neptunium Ions in Aqueous Carbonate and Bicarbonate Solutions*

The radiolytic reactions of neptunium ions in aqueous carbonate, alkaline carbonate, and bicarbonate solutions were examined. It was found that Np(VI) is not oxidized to Np(VII) in carbonate and bicarbonate solutions saturated with nitrous oxide, whereas this oxidation process takes place in alkaline carbonate solutions, in which Np(VI) occurs as a hydroxo complex. It was also found that Np(V) is a radiation-stable neptunium species in carbonate solutions, whereas Np(VI) is stable in bicarbonate solutions.     

Extraction of uranium and transuranium elements with <Emphasis Type="Italic">tert</Emphasis>-butylthiacalix[4]arene from carbonate-alkaline solutions

Extraction efficiency of uranium and transuranium elements (Np, Pu, Am and Cm) with tert-butylthiacalix[4]arene TCA from carbonate-alkaline solutions is studied and compared with that of europium (III). Plutonium (III, IV) extraction efficiency with TCA is found to be lower comparing with that of trivalent americium and europium. Extraction efficiency of studied radionuclides decreases as following: Am ? Eu ? Pu (III), U(VI), Np (V) > Pu (IV) at pH 12. Carbonate concentration increase in aqueous phase suppresses significantly extraction of all studied radionuclides, except americium. This condition can be used for americium individual recovery from complex radioactive carbonate-alkaline solutions.     

Extraction of Americium with Substituted Calix[4]Arenes from Alkaline Solutions

Extraction of ²⁴¹Am and ¹⁵²Eu from carbonate-alkaline medias with solutions of functionalized calix[4]arenes and their nonmacrocyclic analogs in m-nitrobenzotrifluoride was studied. The dependencies of the Am distribution ratios on pH of the aqueous phase in the range 10 -13.5 and on the position and nature of functional groups in the calixarene core were examined. The composition of extractable solvates of americium with functionalized calix[4]arenes was determined. Thiacalixarenes extract americium more efficiently from alkaline solutions, as compared to calixarenes, demonstrating higher Am/Eu separation factor. Functionalization of the calixarenes improves the Am extraction efficiency and Am/Eu separation factor as well. In the case of thiacalixarenes, functionalization considerably decreases the extraction efficiency and selectivity.     

2,2′-Dipyridyl-6,6′-dicarboxylic acid diamides: Synthesis,complexation and extraction properties

New ligands for complexing of the post-transition metals – diamides of 2,2′-bipyridyl-6,6′-dicarboxylic acid were developed, synthesised and characterised. They were proposed to be effective extractants towards americium. The structures of the amides were studied in solid as well as in solution. The extraction of Am and lanthanides depending on diamide structure, chlorinated cobalt dicarbollide (CCD) – diamide ratio, type of diluent was studied. The optimal conditions for Am/REE separation were determined. The properties of new potentiometric sensors on the base of 2,2′-dipyridyl-6,6′-dicarboxylic acid diamides were studied. The correlation structure vs. properties of ionophores (i.e. extractants), their sensitivity and selectivity in sensor analysis and extraction are discussed.     

Use of alcoholic solutions for the isolation and purification of americium and curium with anion-exchangers

The sorption of transplutonium (TPE), rare-earth (RE) and other elements by anion-exchangers (Dowex 1 type) from aqueous alcoholic solutions of nitric acid and ammonium thiocyanate was investigated. This investigation allowed the development of simple and effective methods of americium—curium separation from frradiated plutonium. Plutonium, TPE (in a +3 oxidation state) and RE are firmly sorbed by the anion-exchanger from 1 M HNO₃ in 90% alcohol, Fe, Al and fission products Cs, Sr, Nb, Zr, and Ru pass through the column under these conditions. The RE separation from TPE is achieved by washing the column with 0.5M NH₄SCN in 80% alcohol. The column is then washed with 0.5 M HNO₃ in 85% alcohol, and americium—curium separation proceeds. Use of this method for recovery of an irradiated plutonium target containing 100 mg Pu, Am and Cm is described.     

A procedure for concentrating americium and curium combined with their separation from plutonium and fission products using a chelating resin

The effect of agitation time and resin quantity on the sorption of americium, curium and europium from mineral acid solutions, using a chelating resin based on aminopolystyrene and Arsenazo I was determined, and the behaviour of plutonium and fission products was investigated under optimum conditions with respect to the sorption of americium and curium. A procedure is proposed for concentrating americium and curium from dilute solutions, combined with their separation from iron, plutonium and fission products. The procedure consists of sorption on the chelating resin from 0.1–1N mineral acid solutions, washing of the resin with 0.5M oxalic acid and 11N sulfuric acid, and elution of americium and curium with 2M triammonium citrate.     

Development and Validation of the EXAm Separation Process for Single Am Recycling

The EXAm process aims at recovering americium alone. The principle of the EXAm process is based on the extraction of americium together with some light lanthanides having close values of distribution coefficients in high nitric acidity, while curium and other lanthanides remain in the aqueous phase. The TEDGA amide molecule is added in the aqueous phase to increase Am/Cm and Am/heavy Ln selectivity, because of the preferential complexation of curium and heavy lanthanides by this diglycolamide. The Am is subsequently selectively stripped from the light lanthanides.The full approach aiming at validating this complex process is illustrated in this paper.     

Nitric Acid Extraction into TODGA

Advanced solvent extraction processes, namely DIAMEX, SANEX or GANEX, for the separation of the minor actinides (americium and curium) are under development within Europe. The tridentate diglycolamide ligand, TODGA, shows many interesting properties and is under investigation in conjunction with a variety of other extractants for the DIAMEX and SANEX processes as well as the GANEX process. In order to successfully demonstrate these processes, understanding the acid extraction into the organic phases is critical to process flowsheet design and modelling. Here nitric acid extractions into TODGA have been measured and models produced using an equilibrium based approach accounting for nitric acid activities in the aqueous phase.     

Extraction of Th,Pa, U,and Np by PMBP from aqueous solutions at short phase contact times

The extraction properties of Th/IV/, Pa/V/, U/VI/ and Np from nitric acid solutions into 1-phenyl-3-methyl-4-benzoylpyrazolone-5 /PMBP/ in toluene were studied, using singlestep batch and continuous H-centrifuge methods. The effects of acid and extractant concentration, temperature and phase contact time on the extraction were investigated.     

Rapid method for determination of plutonium, americium and curium in large soil samples

The analysis of actinides in environmental soil and sediment samples is very important for environmental monitoring. There is a need to measure actinide isotopes with very low detection limits. A new, rapid actinide separation method has been developed and implemented that allows the measurement of plutonium, americium and curium isotopes in large soil samples (100–200 g) with high chemical recoveries and effective removal of matrix interferences. This method uses stacked TEVA Resin®, TRU Resin® and DGA-Resin® cartridges from Eichrom Technologies (Darien, IL, USA) that allows the rapid separation of plutonium (Pu), americium (Am), and curium (Cm) using a single multi-stage column combined with alpha-spectrometry. The method combines an acid leach step and innovative matrix removal using cerium fluoride precipitation to remove the difficult soil matrix. This method is unique in that it provides high tracer recoveries and effective removal of interferences with small extraction chromatography columns instead of large ion-exchange resin columns that generate large amounts of acid waste. By using vacuum box cartridge technology with rapid flow rates, sample preparation time is minimized.     

Separation and recovery of Cm from Cm–Pu mixed oxide samples containing Am impurity

Curium was separated and recovered as an oxalate from a Cm–Pu mixed oxide which had been a ²⁴⁴Cm oxide sample prepared more than 40 years ago and the ratio of ²⁴⁴Cm to ²⁴⁰Pu was estimated to 0.2:0.8. Radiochemical analyses of the solution prepared by dissolving the Cm–Pu mixed oxide in nitric acid revealed that the oxide contained about 1 at% of ²⁴³Am impurity. To obtain high purity curium solution, plutonium and americium were removed from the solution by an anion exchange method and by chromatographic separation using tertiary pyridine resin embedded in silica beads with nitric acid/methanol mixed solution, respectively. Curium oxalate, a precursor compound of curium oxide, was prepared from the purified curium solution. 11.9 mg of Cm oxalate having some amounts of impurities, which are ²⁴³Am (5.4 at%) and ²⁴⁰Pu (0.3 at%) was obtained without Am removal procedure. Meanwhile, 12.0 mg of Cm oxalate (99.8 at% over actinides) was obtained with the procedure including Am removals. Both of the obtained Cm oxalate sample were supplied for the syntheses and measurements of the thermochemical properties of curium compounds.     

Extraction chromatographic behaviour of Am/IV/ in the system: H2SO4-K10P2W17O61-Primene JMT

The extraction chromatographic separation of Am/IV/ and Cm/III/ in the H₂SO₄-K₁₀P₂W₁₇O₆₁-Primene JMT system was studied. The elaborated method permitted to purify effectively americium. The yield of americium at an initial concentration of 10^–3M is about 93–95%, the curium content does not exceed 0.1% of the initial concentration.     

Extraction of actinides with heterocyclic dicarboxamides

Extraction of actinides from aqueous nitric acid by three different heterocyclic dicarboxamides (2,6-pyridinedicarboxamide, 2,2′-bipyridine-6,6′-dicarboxamide and 1,10-phenanthroline-2,9-dicarboxamides) was studied. It was shown that all studied ligands extract actinides at different oxidation states (U(VI), Np(V), Pu(IV), Am(III), Cm(III)) from acidic solutions. All studied diamides extract Am(III) better than Cm(III). Et(pHexPh)ClPhen contains electron-withdrawing chlorine atoms at the positions 4 and 7 of the phenanthroline moiety (SF_Am/Cm = 4–6) and possesses the highest separation factor Am(III)/Cm(III). The studied ligands possess high extraction ability to all actinides present in HLW and therefore they could be used for simultaneous extraction of actinides in the GANEX-type process.