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.     

Recovery of americium from nitric acid solutions containing calcium by different co-precipitation methods

Recovery of americium from nitric acid solutions was studied by co-precipitation as hydroxide with various ions like calcium, ferric, nickel using sodium hydroxide and ammonium hydroxide. Studies were also carried out to recover americium using lanthanum fluoride and bismuth phosphate co-precipitation. All the methods are able to co-precipitate Am quantitatively. However, co-precipitation of Am with optimum concentration iron using ammonia is found to be better from nitric acid solutions containing large concentrations of calcium ions. Approximately 2 g of Am was recovered from 150 litres of solution batch wise using iron.     

Separation of AM(III) by extraction from nitrate solutions using some quaternary benzylammonium salts

Benzyldimethyldodecylammonium nitrate and benzyltrioctylammonium nitrate were used for the extraction of Am(III) from aqueous nitrate solutions. The dependence of the extraction performance for Am(III) on the concentration of nitric acid, the kind and concentration of salting-out agents in the aqueous phase, and the kind of solvent was investigated. Americium is extracted by the above quarternary salts as a R₄NAm(NO₃)₄ associate. The extraction of Am(III) is compared with the extraction of lanthanides. The high differences in the distribution coefficients for lanthanides and americium can be utilized for the separation of lanthanides and americium.     

Uptake of actinides by sulphonated phosphinic acid resin from acid medium

The removal of uranium and americium from nitric acid solutions by sulphonated phosphinic acid resin has been investigated. The capacity of the sulphonated resin exceeds the capacities of phosphinic acid resin and commercial cation exchange resin. Other advantages of the sulphonated resin for uranium and americium removal include reduced sensitivity to acidity and inert salt concentration.     

Tributylphosphate extraction behavior of bismuthate-oxidized americium

Higher oxidation states of americium have long been known; however, options for their preparation in acidic solution are limited. The conventional choice, silver-catalyzed peroxydisulfate, is not useful at nitric acid concentrations above about 0.3 M. We investigated the use of sodium bismuthate as an oxidant for Am (3+) in acidic solution. Room-temperature oxidation produced AmO 2 (2+) quantitatively, whereas oxidation at 80 degrees C produced AmO 2 (+) quantitatively. The efficacy of the method for the production of oxidized americium was verified by fluoride precipitation and by spectroscopic absorbance measurements. We performed absorbance measurements using a conventional 1 cm cell for high americium concentrations and a 100 cm liquid waveguide capillary cell for low americium concentrations. Extinction coefficients for the absorbance of Am (3+) at 503 nm, AmO 2 (+) at 514 nm, and AmO 2 (2+) at 666 nm in 0.1 M nitric acid are reported. We also performed solvent extraction experiments with the hexavalent americium using the common actinide extraction ligand tributyl phosphate (TBP) for comparison to the other hexavalent actinides. Contact with 30% tributyl phosphate in dodecane reduced americium; it was nevertheless extracted using short contact times. The TBP extraction of AmO 2 (2+) over a range of nitric acid concentrations is shown for the first time and was found to be analogous to that of uranyl, neptunyl, and plutonyl ions.     

Americium removal from nitric acid waste streams

Separations research at the Rocky Flats Plant /RFP/ has found ways to significantly improve americium removal from nitric acid/7M/ waste streams generated by plutonium purification operations. Partial neutralization of the acid waste followed by solid supported liquid membranes /SLM/ are useful in transferring and concentrating americium from nitrate solutions. Specifically, DHDECMP /dihexyl-N, N-diethylcarbamoylmethylphosphonate/ supported on Accurel polypropylene hollow fibers assembled in modular form transfers >95% of the americium from high nitrate /6.9M/, low acid /0.1M/ feeds into 0.25M oxalic acid stripping solution. Maximum permeabilities were observed to be 0.001 cm sec^–1, consistent with typical values for other systems. The feed:strip volume ratio shows an inverse relationship to the fraction of metal ion transferred. Cation exchangers may be used to concentrate americium from the strip solution. Furthermore, OØD/iB/CMPO /or CMPO/ /octylphenyl-N,N-diisobutylcarbamoylmethylphosphine oxide/ has been tested in an extraction chromatography mode. Preliminary results show CMPO to be effective in removing americium if the feed is neutralized to 1.OM acidity and iron/III/ is complexed with 0.20M oxalic acid.     

Interference of elements upon extraction by macrocyclic extractants

The extraction of cations of a series of alkali and alkali-earth metals, along with Pb(II), Rh(III), and Pd(II) with crown, thiacrown and azacrown ethers from picric and nitric acid solutions was studied. Upon the extraction of metal cations with macrocyclic extractants, the interference of those cations on the extraction of one another was observed in polar solvents. The causes of this phenomenon are revealed, and a mechanism for the suppression of extraction of the microcomponent with the macrocomponent is proposed. Upon the simultaneous extraction of americium (III) and europium (III) with calixarenes the co-extraction was noted for the first time, resulting in the good extraction of Am(III) from nitric acid solutions. We hypothesize on the formation of a mixed nitrate complex of americium and europium that can be effectively extracted into an organic phase with calixarenes.     

Bis(2-ethylhexyl)sulfoxide as an extractant for americium(III) from aqueous nitrate media

Solven extraction separation of americium(III) from dilute aqueous nitrate media into n-dodecane by bis(2-ethylhexyl)sulfoxide (BESO) has been investigated over a wide range of experimentgal conditioins. Very poor extractablity of Am(III), necessitated the use of calcium nitrate as the salting-out agent. Effects of certain variables such as acidity, extractant concentration, salting-out agent concentration, organic diluents on the metal extraction by BESO have been examined in detail. By increasing the concentration of BESO in organic phase or calcium nitrate in aqueous phase, nearly quantitative extraction of americium even from moderate acidity is accomplished. Slope analyses applied to Am(III) distribution experiments from acidic nitrate solutions indicate predominant formation of the risolvated organic phase complex, Am(NO₃)₃)·3BESO for which equilibrium constant is found to be, log K_x=1.99. Extraction behavior of Am(III) has also been evlauated in the presence of several water-miscible polar organic solvents to stuy their possible synergistic effects on its extraction. Extractability of americium increased 5 to 10-fold withi increasing conentration of some of these additives, with maximum enhancement being observed in the presence of acetone or acetonitrile. Recovery of BESO from loaded americium is easily obtained using dilute nitric acid as the strippant.     

Preparation of thorium oxalate–silica sorbent and its application for the sorption of americium from aqueous solutions

A new sorbent, thorium oxalate incorporated in silica gel matrix was prepared. This material was characterized by X-ray, Thermo-gravimetric Analysis, surface area and porosity analysis. The material was obtained in the form of granular particles in the mesh size range of 80–150 American Standard of Testing Materials, yielding good liquid flow, when packed in ion exchange column. This sorbent was investigated for the sorption of americium from various aqueous media such as nitric acid, oxalic acid and sulphuric acid by distribution coefficient studies. Column experiments were carried out to study the practical application of this sorbent for removal of americium from oxalic acid-nitric acid solutions. Elution studies were also carried out for the recovery of americium.     

Studies on the partitioning of actinides from high level liquid waste solution employing supported liquid membrane with Cyanex-923 as carrier

The present paper deals with the studies on the partitioning of actinides from high level liquid waste solution of PUREX origin employing supported liquid membrane technique. The process uses solution of Cyanex-923 in n-dodecane as a carrier with poly tetra fluoro ethylene support and a mixture of citric acid, formic acid and hydrazine hydrate as a receiving phase. Transport studies are carried out for ²⁴¹Am under different experimental conditions to optimize the transport parameters such as feed acidity, carrier concentration, effect of uranium, trivalent metal ion and salt concentration in the feed. Studies indicated good transport of actinides across the membrane from nitric acid medium. Under the optimized conditions the transport of ²⁴¹Am is studied from a uranium depleted synthetic PHWR-HLLW and finally the technique has been used for the partitioning of alpha emitters from an actual research reactor-HLLW. High concentration of uranium in the feed is found to retard the transport of americium, suggesting the need for prior removal of uranium from the waste.     

Extraction of Am(III) in the presence of lanthanides and yttrium by benzyldimethyldodecylammonium nitrate from acidic nitrate solutions

We have investigated the effect of coextraction of lanthanides and yttrium on the distribution coefficients D_Am in the extraction of americium by benzyldimethyldodecylammonium nitrate (BDMLNNO₃) from nitrate solutions. In the coextraction of lanthanides, the extraction of Am(NO₃)₃ is suppressed, which is markedly manifested in the extraction of light lanthanides (La, Ce, Pr); of the series of lanthanides their extraction is the highest. The effect of nitric acid and the possibility of separation of lanthanides and americium by the application of three-stage multiple extraction is discussed.     

Efficient separation between trivalent americium and lanthanides enabled by a phenanthroline-based polymeric organic framework

Separation of the minor actinides (Am and Cm) from lanthanides in high-level liquid wastes (HLLW) is one of the most challenging chemical separation tasks known owing to their chemical similarities and is highly significant in nuclear fuel reprocessing plants because it could practically lead to sustainable nuclear energy by closing the nuclear fuel cycle. The solid phase extraction is proposed to be a possible strategy but all reported sorbent materials severely suffer from limited stability and/or efficiency caused by the harsh conditions of high acidity coupled with intense irradiation. Herein, a phenanthroline-based polymeric organic framework (PhenTAPB-POF) was designed and tested for the separation of trivalent americium from lanthanides for the first time. Due to its fully conjugated structure, PhenTAPB-POF exhibits previously unachieved stability under the combined extreme conditions of strong acids and high irradiation field. The americium partitioning experiment indicates that PhenTAPB-POF possesses an ultrahigh adsorption selectivity towards Am(III) over lanthanides (e.g., SF_{Am(III)/Eu(III)} = 3326) in highly acidic simulated HLLW and relatively fast adsorption kinetics in both static and dynamic experiments. Am(III) can be almost quantitatively eluted from the PhenTAPB-POF packed-column using a concentrated nitric acid elution. The high stability and superior separation performance endow PhenTAPB-POF with the promising alternative for separating minor actinides over lanthanides from highly acidic HLLW streams.     

Liquid-liquid extraction separation and sequential determination of plutonium and americium in environmental samples by alpha-spectrometry

A procedure is described by which plutonium and americium can be determined in environmental samples. The sample is leached with nitric acid and hydrogen peroxide, and the two elements are co-precipitated with ferric hydroxide and calcium oxalate. The calcium oxalate is incinerated at 450 degrees and the ash is dissolved in nitric acid. Plutonium is extracted with tri-n-octylamine solution in xylene from 4M nitric acid and stripped with ammonium iodide/hydrochloric acid. Americium is extracted with thenoyltrifluoroacetone solution in xylene at pH 4 together with rare-earth elements and stripped with 1M nitric acid. Americium and the rare-earth elements thus separated are sorbed on Dowex 1 x 4 resin from 1M nitric acid in 93% methanol, the rare-earth elements are eluted with 0.1M hydrochloric acid/0.5M ammonium thiocyanate/80% methanol and the americium is finally eluted with 1.5M hydrochloric acid in 86% methanol. Plutonium and americium in each fraction are electro-deposited and determined by alpha-spectrometry. Overall average recoveries are 81% for plutonium and 59% for americium.     

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.     

The use of the N,N-diethyldodecanamide for actinide extraction in conjunction with liquid scintillation counting

The extraction properties of N,N-diethyldodecanamide in hydrochloric and nitric acid solutions for uranium, plutonium, thorium and americium have been investigated. As a primary step, the liquid scintillation performances of organic solutions of the amide have been studied revealing a moderate quenching and a lower energy resolution than HDEHP. Nitric acid, uranyl nitrate and thorium nitrate extraction systems have been modeled, taking into account the stoichiometric mean activity coefficients in aqueous solutions. A general protocol to analyse actinides (U, Th, Pu and Am) in soils in conjunction with liquid scintillation counting is proposed using this amide together with other extractants (TOPO, HDEHP).     

Extraction of americium and europium from perchloric acid solutions with N,N′-dialkyl-and N,N,N′,N′-tetraalkylpyridine-2,6-dicarboxamides

Extraction of americium and europium from perchloric acid solutions with pyridine-2,6-dicarboxamides was studied. The solvate numbers and Am/Eu separation factors were determined.     

Effect of gamma-radiation on the extraction of europium and americium by benzyldimethyldodecylammonium nitrate

Previously it was found that in the extraction separation on lanthanides and americium from acidic nitrate solutions of nuclear fission products, benzyldimethyldodecylammonium nitrate gives high values of separation coefficients. The change in the extraction capacity of this agent and its solutions in benzene in the extraction of Eu(III) and Am(III) was investigated as a function of the adsorbed dose of ionizing radiation. The slight reduction in the extraction of both metals is caused mainly by the radiolysis products of nitric acid in the organic phase that enter into secondary reactions with both the solvent and the extractant. Comparison of the radiation stability of benzyldimethyldodecylammonium nitrate systems with tertiary amines show that the changes in distribution coefficients in the range of investigated absorbed doses are significantly lower in the former case. The investigated system may be characterized as radiation stable up to about 100 kGy even in the presence of nitric acid.     

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.     

Synergism of crown ethers in the thenoyl trifluoroacetone extraction of americium(III) in benzene medium

The synergism of the crown ethers (CE) dicyclohexano-18-crown-6 (DC18C6), dibenzo-18-crown-6 (DB18C6) and 18-crown-6 (18C6) has been investigated in the thenoyl trifluoroacetone (HTTA) extraction of americium(III) in benzene medium from an aqueous phase of ionic strength 0.5 and pH 3.50 at room temperature (23°C). The extracted synergistic species have the general formula Am(TTA)₃ · CE except for DC18C6 in which case the species Am(TTA)₃·2CE was also observed at high CE concentrations. The order of synergism was found to be DC18C6>DB18C6>18C6, which is the order of the basicity of CE as indicated by their ability to extract hydrogen ions from nitric acid solutions.     

Preparation, characterization and application of thorium oxalate for sorption of plutonium and americium from aqueous solution

Synthetic inorganic exchangers exhibit good thermal and radiation stability. Thorium oxalate precipitate shows potential for co-precipitation of plutonium and americium from oxalate supernatant generated during plutonium oxalate precipitation. In the present study, efforts were made to prepare thorium oxalate precipitate to be used for column operation. Distribution ratios were determined to optimize conditions for sorption of plutonium and americium on thorium oxalate from nitric acid + oxalic acid solutions with composition similar to that of oxalate supernatant. Column experiments were also performed to evaluate the sorption capacity of thorium oxalate for plutonium and americium from the same medium. The result showed that, thorium oxalate prepared in 1.75M HNO₃ at 70 °C is suitable for column operations. These studies showed that plutonium and americium could be simultaneously removed from aqueous solutions with composition similar to plutonium oxalate waste using glass column packed with thorium oxalate and these nuclides could be recovered by eluting with 3M HNO₃.