Americium distribution ratios

Am(iii) is sorbed much more strongly onto anion-exchange resins from concentrated lithium chloride solutions than are the lanthanides [61], Americium distribution ratios increase with increased lithium chloride concentration (Fig. 8.1), whereas increased temperature enhances the separation of americium from rare earths. A lithium-chloride-based anion-exchange process for separating multigram amounts of americium and curium from lanthanide fission products and to isolate an Am-Cm fraction free of heavier actinides is routinely operated at the Oak Ridge facility [14]. 

These compounds, tested in NPHE at Cadarache, were used as reference compounds for the extraction of actinides by functionalized calixarenes (see below). The distribution ratios for neptunium mainly at the oxidation state (V), plutonium at the oxidation state (IV), and americium (III) are shown in Table 4.21 for OOCMPO. They were also used as references for the americium over europium selectivity (Table 4.22). 

Extraction of neptunium, plutonium, and americium from simulated radioactive liquid waste was carried out in particular with tert-butyl and dealkylated tetramers, hexamers, and octamers of calixarene [ethoxy(diphenylphosphine oxide)]. Among these six calixarenes, the highest distribution ratios were obtained with the dealkylated calix[8]arene. Using a different sample of the dealkylated hexamer, the Strasbourg group concluded that this compound is the most efficient. This discrepancy can be explained by the presence of impurities, detected by NMR, which were probably responsible for the poor performances of the dealkylated hexamer tested at Cadarache. 

Distribution Ratios for the Extraction of Several Lanthanides and Americium from Aqueous Solutions (3-4 M HN03) into NPHE by CPw3 and CPw17 Calixarenes... 

To increase the distribution ratios, a solution of lithium nitrate 1M was used. This salt, which has a common anion with europium and americium to be extracted but a cation which is usually negligibly extracted by other calixarenes, should increase the distribution ratios according to the relation Du = A (JU "[N03- ". It seems that these calixarenes, as several nitrogen ligands do, present a certain affinity for this lithium cation. The lipophilic dicarbollide anion (BrCosan), which is known to facilitate cation extraction, was implemented and led to a strong increase of the extraction of cations from 10 3 M HN03 solutions. Under these conditions, only thiopicolinamide was not able to significantly extract trivalent actinides.187... 

Under the same conditions, in contrast to what is observed for calix[4]arene-bearing CMPO moieties, with CPil2, distribution ratios of lanthanides increase from the lightest lanthanide, lanthanum, to europium. Americium can be easily separated from the lightest lanthanides (separation factor DAm/La > 20, DAm/Ce =15, /lAlll,Nd = 10, UAi /si = 7.5, DAm/Eu = 6), which are the most abundant lanthanides in fission-product solution. Cavitands bearing picolinamide (Cv5) or thiopicolin-amide (Cv6) residues seems much less selective than their calixarene counterparts, giving SAm/Eu < 2.18... 

The numerical values of the extraction factor (a) are set by selecting the desired separation, the mode of extraction, and the number of extraction stages. For example, the value of a required for 90% americium removal by two stage countercurrent extraction is a = 2.54 (see Table I). As shown previously, the value of the americium distribution coefficient is a function of the salt composition i.e., the MgCl2 content of the salt and the composition of the diluent salt system. The value of the salt-to-metal ratio is set by the weight of salt and metal fed to the extraction. 

The stoichiometry shown in Equation (1) is similar to that followed during extraction of trivalent americium by TBP and other monofunctional neutral organophosphorus extractants. Distribution ratio data plotted in Figure 2 show that DBBP extracts Am(III) more strongly than TBP from HNO3 media. The equilibrium constant for Reaction (1) (at zero ionic strength) is 7.4 compared to a value of only 0.4 for the similar reaction with TBP.(5)... 

The DBBP extraction scheme provides excellent decontamination of plutonium and americium from all the other metals in the neutralized CAW solution. Kingsley(7) reports that distribution ratios for Fe3+, Al3+, Ca2+, and Mg between 30 vol% DBBP-CCI4 and neutralized CAW are, respectively, 0.11, <0.003, 0.025, and <0.0005. Primarily because of entrainment but partly because of extraction, small amounts of aluminum, iron, and sodium accompany 21tlAm into the dilute HNO3 strip solution. Richardson(9 ) in nonradioactive tests of the countercurrent DBBP 2 1Am recovery process observed decontamination factors in the range 80-180 for iron and in the range 2 x 103 - 1.5 x 105 for aluminum. 

As shown in Figure 6, diisodecylphosphoric acid (DIDPA) gives higher distribution ratios compared with those of DEHPA, being more suitable for using as extractant at higher acid concentrations. One M DIDPA diluted with normal paraffins can be used to recover americium and lanthanides from the HLW of PUREX process. 

The separation of americium from the lanthanides is feasible by either solvent extraction or ion-exchange process. For the solvent extraction, 0.25 M DIDPA diluted with diisopropylbenzene (DIPB) provides appropriate distribution ratios of americium between the organic phase and the aqueous phase composed of 0.05 M NasDTPA and 1 M lactic acid. 

Investigation of the extractant dependence of D values for Am and Eu with AC, XO, and OPIDA indicated that distribution ratios increase sharply in the complexant concentration range 0.001—0.01 M and level off if the concentration of extractant is higher than 0.01 M [191- The concentrations of KaCOi used to generate the biphasic systems have little effect on the trivalent ions of americium and europium, and 40% K2COi was suggested to achieve an effective extraction. 

Distribution ratio experiments with Inorganic cation exchangers were performed by equilibrating 1.0 g of the exchanger with 10 ml of either 0.25M oxalic acid or water In a 14 ml vial on a rotating wheel for up to 72 hours. The aqueous pH was adjusted with 50Z NaOH or 7M HNO. The Initial and final americium and plutonium concentrations were determined radlometrlcally. The volume distribution ratio, D, Is defined as [Am]/g solid r [Am]/ml aqueous phase,and Is calculated from 10 x ([Am]aq - [Amjaq /[Am]aq... 

Evaluation of Inorganic Ion Exchangers. The materials In Table II were evaluated for their ability to sorb americium from both 0.25M oxalic acid and water, each adjusted to an Initial pH of 1.0. High distribution ratios under these conditions are essential to accomplish the Immobilization and disposal of americium. 

On the other hand, Dyrssen and Liem (1960) report (table 7) greater variation in both distribution ratios [for americium and europium extraction by dibutyl phosphoric acid (HDBP)] and in separation factors as a function of diluent. The separation factors and distribution coefficients are correlated (more or less consistently) inversely with the distribution ratio of the extractant between the phases. In this system, the largest separation factors are observed in n-hexane, chloroform, and carbon tetrachloride. Diluents capable of direct coordination (i.e., those possessing potential oxygen-donor atoms) are correlated with reduced distribution ratios and separation factors. The observations of greater separation factors in non-complexing diluents suggest that more effective separation is observed when the inner-coordination sphere of the hydrophobic complex is not disturbed. 

Distribution ratios and separation factors for americium/ europium extraction by 4-benzoyl-2,4-dihydro-5-methyl-2-phenyl-3H-pyrazol-3-thione/toluene (0.0297 M)/0.1 M NaClO as a function of 4,7-diphenyl-l,10-phenanthroline (synergist) from Ensor et al. (1988). 

There are two apparent artifacts in this correlation. First, one would not expect based on these arguments that the acidic phosphoric acid esters HDOP, HDBP, and HDEH P (bars U, V, and W) would demonstrate as great a selectivity for europium as is observed. Similarly, there is no apparent reason for the enhanced selectivity demonstrated by 100% TBP for americium for extraction from 13 M HNOj (bar G). In the case of the phosphoric-acid extractants, the apparent anomaly is a manifestation of the steep slope of the linear relationship between distribution ratios and atomic number (cation radii) as shown in figs. 4 and 5, and a mismatch of the ionic radii of americium and europium. It is generally believed that the cation radius of americium is more nearly comparable to that of promethium or neodymium than europium (see table 1). The logSi J calculated from the the same data is —0.35. 

Some extraction systems are able to extract metals by both the solvation and ion exchange mechanisms an example of such a system is the americium (and lanthanide) extraction from nitric acid by a combination of 6,6 -Z>A-(5,6-dipentyl-l,2,4-triazin-3-yl)-2,2 -bipyridine and 2-bromohexanoic acid in tert-h xt benzene. At both high- and low-nitric acid concentrations, the metal distribution ratio is higher than it is for an intermidate nitric acid concentration. 

Although americium (Am) exists in seawater exclusively in the trivalent oxidation state, its profiles in Fig. 12.4 contrast sharply with those of the trivalent lanthanides. Assessments of Nd isotopic ratios in seawater (e.g. Bertram and Elderfield, 1993) indicate that more than 1000 years are required for attainment of steady-state distributions of lanthanides and chemically similar elements in seawater. On such a basis it is expected that, in spite of substantial chemical similarities to the lanthanides, 241Am, a relatively short-lived isotope (half-life 470 years) with variable and recent anthropogenic inputs, will not exhibit profiles similar to those of the lanthanides. 

Continental margins are important areas to study concerning the distribution and deposition of pollutants, because coastal sediments accumulate most of those contaminants. The main source of radioactive pollutants is fallout from stratospheric nuclear testing performed from the late 1950s until the early 1960s. Northeastern Spain has also been affected by the 1986 Chernobyl accident, as the Cs/ Cs ratio calculated for seawater samples has proven. However, no sign of Cs has been detected in sediments collected in the area. In the Mediterranean Sea, the mean residence times in shallow waters are estimated as 11 years for plutonium and 5.5 years for americium. The inventory ranges from 9 Bq/m to... 

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