Americium trivalency

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

One of the main differences, therefore, between actinides and rare earths is the steeper change of the f-d energy difference along the actinide series (fig. 3). This first of all makes americium trivalent, but also implies that a series of elements at the end of the actinide series will be divalent. Indeed, the divalent state of nobelium is so relatively stable that, of the possible halide compounds, the only trivalent compound it will be able to form is the trifluoride (Johansson 1977a). 

Americium metal has been obtained by heating americium oxide, Am203, with lanthanum at 1,200 °C americium, which is more volatile than other actinides, volatilizes and can readily be separated from other actinides. Am02 can be obtained by igniting most trivalent americium compounds (Budavari 1996 Cotton and Wilkinson 1980 UIC 1997). 

Americium will occur in soil in the trivalent state. The transformations that may occur would involve complexation with inorganic and organic ligands (see Section 6.3.1) and precipitation reactions with anions and other substances present in the soil solution. The 241 Am occurring as an ingrowth progeny of 241Pu and trapped in a plutonium matrix will exhibit solubility and biokinetic characteristics of the plutonium, rather than americium. 

Fuger, J. and Cunningham, B. B. (1964). Microcalorimetric determination of the enthalpy of formation of the complex ions of trivalent plutonium, americium and lanthanum with EDTA, J. Inorg. Nucl. Chem. 27,1079. 

As trivalent americium has a smaller ionic potential than the ions of plutonium it hydrolyses to a much lesser extent than the various plutonium ions. However, like Pu3+, hydrolytic reactions and complex formation are still an important feature of the aqueous chemistry of Am3+. Starik and Ginzberg (25) have shown that Am(III) exists in its ionic form from pH 1.0 to pH 4.5 but above pH 4.5 hydrolysis commences and at pH 7.0 colloidal species are formed. The hydrolytic behaviour of Cm(III) resembles that of Am(III). 

Of the many radioactive and inert constituents in waste solutions, only Tc and lanthanides coextract with actinides into the Truex process solvent. Trivalent lanthanides follow Am and Cm when the latter are stripped with dilute HNO3 depending on the disposition of the americium and... 

Dr. Glenn T. Seaborg proposed the term actinide for the new heavy elements that were predicted to follow the lanthanide series (Z-57 to Z-71). Dr. Seaborg believed that the actinides would be difficult to discover, and he proposed they would be trivalent homo-logues to the elements in the lanthanide series in which the 4f orbitals would be filled. His team at the Lawrence Berkeley National Laboratory (LBNL), located at the University of California s Berkeley campus, separated Z-95 (americium) and Z-96 (curium) as trivalent homologues of two of the elements in the lanthanide series located just above them in the periodic table. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

In solution Am4+ ion is not so stable, slowly reducing to trivalent Am3+. However, simple and also complex tetravalent compounds of americium are known. Some examples are Am(OH)4, AmF4, LiAmFs, andK2AmF4. Am(OEf)4 is stable in basic solution and results from the oxidation of Am(OH)3 by hypochlorite ion. 

This paper describes a new reaction which may yield useful amounts of the product isotope following neutron capture by lanthanide or actinide elements. The trivalent target ion is exchanged into Linde X or Y zeolite, fixed in the structure by appropriate heat treatment, and irradiated in a nuclear realtor. The (n, y) product isotope, one mass unit heavier than the target, is ejected from its exchange site location by y recoil. It may then be selectively eluted from the zeolite. The reaction has been demonstrated with several rare earths, and with americium and curium. Products typically contain about 50% of the neutron capture isotope, accompanied by about 1% of the target isotope. The effect of experimental variables on enrichment is discussed. 

Horwitz et al. showed that the trivalent americium is coordinated to three CMPO molecules and three nitrate anions in an overall neutral complex, another molecule of nitric acid being hydrogen bound to each of the carbamoyl oxygen atoms.157 158 Several molecules of CMPO are included in complexes formed with other cations such as plutonium. 

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... 

Enarsoson, A., Spjuth, L., Liljenzin, J.O., Kallvenius, G., Hudson, M.J., Iveson, P.B., Russell, M.L., Madic, C., Cordier, P.Y. 1999. Separation of trivalent americium and europium with some substituted oligopyridines and triazines in synergy with 2-bromo-decanoic acid. In Solvent Extraction for the 21st Century, Proceedings of International Solvent Extraction Conference, ISEC 1999, Cox, M., Hidalgo, M., Valiente. M. Eds. Society of Chemical Industry (SCI) London, Vol. 1, 323-328. 

The total concentration of trivalent americium species produced in the dissolution process is equivalent to [02(g)]0-25. If all of these species are assumed to remain in solution, the following relation can be postulated ... 

Direct speciation of dissolved americium and plutonium is possible only for solutions with appreciable concentration ([Am(DI)] > 10-6 M and [Pu(VT)] s 10-5 M) using a spectrophotometer with cumulative data recording (21). Typical spectra measured for the Am3+ ion at pH = 6.5 are shown in Figure 5 for 1, 5, 10, and 40 times cumulation at 503 nm. With this method it is shown that only trivalent americium ions are present in both equilibrium solutions from Am02 and Am(0H)3>nH20 solids. For plutonium solutions, the spectrophotometric study indicates the presence of polymers as shown in Figure 4. 

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. 

Treatment of irradiated targets. The chemical operations relative to the production of transplutonium elements (americium 243, curium 244) are all performed using a nitric acid medium. The highly corrosive nature of the solutions concentrated with Cl" ions, which were used in the USA for the development of the Tramex process (JO, and the instability of SCN" ions to radiation (12), led us to select nitric acid solution to perform the chemical separations. Once the medium was selected, it was necessary to find an adequate additive which, in combination with a suitable extractant, would allow solution of the main problem namely separation of the trivalent actinides from triva-lent lanthanides. 

This study shows that by diluting the salt solution, the selectivity for plutonium and americium is greatly increased whereas in undiluted solutions the actinides tend to migrate through the resin with the other salt constituents. Selectivity between different valencies is a function of the ionic strength, and the selectivity of the resin for the trivalent ion over the divalent ion (such as Pu3+ over Ca2+) is inversely related to the total concentration of the solution (16). Furthermore, the selectivity of the resin for the trivalent ion over the monovalent ion (such as Pu over K or Na"1") is inversely related to the square of the total concentration. 

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)... 

Proper adjustment (neutralization) of CAW solution acidity is crucial to satisfactory DBBP extraction of trivalent americium. Kingsley s(7) results (Figure 3) show that the CAW acidity must be adjusted to a concentration in the range 0.1-0.3M to achieve adequate distribution of Am + to the DBBP phase. 

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