Americium species

Spent Zirflex Process Decladdent. The basic perceived need is to devise an3 develop a simple process for selective and efficient removal of plutonium (and 21 1Am) from spent Zirflex process decladdent solution. To satisfy this need, it may be necessary--or prove beneficial—to determine, by appropriate physiochemical methods, the nature of the plutonium (and americium) species in the decladding solution. Availability of a satisfactory transuranium removal scheme may be one of the key factors in devising an alternative to storage in expensive double-shell tanks for spent Zirflex process solution at the Hanford site. 

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 ... 

Predominance-Zone Diagrams for Chemical Species, Fig. 1 Pourbaix diagram of americium species for —log [Am]To,ai = 10.0 [24], Dashed lines represent the oxidation (upper) and reduction (lower) of water... 

Predominance-Zone Diagrams for Chemical Species, Fig. 2 Pourbaix-type diagram of americium species at —logfSOy] = 0.523 in the Eh/pH space, constructed with the MGSE. SO4 = HS04 + SO4 . Dashed black zones represent the coexistence of condensed phases for... 

Electron configurations of gaseous americium species as determined from spectroscopic and atomic-beam experiments show a Sf 7s ground state and (Sf 7 2 + 2 and Tables 14.3 and 15.6], Americium is the sixth member of the... 

Table 9.23 Thermodynamic data for americium species and phases at 25 °C determined in the present review and comparison with those given in the literature.

Most smoke alarms (Figure 19.1, p. 517) use a radioactive species, typically americium-241. A tiny amount of this isotope is placed in a small ionization chamber decay of Am-241 ionizes air molecules within the chamber. Under the influence of a potential applied by a battery, these ions move across the chamber, producing an electric current. If smoke particles get into... 

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. 

Although the relationship of sediment adsorption to water concentration appears to be a controlling feature of shallow water systems such as lakes and coastal shelf water, the open ocean is more likely to contain soluble plutonium which seems to be unaffected by particulate matter. This is particularly evident in two oceanographic studies. Bowen et al have discovered a stratum of plutonium in the North Pacific at about 500m that has not changed depth appreciably from 1973 to 1980. How it arrived at this depth is subject to conjecture but it appears to be soluble plutonium which is not settling(17). Fukai et al have delineated plutonium maxima in the Mediterranean Sea which seem to be due to soluble species(18). Comparison of americium to plutonium ratios in this... 

A large degree of variation is apparent in retention rates for americium in the liver among various animal species (Durbin 1973), as indicated by measured or estimated liver clearance half-times of approximately 5-16 days in rats, 152 days in baboons, 1-10 years in dogs, and 10 years in Chinese hamsters. A liver clearance half-time of 2 years has been estimated for humans (Griffith et al. 1983). Refer to Section 3.5.1 for information regarding toxicokinetic mechanisms that may play a role in interspecies differences in liver retention of americium. 

The model is designed for applications to human dosimetry however, it predicts reasonably well the retention of americium in beagle dogs as well (Mewhinney and Griffith 1982). The model cannot be applied to other species without modification. 

Distribution. Bone constitutes the largest fraction of the deposited body burden of americium in all mammalian species that have been studied. The mechanisms by which americium is taken up and retained in bone are only partially understood. The distribution of americium in bone initially is confined to bone surfaces, including endosteal and periosteal surfaces, and adjacent to vascular canals in cortical bone (Polig 1976 Priest et al. 1983, 1995 Schlenker et al. 1989). Deposition appears to be favored at sites of active... 

Chronic-Duration Exposure and Cancer. No data were located regarding chronic-duration exposure of humans or animals to americium. Chronic-duration inhalation and oral MRLs were not derived for americium due to the lack of human or animal data. To generate appropriate data for deriving chronic-duration inhalation and oral MRLs for americium, at least one comprehensive chronic-duration inhalation and one chronic-duration oral toxicity study of at least one animal species exposed to several dose levels would be needed. Such studies could be designed to also generate data regarding potential age-related differences in toxicity. However, since americium is not found naturally, and is produced and used... 

Schreckhise RG, Cline JF. 1980. Comparative uptake and distribution of plutonium, americium, curium and neptunium in four plant species. Health Phys 38 817-824. 

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). 

There is tracer scale evidence455 for the formation of the Cf02 ion during ozonization of 249Bk and subsequent jS decay to 249Cf. However, the only compounds isolated in this oxidation state are all americium(V) species. 

The nitratocomplexes MIAm02(N03)3 (M1 = Rb or Cs424) are precipitated from nitric acid solutions of americium(VI). The IR spectrum of RbAm02(N03)3 has been reported.249 Hydrated phosphato, arsenato and sulfato complex salts are included under Aqua species in the preceding section. 

In an analysis of the hazards of the alpha emitters from reactor operations it has been pointed out (25) that the most significant and hazardous species are plutonium, americium, curium, and neptunium. Plutonium is as hazardous as such fission products as ruthenium-106, cesium-137, cerium-144, and promethium-147, depending on the kind of fuel, the power of the reactor, the storage time of the waste, and whether it is released to the atmosphere or to water. If strontium-90 is removed... 

The oxidation-reduction behaviors of neptunium, plutonium and americium in basic solution have been determined via polarographic and coulometric studies (6-9). These studies, which showed that the more soluble (V), (VI), and (VII) oxidation states of these actinides are stable in alkaline solution under certain redox conditions, helped identify possible actinide species and oxidation states in our experiments. Actual identification of radioelement oxidation states was not done in the present experiments. 

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