Speciation americium

Bulman RA, Johnson TE, Ham GJ, et al. 1993. Speciation of plutonium in potato and the gastrointestinal transfer of plutonium and americium from potato. Sci Total Environ 129 267-289. 

Cleveland, J.M., K.L. Nash, and T.F. Rees Neptunium and Americium Speciation... 

Industrial utilization of neptunium has been very limited. The isotope 1 Np has been used as a component in neutron detection instruments. Neptunium is present in significant quantities in spent nuclear reactor fuel and poses a threat to the environment. A group of scientists at the U.S. Geological Survey (Denver, Colorado) has studied the chemical speciation of neptunium (and americium) in ground waters associated with rock types that have been proposed as possible hosts for nuclear waste repositories. See Cleveland reference. 

Cleveland. J.M., K.L. Nash, and T.F. Rees Neptunium and Americium Speciation in Selected Basalt, Granite, Shale, and Tuff Ground Waters, Science, 221, 271-273 (1983). 

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. 

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

The interaction of radiation with matter can have profound effects. Whether in solid, solution, or gaseous states, radioactivity can impact the environment and therefore change the molecular speciation of the actinides. To put this into perspective, three examples are discussed below plutonium metal, americium crystals, and an aqueous solution of plutonium. 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

Oxidation state. Differences among the potentials of the redox couples of the actinides account for much of the differences in their speciation and environmental transport. Detailed information about the redox potentials for these couples can be found in numerous references (e.g., Hobart, 1990 Silva and Nitsche, 1995 Runde, 2002). This information is not repeated here, but a few general points should be made. Important oxidation states for the actinides under environmental conditions are described in Table 4. Depending on the actinide, the potentials of the III/IV, IV/V, V/VI, and/or IV/VI redox couples can be important under near-surface environmental conditions. When the redox potentials between oxidation states are sufficiently different, then one or two redox states will predominate this is the case for uranium, neptunium, and americium (Runde, 2002). The behavior of uranium is controlled by the predominance of U(VI) species under... 

Table 7 contains the results of actinide solubility and speciation calculations for the J-13 and SKI-90 reference waters carried out using the MINTEQ2A code (Allison et al., 1991) as described by Langmuir (1997a). The MINTEQ2A thermodynamic database of Turner et al. (1993) was used with revised data for americium (Silva et al., 1995) and modifications for uranium... 

In the WIPP speciation and solubility calculations, the solubility-controlling solid phase for americium, and by analogy, for all - - III actinides under WIPP conditions, was Am(OH)C03(cr> (Novak, 1997 US EPA, 1998a,b,c,d WaU et al, 2002). Am(OH)J was the most abundant aqueous species, and estimated americium solubilities in the reference Salado and Castile brines (Table 8) were 9.3 X 10 and 1.3X10 M, respectively (Novak, 1997 US EPA, 1998d). 

S., and Carpenter S. A. (1993) Measured solubilities and speciations of neptunium, plutonium and americium in a typical groundwater (J-13) from the Yucca Mountain region. Los Alamos National Laboratory. 

Rees T. E., Cleveland J. M., and Nash K. L. (1983) The effect of composition of selected groundwaters from the basin and range province on plutonium, nepmnium, and americium speciation. Nuclear Technol. 65, 131-137. 

Studies of the speciation of actinides in environmental waters are made difficult by the very low concentrations involved and the possibility that minor, undetected contaminants may dominate the binding of a particular metal ion. The environmental behaviour of the actinides has been reviewed. Americium and thorium exhibit simpler behaviour than other actinides since their oxidation states under such conditions are limited to Am and Th. Both are readily adsorbed by granitic rocks and tend to exhibit low solubilities, The thermodynamic solubility product of amorphous Am(OH)3 has been measured as log = 17.5 0.3 and no evidence for amphoteric behaviour or the formation of Am(OH)4 was found below pH 13. Stability constants for the binding of Am to humic acid have been found to vary with the degree of ionization, a, and were given by log = 10.58a -1-3.84 and log 2 = 5.32a -b 10.42. These were larger than the corresponding values for Eu. Humic acids also bind Th as described in Section 65.2.1. 

Preparation of diis manuscr t was siqiported by the EMSP under a high level waste project entided Chemical Speciation of Strontium, Americium, and Curium in Hi Level Waste, Project No. 267S3, and an EMSP Subsur ce Science Project The Aqueous Thermodynamics and Conqilexation Reactions of Aqueous Silica Species to High Concentration, Project No. 30944. 

Nitsche, H., R.C. Gatti, E.M. Standifer, S.C. Lee, A. Muller, T. Prussin, R.S. Deinhammer, H. Maurer, K. Becraft, S. Leung, and S.A. Carpenter. 1993. Measured solubilities and speciations of neptunium, plutonium, and americium in a typical groundwater (J-13) from the Yucca Mountain Region. YMP Milestone Rep. 3010 LA-12562-MS. Los Alamos National Laboratory, Santa Fe, NM. 

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