Americium solubility

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

Judging from the R2 values, plutonium solubility was described adequately by its prediction equation while the strontium and americium solubility equations were less satisfactory. Analysis of the individual test data was found to be more useful than the prediction equations in assessing strontium and americium behavior. 

Inspection of the americium test data showed that the entire spike (about 8xlO 6M americium) dissolved in the presence of 0.1M HEDTA. Subsequent synthetic HLW test solutions having 0.1M HEDTA completely dissolved an 8xlO"4M americium spike (0.6 Ci 241Am/L), tnus confirming the strong effect of HEDTA on americium solubility. Citrate and hydroxyacetate also increased americium solubility. At 0.03M citrate, with <. 10 5M HEDTA,... 

Additional work in progress includes optimization of parameters affecting the oxalate precipitation step this includes determination of the chloride concentration required to solubilize lead the oxalate ion concentration required for maximum americium recovery with minimum impurity precipitation precipitate aging and hydrogen ion concentrations that will minimize americium solubility yet maximize impurity solubilization. 

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). 

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... 

Methods for reducing peak absorption of americium after inhalation or oral exposure have not been described. Topical applications of saline containing DTP A, tartaric acid, or citric acid (e.g., Schubert s solution) have been used to remove americium from the skin and wounds after accidental dermal exposures (Breitenstein 1983). These agents form stable, water soluble complexes with americium. 

Amendation of soil with lime and organic matter decreases uptake (Adriano 1979 Hoyt and Adriano 1979). This is thought to reduce the solubility or increase americium binding, thereby reducing its availability. 

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. 

Denmark 1.5 days after the explosion. Air samples collected at Roskilde, Denmark on April 27-28, contained a mean air concentration of 241Am of 5.2 pBq/m3 (0.14 fCi/m3). In May 1986, the mean concentration was 11 pBq/m3 (0.30 fCi/m3) (Aarkrog 1988). Whereas debris from nuclear weapons testing is injected into the stratosphere, debris from Chernobyl was injected into the troposphere. As the mean residence time in the troposphere is 20-40 days, it would appear that the fallout would have decreased to very low levels by the end of 1986. However, from the levels of other radioactive elements, this was not the case. Sequential extraction studies were performed on aerosols collected in Lithuania after dust storms in September 1992 carried radioactive aerosols to the region from contaminated areas of the Ukraine and Belarus. The fraction distribution of241 Am in the aerosol samples was approximately (fraction, percent) organically-bound, 18% oxide-bound, 10% acid-soluble, 36% and residual, 32% (Lujaniene et al. 1999). Very little americium was found in the more readily extractable exchangeable and water soluble and specifically adsorbed fractions. 

Bioavailability from Environmental Media. The absorption and distribution of americium as a result of inhalation and ingestion exposures have been discussed in Sections 3.3.1 and 3.3.2. EPA lists identical uptake factors for inhaled and ingested americium (and all the other transuranics other than plutonium) regardless of compound solubility, indicating that the knowledge base for americium is not sufficiently developed to quantify the differences that are recognized for most other elements. 

In vitro analyses of americium are routinely performed in situations where in vivo analyses cannot be obtained or in support of an in vivo monitoring program. Urine is the preferred sample for in vitro analyses of americium, although other sample types, such as feces, tissue, bone, or blood, can also be used on a more limited basis. Urine provides for an analysis of soluble or transportable americium, fecal analysis can be used to measure ingestion or clearance of americium, and tissue is used to assess whole or regional body burdens of americium (Guilmette and Bay 1981 Ide 1986 Ide et al. 1985 Mclnroy et al. 1985). 

Schell et al. [ 57] have described a sorption technique for sampling plutonium and americium, from up to 4000 litres of water in 3 h. Battelle large-volume water samples consisting of 0.3 xm Millipore filters and sorption beds of aluminium oxide were used. Particulate, soluble, and presumed colloidal fractions are collected and analysed separately. The technique has been used in fresh and saline waters, and has proved to be reliable and comparatively simple. 

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 its precipitation reactions ameiicium(IIl) is very similar to the other tripositive actinide elements and to the rare earth elements. Thus the fluonde and the oxalate are insoluble and the phosphate and iodate are only moderately soluble in acid solution, whereas the nitrates, halides, sulfates, sulfides, and perchlorates are all soluble. Americium(VI) can be precipitated with sodium acetate giving crystals isostructural with sodium uranyl acetate,... 

Tin and americium were so extensively sorbed under all conditions that isotherm data could not be obtained. These elements are not significantly mobile in the Mabton Interbed aquifer. Values of Freundlich constants for technetium, radium, uranium, neptunium, and plutonium are given in Table IV. The Freundlich equation did not fit the selenium sorption data very well probably because of slow sorption kinetics or precipitation. Precipitation was also observed for technetium at 23°C for concentrations above 10 7M. This is about the same solubility observed for technetium in the sandstone isotherm measurements. Linear isotherms were observed only in the case of radium sorption. In general, sorption on the Mabton Interbed was greater than on the Rattlesnake Ridge sandstone. This is probably due to the greater clay content of the Mabton standard. 

Effects of Hanford High-Level Waste Components on the Solubility and Sorption of Cobalt, Strontium, Neptunium, Plutonium, and Americium... 

Chemical components in the waste solutions potentially could affect radioelement solubility and sorption reactions, and thus enhance or reduce radionuclide transport. The effects of 12 chemical components on the solubility and sorption of cobalt, strontium, neptunium, plutonium, and americium were studied to... 

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. 

The solubility results were reflected in the sorption behaviors observed for the five radioelements. Sorption of strontium and americium was decreased by HEDTA and EDTA. Cobalt sorption also decreased in the presence of HEDTA. Sorption of both cobalt and strontium was decreased by increasing sodium ion concentration in the HLW due to competition of the cations for sorption sites. 

In summary, the solubility and sorption reactions of cobalt, strontium, neptunium, plutonium, and americium were found to be dependent on HLW compositions. Evidence revealed the formation in HLW of organic complexes of cobalt, strontium, and americium, and of hydroxide complexes of neptunium(V) and plutonium(V). Sorption reactions were dependent on radioelement complex formation and suspected waste/sediment reactions. These data can aid in assessing effects of future HLW processing operations as well as in judging the feasibility of continued storage of HLW in existing tanks. 

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