Actinides 523 solubility limiting

Th-oxyhydroxide species readily dissolve upon dilution below the solubility limit, it is not veiy likely that such actinide(IV) colloids play a role away from the source in the far field of a repository. In the near field of a repository, however, they may be predominant species controlling the solubility of tetravalent actinide species such as U(IV) and Pu(IV) and thus the source term. Unusual stability at high ionic strength has been also reported for amorphous SiOz colloids (Iler 1979 Healy 1994) which also cannot be explained solely by electrostatic repulsion. Formation of oligomeric or polymeric silicate species at the colloid-water interface are thought to exert additional steric stabilization by preventing close approach of those particles. 

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). 

Actinide Solubility (M) Solubility-limiting phase Dominant aqueous phases... 

Novak C. F. (1997) Calculation of actinide solubilities in WIPP SPC and ERDA-6 brines under MgO backfill scenarios containing either nesquehonite or hydromagnesite as the Mg-COs solubility-limiting phase. Sandia National Labs, WIPP Records Center. 

Since LPAS application to actinide chemistry is in its infancy, only a limited number of works are available in the published literature. Experiments hitherto performed are confined to either hydrolysis, complexation reactions with carbonate, EDTA and humate ligands and a variety of speciation works for Am(III) and to much lesser extent for U(IV), U(VI) Np(IV), Np(V), Np(VI) Pu(IV), Pu(VI). Of considerable interest is the LPAS application to the direct speciation of actinides in natural aquifer systems, where the solubility of actinides is in general very low and multi-component constituent elements as well as compounds are in much higher concentrations than actinide solubilities. The study of the chemical behaviour of actinides in such natural systems requires a selective spectroscopic method of high sensitivity. LPAS is an invaluable method for this purpose but its application to the problem is only just beginning. 

Summary. Spectroscopic techniques, in particular laser induced fluorescence, appear as a good analytical tool for complexation measurements since they are non-intrusive methods, they allow to work at low level of cation concentration (in the case of TRLIF), in particular below the solubility limits permitting to cover a large range of pH and they also allows the use of low humic acid concentrations to avoid aggregation phenomena. From the conditional interaction constants measured by these techniques, the following trend of actinides for humic acids is deduced ... 

At the PWR primary coolant pH of 7 to 8, the fission product isotopes of the tri- and tetravalent elements show strong hydrolysis, resulting in very low solubilities. This macrochemical behavior is consistent with the observations made in coolant analyses that these radionuclides can be almost quantitatively isolated together with the suspended corrosion products by filtration. However, this behavior does not necessarily indicate the presence of particular oxides or hydroxides of these fission products, since due to their very low element concentrations in the coolant their solubility limits are probably not exceeded. Presumably, these element traces are attached to the corrosion product oxides either by adsorption onto their surfaces or by formation of mixed crystals. A significant fraction of the longer-lived tri- and tetravalent fission products, as well as of the actinides, is incorporated into the contamination layers which cover the primary circuit surfaces. However, because of the usually very low actiAuty concentrations of these radionuclides in the coolant and, consequently, in the contamination layers, their contribution to the contamination dose rates is negligible. 

Because of the ease of oxidation of protactinium(IV) and uranium(IV), peroxides and peroxo complexes are limited to their higher oxidation states. The compounds M04"JcH20 precipitated from dilute acid solutions of neptunium(IV) and plutonium(IV) by hydrogen peroxide appear to be actinide(IV) compounds. Soluble intermediates of the type [Pu( U-02)2Pu]4+ are formed at low hydrogen peroxide concentrations. 

Currently proposed licensing regulations for geologic nuclear waste repositories require a performance assessment involving long-term predictive capabilities. Previous work (J- 5) has shown the importance of solubility controls for modeling maximum actinide concentrations in repository groundwaters. However, until reliable data are available on the actinide solid phases that may be present or that may precipitate in the environment, the solubility of solid phases such as hydrous oxides that have fast precipitation kinetics can be used to initially set maximum solution concentration limits. 

The cyclopentadienyl reactions have been illustrated for early as well as late transition metals and also the actinides. It is remarkable that the reactions are very similar across the entire group. This methodology results often in sandwich-type complexes of the Et units and has been found successful for both phosphorus and arsenic, but only one example has been reported for antimony. There are no reports of bismuth metal reacting with cyclopentadienyl metal fragments. Neither Sb nor Bi readily form the soluble E4 species as do P and As, and so their reactivities may be limited by solubility factors (Table XLI). 

Actinide(III) precipitates dissolve to a limited extent in solution containing 1M (or more) (NH J CO. However, the actinide (I I I) carbonato complex ion precipitates very slowly by adding hexamminecobalt(III) chloride and the yield of precipitation is not high. Separations of actinide(IV) and (VI) ions from actinide (III) and (V) ions are thus achieved by taking advantage of their different solubilities in ammonium carbonate solution. Hexamminecobalt (III) salt is used as a precipitant to recover U(VI), Pu(IV) and Am(VI) ions from ammonium carbonate solution. 

Protactinium tetrafluoride, like the other actinide tetrafluorides, possesses the 8-coordinate UF4-type of structure (Table III) but no bond distances are available. It is easily the most stable tetravalent halide of protactinium and can be handled in the atmosphere, at least for a limited period, without hydrolysis or oxidation occurring. As mentioned earlier it is the usual starting material for the preparation of protactinium metal. Tetrafluoride hydrates have not been fully characterized, but a mixed fluorosulfate, PaF2S04 2H20 can be precipitated from aqueous solution (131). Protactinium tetrafluoride is soluble in aqueous ammonium fluoride solutions, for which some spectral properties have been recorded (4, 83). 

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