Uranium oxide surface oxidation states

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

G. (1999) Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine. Chem. Geol. 158 81-103 Allen, G.C. Kirby, C. Sellers, R.M. (1988) The effect of the low-oxidation-state metal ion reagent tris-picolinatovanadium(II) formate on the surface morphology and composition of crystalline iron oxides. J. Chem. Soc. Faraday Trans. I. 84 355-364... 

Margolis [203] confirms such results for antimonates and reports the existence of a surface compound containing Sb3+—O—C. Aykan and Sleight [34] examined the system U—Sb—O in air up to 1000°C by different techniques (e.g. ESR) and found the ternary components USbOs and USb3Oi0. Since USb03 is paramagnetic, the formal oxidation state of U must be 5+, hence Sb must also be in the 5+ state. The authors conclude that USb3O10 also contains pentavalent uranium. 

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... 

In the earlier history of the earth, up to perhaps 1.4x 10 years ago, there was almost no oxygen in the atmosphere and uranium oxides could exist at the surface of the earth as grains or nuggets without being oxidized from their 4+ state into the soluble 6+ state. As such, these uranium oxide particles could travel in streams and because of their density they were segregated from less dense materials in streams, in the same way that Au collects in placer deposits. With time these stream deposits were buried, thrust deep into the ground and metamorphosed into the type of accumulation called a quartz... 

The single-crystal snrfaces of UO have been studied to a lesser extent, most likely because they are not readily available and the perceived irrelevance of their catalytic activity. Nnmerons investigations of polycrystalline and thin-fihn uranium surfaces have been stndied, due to their relevance in nuclear technology, [26-30]. It is however ironic that the first published LEED pattern of any metal oxide surface was that of a U02(1H) single crystal [28]. The reduced UOj surface, like TiO contains a distribution of oxidation states less than +4 [26]. Contrary to TiO, the UO strnctnre, Eig. 7.2, can also accommodate additional oxygen atoms making it a potentially good candidate for oxidation reactions [31]. 

Eh are usually less than 10 M because of the extremely low solubilities of these solids. In the U(V) oxidation state, uranium occurs as the UOJ ion which forms relatively weak complexes (Grenthe et al. 1992). This species is only found at intermediate oxidation potentials and low pH s and is unstable relative to U(IV) and U(VI). In oxidized surface- and groundwater-uranium is transported as highly soluble uranyl ion (UOf ) and its complexes, the most important of which are the carbonate complexes. The thermodynamic properties of these minerals and aqueous species must be known if we are to understand the reactions that may control U concentrations in natural waters. 

Applications of EPMA include elemental analysis of surfaces and of micron-sized features at a surface. The sensitivity of the method is about 0.2 atom%. It provides a rapid, accurate method for compositional analysis of microscopic features. Elemental mapping of the elements present at the surface can also be done, and the composition correlated to topographical maps obtained from an analytical microscopy method, allowing correlation of topographical features of a surface with elemental composition. Like XRF, EPMA is strictly an elemental analysis technique no information on chemical spe-ciation or oxidation state is obtained. AU elements from boron to uranium can be determined. Given that X-rays can escape from depths of 1000 A or so, EPMA has the deepest definition of surface of the techniques discussed in this chapter. In fact, like XRF, it can be considered to be a bulk analysis technique assuming the sample is homogeneous. 

Uranium displays multiple oxidation states from + 3 to +6, with +4 and +6 being the most common. Uranium(IV) is largely insoluble, as it reacts readily with particle surfaces. Uranium(VI), on the other hand, exists as a di-oxo cation, UOi+ in... 

The chemical properties span a range similar to the representative elements in the first few rows of the periodic table. Francium and radium are certainly characteristic of alkah and alkaline earth elements. Both Fr and Ra have only one oxidation state in chemical comhina-tions and have little tendency to form complexes. Thallium in the 1+ oxidation state has alkah-like properties, but it does form complexes and has extensive chemistry in its 3+ state. Similarly, lead can have alkaline earth characteristics, hut differs from Ra in forming complexes and having a second, 4+, oxidation state. Bismuth and actinium form 3+ ions in solution and are similar to the lanthanides and heavy (Z > 94) actinides. Thorium also has a relatively simple chemistry, with similarities to zirconium and hafiuum. Protactinium is famous for difficult solution chemistry it tends to hydrolyze and deposit on surfaces unless stabilized (e.g., by > 6 M sulfuric acid). The chemistry of uranium as the uranyl ion is fairly simple, hut... 

Thoria-based fuels are appealing from a wasfe managemenf perspective because Th02 is chemically stable and almost insoluble in groundwater. By far fhe most important chemical difference between Th02 and UO2 is the fact that thorium is present in its maximum oxidation state, Th(IV), whereas uranium is not. Therefore, oxidative dissolution of the matrix is not an issue with thoria fuel. Redox conditions could affect the leachability of 233U from irradiated thoria, but this would be limited to surface dissolution and is unlikely to be a major concern. 

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