Uranium aqueous chemistry

KEYWORDS uranium, aqueous chemistry, redox equiiibria, radon, hydromorphic dispersion... 

As a result of its four oxidation states, its tendency to complex with anions and the polymeric ions formed as a result of hydrolysis, the aqueous chemistry of uranium is complex. In aqueous solutions uranium salts exhibit acidic properties as a result of hydrolysis which increases in the order U3+ < UO + < U4+. In the case of UO + at 25° the principal hydrolised species are U020H+,(U02)2 (OH) + and (UO fOHj. 

Fluoroalkyl copper compounds, 17 143, 144 a-Fluoroalkylidinehydrazines, 33 167 2-Fluoroalky 1 -2 phospha-1,3-dialky 1 -1,3-diaze-tidinones, synthesis of, 14 87 Fluoroanions, trivalent, uranium, 34 94—95 Fluoroantimony compounds, 7 17 Huoroaryl copper compounds, 17 143, 144 Fluoroberyllates anhydrous, 14 267-276 aqueous chemistry of, 14 274-278 glasses, 14 265-267 preparation and properties of, 14 265-267... 

Uranium exists in the +3, +4, +5, and +6 oxidation states. The +5 state dispropor-tionates to the +4 and +6 states and is of little importance. Trivalent uranium reduces water and therefore there is no stable aqueous chemistry of U3+ although compounds do exist. 

HAA/WIL] Haacke, D. F., Williams, P. A., The aqueous chemistry of uranium minerals Part 1. Divalent cation zippeite, Mineral. Mag., 43, (1979), 539-541. Cited on page 160. 

The chemical properties of the actinides are much less similar to each other than those of the lanthanides, because the additional electrons added to the 5/ and 6d are bound less t tly than those of the 4/and 5d shells of the lanthanides. As shown in Table 9.4, the lanthanides in aqueous solutions exist principally in a single, tiivalent oxidation state, whereas four or more oxidation states are observed in the aqueous chemistry of uranium, neptunium, and plutonium. The actinide ions normally formed in solution by the oxidation states II through VI are M, M, M, MO2, MOj , respectively. 

This report contains a detailed evaluation of the thermochemistry of 142 uranium-halogen containing compounds. The properties given are A H , A G . S , C , and (Hj-Hq), all at 296.15 K, and A,H at 0 K. The analysis of much of the dSta involves the consideration of the aqueous chemistry of uranium containing compounds (also see item [149]). The recommended values are consistent with the CODATA scale (item [23]). 

Because of the technical importance of solvent extraction, ion-exchange and precipitation processes for the actinides, a major part of their coordination chemistry has been concerned with aqueous solutions, particularly that involving uranium. It is, however, evident that the actinides as a whole have a much stronger tendency to form complexes than the lanthanides and, as a result of the wider range of available oxidation states, their coordination chemistry is more varied. 

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

Since transport by water is virtually the only available mechanism for escape, we will be predominantly concerned with the chemistry of aqueous solutions at the interface with inorganic solids - mainly oxides. These will be at ordinary to somewhat elevated temperatures, 20-200 C, because of the heating effects of radioactive decay during the first millennium. The elements primarily of interest (Table I) are the more persistent fission products which occur in various parts of the periodic table, and the actinides, particularly uranium and thorium and, most important of all, plutonium. 

At unit activities of the oxidant and reductant, the potential depends only on pH the slope of the line for a plot of potential versus pH is governed by the ratio m/n. Potential-pH diagrams are a concise means to display the redox properties of a system. We will take uranium as an example. The +6, +5, +4, and + 3 oxidation states are known in aqueous solution. The determination of +6 uranium by coulometric titration has been investigated by many workers and the lower oxidation states have all been used as coulometric titrants. Hydrolyzed uranium species exist in a noncomplexing solution, but the chemistry is simplified considerably if the discussion is limited to solutions more acidic than about pH 4. Some of the half-reactions to be considered are listed next with E° vs. NHE ... 

The solution chemistry of uranium is that of the +4 and +6 oxidation states, that is, U4+ and U02+. The formal reduction potential of uranium in aqueous solution (i.e., 1 M HC104) is... 

The above processes involve separation based either on bulk properties (for example, size, density, shape, etc.) directly or by subtle control of the chemistry of the narrow interfacial region between the mineral particle and the aqueous solution in which it is suspended. In the processing of certain ores, such as those of uranium, gold or oxidized copper, chemical alteration of the minerals may be required to recover the valuable metals. These techniques are not discussed here, except to include those aspects which are directly related to surfaces and interfaces. 

Allard, B. Kipatsi, H. LiTjinzin, J. O. "Expected Species of Uranium, Neptunium, and Plutonium in Neutral Aqueous Solutions, J. Inorg. and Nuclear Chemistry 1982,42,1015-1027. 

Newton, T. W and F. B. Baker Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium. In if. F. Gould (Ed.), Lanthanide/Actinide Chemistry, Advances in Chemistry Series, Vol. 71, p. 268. Washington American Chemical Society 1967. 

Investigations of the solid-state chemistry of the americium oxides have shown that americium has properties typical of the preceding elements uranium, neptunium, and plutonium as well as properties to be expected of a typical actinide element (preferred stability of the valence state 3-j-). As the production of ternary oxides of trivalent plutonium entails considerable difficulties, it may be justified to speak of a discontinuity in the solid-state chemical behavior in the transition from plutonium to americium. A similar discontinuous change in the solid-state chemical behavior is certainly expected in the transition Am Cm. Americium must be attributed an intermediate position among the neighboring elements which is much more pronounced in the reactions of the oxides than in those of the halides or the behavior in aqueous solution. 

Davis J. A., Kohler M., and Payne T. E. (2001) Uranium (VI) aqueous speciation and equilibrium chemistry. In Surface Complexation Modeling of Uranium (VI) Adsorption on Natural Mineral Assemblages NUREG/CR-6708. US Nuclear Regulatory Commission, Washington, DC,... 

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