Solubility uraninite

Geochemical Nature and Types of Deposits. The cmst of the earth contains approximately 2—3 ppm uranium. AlkaHc igneous rock tends to be more uraniferous than basic and ferromagnesian igneous rocks (10). Elemental uranium oxidizes readily. The solubiHty and distribution of uranium in rocks and ore deposits depend primarily on valence state. The hexavalent uranium ion is highly soluble, the tetravalent ion relatively insoluble. Uraninite, the most common mineral in uranium deposits, contains the tetravalent ion (II). 

Figure 4. Solubility of uraninite as a function of Eh and PCO2 at pH = 8 and 25°C. The increase of uraninite solubility at high Pco2 results from the formation of uranyl carbonate complexes. [Used with permission of Elsevier Science, from Langmuir (1978) Geochim Cosmochim Acta, Vol. 42, Fig. 15, p. 561].

The incorporation of anions, as for example, S04 , CO2-, etc., makes leaching possible through the formation of stable uranyl (VI) oxyanions. In sulfate leaching, an observation of the potential-pH diagram for the uranium system reveals that uranium species in solution may be in the form of cations U02+, neutral species U02(S04)2 or anions U02(S04)4-. The oxidation of uraninite, U02, in acid solutions, transforming U(IV) to U(VI), yields soluble uranyl sulfate through the reaction as shown below ... 

Alkaline leaching is carried out by using sodium carbonate solution. In this case any U(IV) present in the ore must also be oxidized to U(VI). The uranium species soluble in carbonate leach solutions in the uranyl tricarbonate ion. The formation of this ion by solubilization of a hexavalent uranium mineral such as camotite, or a tetravalent uranium mineral such as uraninite, may be represented by the following reactions ... 

Once this common thermodynamic framework is established for the solubility of U02 under nominally reducing conditions, we have to ascertain the most probable pathway for the oxidative alteration of U02 spent fuel in geological repository conditions. There is a large body of evidence on the processes involved in the oxidative alteration of natural uraninites and unirradiated U02. Long-term unsaturated tests performed by Wronckiewicz et al. (1992) on groundwater from Yucca Mountain (the so-called J-13 groundwater), indicated that the formation of schoepite, as described by process (20) and (21), occurs, but is a transient event and that the alteration proceeds towards the precipitation of... 

Fig. 9. Experimental solubilities as total uranium concentration in solution for experiments on dissolution of uraninite samples from Oklo and Cigar Lake. Solid lines correspond to the calculated solubilities. Calculations performed with PHREEQC geochemical code (Parkhust Appelo 1999) and uranium database taken from Grenthe et al. (1992) and Bruno Puigdomenech (1989).

Th(IV) and U(IV) in neutral and alkaline solutions (the pH region of interest in performance assessment). Hence, the thermodynamic constants selected for the data base update (Hummel et al. 2002) do not refer to well-defined thorianite, Th02(cr), and uraninite, U02(cr), used in calorimetric measurements, but to the still poorly defined solids Th02(s) and U02(s) encountered in solubility studies at pH > 6 and pH > 5, respectively. Therefore, the thermodynamic constants selected in the data base update cannot be used to represent the widely varying solubilities of Th02 and U02 at pH < 5. 

Parks, G. A. Pohl, D. C. 1988. Hydrothermal solubility of uraninite. Geochimica et Cosmochimica Acta, 52, 863-875. 

Uranium U(VI) minerals are most often products of the oxidation and weathering of nearby primary U(IV) ore minerals such as uraninite [U02(c)I and coffinite [USi04(c)l (cf. Pearcy et al. 1994). They also form by evaporative concentration of dissolved U(VI), particulary under arid conditions. Schoepite (/J-UOj 2H2O) is fairly soluble and, therefore, is a rare mineral, whereas carnotite K2(U02)2(V04)2j and tyuyamunite (Ca(U02)2(V04)2j, which have lower solubilities (particularly above pH 5) are the chief oxidized ore minerals of uranium. The plots in Figs. 13.5 and 13.6 indicate that uranyl minerals are least soluble in I0W-CO2 waters, and, therefore, are most likely to precipitate from such waters. This is con.sistent with the occurrence of carnotite and tyuyamunite in oxidized arid environments with poor. soil development (Chap. 7), such as in the calcrete deposits in Western Australia (cf. Mann 1974 Dall Aglio et al. 1974), and in the sandstone-hosted uranium deposits of the arid southwestern United States (cf. Hostetler and Carrels 1962 Nash et al. 1981). The... 

Many researchers have attempted to measure the solubility of amorphous to crystalline UO2 (uraninite) as a function of pH. Some of this work is summarized in Table 13.4 and Fig. 13.7. Solubility measurements have been complicated by the fact that the UO2 solids were often of different and poorly known crystallinity and particle size. Further, oxygen (and possible CO2) contamination invalidated the results of most early measurements. With oxygen contamination, the measured solubility becomes that of a mixed oxidation-state oxide or a U(VI) solid such as schoepite. Oxygen contamination apparently invalidates the results of Gayer and Leider (1957) and Bruno et al. (1987) who obtained solubilities roughly equal to that of UO3 H2O (as reported by Gayer and Leider 1955) or to the solubility of schoepite as shown in Fig. 13.5. Measurements by Rai et al. (1990), Torrero et al. (1991), and Yajima et al. (1995) (see also Parks and Pohl 1988) indicate that the solubility of amorphous to more crystalline UO2 is independent of pH above about pH 4 to 4.5. This indicates that the dissolution reaction is... 

Alteration of uraninite U02(c)] in the deposit produces U(V1) oxyhydroxides and subsequently uranophane. The local geology is welded silicic tuffs, similar to the geology of Yucca Mountain, Nevada. Assuming 25 C and oxidizing conditions, and assuming that the solubility of uranophane limits U concentrations, what U(aq) concentration would you expect to find in the borehole water ... 

II). Manganese (II) once formed makes manganese carbonate (Kashefi and Lovley, 2000). The resulting compounds produce ores and the ores formed are deposited around the hydrothermal vents. From the vents, uranium (VI) compounds come forth which are water soluble, and are reduced by P. islandicum to uranium (IV) compounds which are in turn insoluble and are deposited around the vents to make uranium ores (e.g., uraninite). Furthermore, the bacterium is able to reduce gold... 

Unlike iron and other common metals, the higher oxidation state of uranium, U(VI), is water soluble, and its lower oxidation state, U(IV), is insoluble. So during the deposition of detrital uraninites, which are mostly older than 2 Gyr, the level of atmospheric oxygen must have been low, <0.01 PAL (Kasting 1993 Rasmussen Buick 1999). Large-scale deposition of detrital pyrite (FeS2) similarly mostly occurred before 2 Ga. 

This conclusion seems inconsistent with mineralogical observations indicating that uraninite is in fact present in the Tono Uranium Deposit (Section 2). A possible explanation for this apparent dichotomy is the experimental observation of Neck and Kim [28] that uraninite surfaces in contact with an aqueous phase at pH >3 may be coated with a thin layer of U02(am). Under such conditions, uraninite dissolution is effectively irreversible, and solubility is controlled by the amorphous surface layer. Additional experimental studies and observations of relevant natural systems are needed to test this hypothesis. 

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