Schoepite

Weathering of natural uraninite (Finch Ewing 1992 Finch Murakami 1999) and synthetic U02 (Wronkiewicz et al. 1992, 1996) under oxidizing conditions can result in the formation of a series of U6+ secondary phases, such as U6"1" oxide hydrates (e.g., dehydrated-schoepite, UO3-0.8H2O), alkali U6+ oxide hydrates (e.g., compreignacite, K2[(U02)604(0H)6j 8H20), U + silicates (e.g., uranophane, Ca(U02)2(Si03)2... 

Fig. 6. SEM images of meta-schoepite (a) Precipitation of meta-schoepite after 1 y of reaction (b) After 2 y, preferential corrosion of the U64 phase occurred along specific crystallographic directions.

Fig. 7. SEM image of studtite from corroded SNF. Studtite replaces meta-schoepite, which is not visible in the image.

Salbu et al. (2003) used micro-XAS to examine oxidation of depleted uranium (DU) munitions. Interestingly, these studies revealed the presence of U02 and U3Os but no U6+ oxide hydrate phases. Brock et al. (2003) examined the corrosion of DU penetrators in an arid environment. Using SEM, they observed aggregates of tabular, hexagonal schoepite and meta-schoepite crystals with clay/silt particles that were coated with amorphous silica. Brock et al. (2003) suggested that as the schoepite/meta-schoepite phases were coated with amorphous silica/clays, further dissolution was inhibited. 

Zhao Ewing (2000) examined altered uraninite from the Colorado Plateau with quantitative electron microprobe analysis in order to determine the fate of trace elements, including Pb, Ca, Si, Th, Zr, and REE, during corrosion under oxidizing conditions. The alteration phases identified included schoepite, calciouranoite, uranophane, fourmarierite, a Fe-rich U phase, and coffinite. The primary uraninites and alteration phases generally had low trace element contents. The electron microprobe analyses indicated that the trace elements preferentially entered the secondary U phases. Alteration caused the loss of U, Pb, and Zr, and incorporation of Si, Ti, Ca, and P into U phases. 

The released U(VI) from the U()2 matrix will continue to be dissolved until saturation with secondary U(VI) solid phases is reached. The observations from both laboratory and natural systems would indicate that the kinetically preferred phase is hydrated schoepite. This will be denoted as U02(0H)2(s) for the sake of description of the model, although the correct notation would be U03-xH20, with x oscillating between 0 and 2. Depending on the presence of carbonates in the contacting solution, the reactions can be described as ... 

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. 11. (a) Thermodynamic reaction pathway for the initial oxidative alteration of the spent fuel matrix at pH 8, calculated by using the PHREEQC code (adapted from Bruno etaL 1995). (b) Thermodynamic reaction pathway for the alteration of schoepite in granitic/bentonite groundwater at pH 8, calculated by using the PHREEQC code (adapted from Bruno et al. 1995 with permission). 

Figure 6 Aqueous speciation of uranium(VI) and the solubility of crystalline schoepite ( 3-U02(0H)2) under atmospheric conditions (Pco = 10 atm (Davis (2001) reproduced by permission of Nuclear Regulatory Commission from Complexation Modeling of Uranium(VI) Adsorption on Natural Mineral Assemblages...

Uncertainties in the identity and solubility product of the solubility-limiting uranium solid in laboratory studies lead to considerable uncertainty in estimates of the solubility under natural conditions. If amorphous U02(0H)2 is assumed to limit the solubility in solutions open to the atmosphere, then the value of the minimum solubility and the pH at which it occurs change. If crystalline schoepite (/3-U02(0H)2) controls the solubility, then the minimum solubility of 2 X 10 M occurs at about pH 6.5. If amorphous U02(0H)2 controls the solubility, the solubility minimum of 4 X 10 M occurs closer to pH 7.0 if the solubility product of Tripathi (1983) is assumed (Davis et al, 2001). 

Figure 7. The fourmaiieiite sheet anion-topology generated by stacking chains (a), and the sheet of uranyl pentagonal bipyramids that occurs in fourmarierite, schoepite and meta-schoepite (b).

Hydrates of UO3 include U03.iH20, UO3.H2O, and UO3.2H2O. The structure of the hemihydrate is not known. There appear to be three forms of the monohydrate , which is actually U02(0H)2, and possibly two of the dihydrate (U02(0H)2. H2O). The latter has been studied as the mineral schoepite it consists of U02(0H)2 layers with H2O molecules interleaved between them. The type of... 

Figure 10.7 Adsorption isotherm describing the adsorption of uranyl (UO +) species onto suspended amorphous ferric hydroxide at pH 7.23 and 25°C. The vertical line denoting saturation with respect to schoepite [U02(0H)2 H2OI has been computed from the pH and dissolved uranyl concentration. The enrichment factor, E.F., equals K,. Reprinted from Geochim. et Cosmochim. Acta, Vol. 53(6), D. Langmuir, Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits, pp. 547-569,...

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... 

Fig. 13.10, along with stability fields of stoichiometric uraninite and schoepite. The plot indicates that the intermediate oxides have a small stability range in Eh-pH space, however their stability fields occur under conditions commonly encountered in groundwater. Ahonen et al. (1993) have suggested, in fact, that their measured Eh and pH values in three drill holes in the Finnish Palmottu uranium deposit may be in equilibrium with UOj33 (U3O7) (Fig. 13.11). (See also Cramer and Smellie 1994, regarding the Cigar Lake deposit.)...

The Pena Blanca deposit in northern Mexico occurs in unsaturated and oxidized rhyolitic tuffs, in a geologic and climatic setting similar to that of the proposed U.S. repository at Yucca Mountain, Nevada. Much of the original UO2 ore has been oxidized and altered, sometimes first to form U(VI) oxide hydrates such as schoepite, and later to precipitate as more stable and abundant U(VI) silicate... 

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