Aqueous Speciation and Solution-Mineral Equilibria

Above about pH 5, U(VI) generally occurs as aquocomplexes in natural waters. The relative importances of the U(VI) hydroxyl complexes are shown in Fig. 13.3 for a typical groundwater U con- 

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 

Actinides and Their Daughter and Fission Products Chap. 13 

Waychunas, and N. Xu, Uranium (VI) adsorption to ferrihydrate Application of a surface complexation model, pp, 5465-78, 1994, with permission from Elsevier Science Ltd., Tbe Boulevard, Langford Lane, KidlingtonOXSlGB, U.K. 

Yucca Mountain, Nevada, is being considered as the site for deep geological disposal of U.S. high-level nuclear wastes. Any release of uranium (or other radionuclides) from the waste to 

---