Aqueous Geochemistry of Uranium

TABLE 13.3 Specific activities for radionuclides in the decay series 

As the most abundant actinide element, U averages 1.2 to 1.3 yug/g in sedimentary rocks, ranges from 2.2 to 15 /xg/g in granites, and from 20 to 120 yug/g in phosphate rocks (Langmuir 1978 Eisenbud 1987 see also Table 13.1). Uranium occurs in 4+, 5+, and 6+ oxidation states, which are usually written U(IV), U(V) and U(V1). Most important in nature are the uranous [U(IV)J and uranyl [U(VI)] oxidation states. 

Of concern in many countries is the proposed disposal of spent fuel (largely UO2) from nuclear power plants in a geological respository. Leaching of the spent fuel by groundwater could release to the environment the U and associated radionuclides, including Sr, Tc, 1, and Cs, and long-lived radioisotopes of Am, Np, and Pu. Because of their importance in nuclear waste disposal, the geochemistry of I, Tc, Am, Np, and Pu are considered briefly later in this chapter. 

Uranous ion (U ) and its aqueous complexes predominate in groundwaters of low Eh. U(IV) is the major oxidation state in the most common uranium ore minerals uraninite [U02(c)]—pitchblende is roughly U02(am)—and coffinite (USi04). The U(IV) concentrations in groundwater at low 

Actinides and Their Daughter and Fission Products Chap. 13 

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AH Welch, MS Lico. Aqueous geochemistry of ground-water with high concentrations of arsenic and uranium, Carson River basin, Nevada. Chem Geol 70 19-19, 1988. 

Langmuir D (1997) Aqueous Environmental Geochemistry. Prentice Hall, New Jersey, USA Latham AG, Schwarcz HP (1987a) On the possibihty of determining rates of removal of uranium from crystalline igneous rocks using U-series disequilibria - 1 a U-leach model, and its apphcabihty to whole-rock data. Appl Geochem 2 55-65... 

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