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Experimental determination of uranium (IV) speciation in HF solutions at 500°C and 1000 bar

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Abstract

Uraninite solubility in HF solutions (0.0001–0.5 m) was experimentally studied at 500°C, 1000 bar, and hydrogen fugacity corresponding to the Ni/NiO buffer. It was shown that the predominant U(IV) species in aqueous solution are U(OH) 04 , U(OH)3F0, and U(OH)2 F 02 . Using the results of uraninite solubility measurement, the Gibbs free energies of the uranium (IV) species were calculated at 500°C and 1000 bar (kJ/mol): −986.55 for UO2(aq), −1712.42 for U(OH)3F0, −1755.53 for U(OH)2F 02 , and the equilibrium constants of the uraninite solubility in water and HF solutions were estimated: UO2(κ) = UO2(aq), which is similar to UO2(cr) + 2H2O = U(OH) 04 , pK0 = 6.64; UO2(cr) + HF0 + H2O = U(OH)3F0, K1 = 0.0513; UO2(cr) + 2HF0 = U(OH)2F 02 K2 = 7.00 × 10−4. Approximate values K3 = 5.75 × 10−3 and K4 = 6.7 × 10−2 were obtained for equilibria UO2(cr) + 4HF0 =UF 04 + 2H2O and UO2(cr) + 4HF = UF 04 + 2H2O. Maximum observed in the uranium concentration curve as a function of HF concentration can be explained by the decrease (to < 1) of activity coefficient ratio of HF0 to U(OH)3F0 with increasing HF concentrations.

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Authors and Affiliations

  1. Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119991, Russia

    N. I. Kovalenko, B. N. Ryzhenko, N. I. Prisyagina & Ya. V. Bychkova

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  1. N. I. Kovalenko
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  2. B. N. Ryzhenko
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  3. N. I. Prisyagina
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  4. Ya. V. Bychkova
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Correspondence to B. N. Ryzhenko.

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Original Russian Text © N.I. Kovalenko, B.N. Ryzhenko, N.I. Prisyagina, Ya.V. Bychkova, 2012, published in Geokhimiya, 2012, Vol. 50, No. 1, pp. 21–29.

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Kovalenko, N.I., Ryzhenko, B.N., Prisyagina, N.I. et al. Experimental determination of uranium (IV) speciation in HF solutions at 500°C and 1000 bar. Geochem. Int. 50, 18–25 (2012). https://doi.org/10.1134/S0016702912010053

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  • DOI: https://doi.org/10.1134/S0016702912010053

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