Uranyl speciation in sulfate-bearing hydrothermal solutions up to 250 °C

Journal Article · Wed Aug 28 00:00:00 EDT 2019 · Geochimica et Cosmochimica Acta

DOI:https://doi.org/10.1016/j.gca.2019.08.027· OSTI ID:1561080

Kalintsev, Alexander  ^[1]; Migdisov, Artaches  ^[2]; Xu, Hongwu ^[2]; Roback, Robert ^[2]; Brugger, Joël  ^[3]

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1. Los Alamos National Lab. (LANL), Los Alamos, NM (United States); Monash Univ., Melbourne, VIC (Australia)
2. Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
3. Monash Univ., Melbourne, VIC (Australia)

The speciation of uranyl in sulfate-bearing solutions at temperatures up to 250 ºC has been investigated using in situ UV-Visible spectrophotometry. Formation constants for the reactions UO$$^{2+}_{2}$$ + SO$$^{2–}_{4}$$ $$\Longleftrightarrow$$ UO₂SO$$^{0}_{4}$$ (logβ₁ = 3.38, 4.40, 5.44, 6.33, 7.74) and UO$$^{2+}_{2}$$ + 2SO$$^{2–}_{4}$$ $$\Longleftrightarrow$$ UO₂(SO₄)$$^{2–}_{2}$$ (logβ₂ = 4.10, 5.26, 6.83, 8.00, 9.70) derived at 25, 100, 150, 200 and 250 ºC respectively. These formation constants were fitted to the modified Ryzhenko – Bryzgalin (MRB) model to derive pK(298), A(zz/a) and B(zz/a) values for both complexes: 3.262, 2.212, -197.96 (UO₂SO$$^{0}_{4}$$) and 4.189, 3.21, -473.14 UO₂(SO₄)$$^{2–}_{2}$$ respectively. Compared with values extrapolated using previously available data for 25, 70 and 75 ºC, our new data suggest higher stability of UO₂SO$$^{0}_{4}$$ at temperatures above 150 ºC and significantly lower stability of UO₂(SO₄)$$^{2–}_{2}$$ at all temperatures above 25Cº. Molar absorbances of both sulfate species were also derived. At 25 Cº we found our molar absorbance for UO₂SO$$^{0}_{4}$$ agreed well with those reported previously in the literature, however we report lower peak amplitudes for UO₂(SO₄)$$^{2–}_{2}$$ . We also noted significant temperature-dependent red shifts in the molar absorbance of UO₂SO$$^{0}_{4}$$. We imply that these shifts could be explained by changes in sulfate bonding behaviour around the uranyl O04 ion - namely shifts in the distribution of complexes with sulfate bound in either a monodentate or bidentate configuration. Simple models calculated with our new data suggest that sulfate complexes may readily predominate over chloride complexes even in solutions containing upwards of 20 wt% chloride and only 100 ppm sulfate. In addition, moderate concentrations of sulfate (~1 wt%) can increase the solubility of uranium by an order of magnitude. Therefore, we imply that sulfate could play a vital role in mobilising uranium in hydrothermal systems, and that the removal of sulfate via precipitation of sulfate minerals may act as a means of depositing uranium under oxidizing conditions. This in turn might explain the associations between sulfate minerals and uranium mineralisation occasionally seen in large uranium ore deposits such as Olympic Dam and MacArthur River.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• Grant/Contract Number: 89233218CNA000001
• OSTI ID: 1561080
• Alternate ID(s): OSTI ID: 1693461
• Report Number(s): LA-UR-19-20322; TRN: US2000587
• Journal Information: Geochimica et Cosmochimica Acta, Vol. 267; ISSN 0016-7037
• Publisher: Elsevier; The Geochemical Society; The Meteoritical Society
• Country of Publication: United States
• Language: English

Related Subjects

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY
Uranium
sulfate
hydrothermal
spectroscopy