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

Abstract The speciation of uranyl in sulfate-bearing solutions at temperatures up to 250 °C has been investigated using in situ UV-Visible spectrophotometry. Formation constants for the reactions U O 2 2 + + S O 4 2 - ⇔ U O 2 S O 4 0 ( l o g β 1 = 3.38, 4.40, 5.44, 6.33, 7.74) and U O 2 2 + + 2 S O 4 2 - ⇔ U O 2 S O 4 2 2 - ( l o g β 2 = 4.10, 5.26, 6.83, 8.00, 9.70) were derived at 25, 100, 150, 200 and 250 °C 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 ( U O 2 S O 4 0 ) and 4.189, 3.21, -473.14 U O 2 S O 4 2 2 - respectively. Compared with values extrapolated using previously available data for 25, 70 and 75 °C, our new data suggest higher stability of U O 2 S O 4 0 at temperatures above 150 °C and significantly lower stability of U O 2 S O 4 2 2- at all temperatures above 25 °C. Molar absorbances of both sulfate species were also derived. At 25 °C we found our molar absorbance for U O 2 S O 4 0 agreed well with those reported previously in the literature, however we report lower peak amplitudes for U O 2 S O 4 2 2 - . We also noted significant temperature-dependent red shifts in the molar absorbance of U O 2 S O 4 0 . We suggest that these shifts could be explained by changes in sulfate bonding behaviour around the uranyl 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. Thus we suggest that sulfate could play an important 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.