Uranium(VI) hydrolysis up to 250 °C and its geological implications

The hydrolysis of uranium(VI) has been investigated by solubility experiments conducted on sodium uranate (Na2U2O7) over a pHT range of 4.5–8 at 200 and 250 °C. Equilibrium constants for the UO2OH+, UO2OH20, UO2OH3- and UO2OH42- complexes have all been derived at both temperatures. Alongside this investigation a comprehensive literature review has been conducted to document previous investigations into U(VI) hydrolysis at temperatures above 25 °C. Using the new data reported here alongside those available in the literature, modified Ryzhenko-Bryzgalin equation of state parameters have been generated for the UO2OH+, UO2OH20, UO2OH3- and UO2OH42- complexes along with the polynuclear UO22OH22+ and UO23OH5+ complexes. This permits future works to extrapolate thermodynamic properties for these complexes to pressure–temperature conditions beyond those for which experimental data are available. Results reported here highlight the potential inadequacies of using the Ryzhenko-Bryzgalin and revised Helgeson-Kirkham-Flowers equations of state to extrapolate thermodynamic properties solely from room temperature data, with discrepancies ranging over 2–6 orders of magnitude being observed between extrapolations and the equilibrium constants experimentally derived at 200/250 °C. Using the newly derived properties, we report the results of thermodynamic calculations of uranium solubility and aqueous speciation under hydrothermal conditions relevant to a range of geological systems. These calculations suggest that uranyl hydroxide complexes may be responsible for uranium transport in a wide range of crustal fluids potentially obviating the need for high ligand concentrations.