The hydrolysis of uranium(VI) has been investigated by solubility experiments conducted on sodium uranate (NaUO) over a pH range of 4.5–8 at 200 and 250 °C. Equilibrium constants for the , , and 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 , , and complexes along with the polynuclear and 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.

中文翻译：

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铀 (VI) 在 250 °C 时水解及其地质意义


通过在 200 和 250 °C、pH 值 4.5–8 范围内对铀酸钠 (NaUO) 进行溶解度实验，研究了铀 (VI) 的水解。 、 和 配合物的平衡常数均已在两种温度下导出。除了这项调查之外，还进行了全面的文献综述，记录了之前对 25 °C 以上温度下 U(VI) 水解的研究。使用此处报告的新数据以及文献中提供的数据，已为 、 、 和 配合物以及多核和配合物生成了修正的 Ryzhenko-Bryzgalin 状态参数方程。这使得未来的工作能够将这些配合物的热力学性质推断到超出实验数据可用的压力-温度条件。这里报告的结果强调了使用 Ryzhenko-Bryzgalin 和修正的 Helgeson-Kirkham-Flowers 状态方程仅从室温数据推断热力学性质的潜在不足，外推法与平衡态之间存在超过 2-6 个数量级的差异200/250°C 下实验得出的常数。利用新导出的特性，我们报告了与一系列地质系统相关的热液条件下铀溶解度和水形态形成的热力学计算结果。这些计算表明，氢氧化铀络合物可能负责多种地壳流体中的铀运输，从而可能避免对高配体浓度的需要。