The mobility of rare earth elements (REE) in natural hydrothermal systems can be assessed using geochemical modeling, which requires reliable thermodynamic data of relevant aqueous species. In this study, we evaluate the controls of pH and temperature on La speciation and the role of hydroxyl complexes in REE transport at hydrothermal conditions. Batch-type hydrothermal solubility experiments were conducted using synthetic La hydroxide powders equilibrated in perchloric acid-based aqueous solutions at temperatures between 150 and 250 °C and starting pH of 2 to 5. The La hydroxide solubility is retrograde with temperature and displays a strong pH dependence with a decrease in La concentrations from acidic to mildly acidic pH spanning between 3 and 5 orders of magnitude (e.g. log La molality of −2.5 to −7.2 at 250 °C). Thermodynamic optimizations using GEMSFITS allow to retrieve the standard partial molal Gibbs energies for the La3+ aqua ion and the formation constants for the La hydroxyl species (i.e., LaOH2+, La(OH)2+, La(OH)30) between 25 and 250 °C. A comparison between the experimentally derived thermodynamic properties with the calculated values from the Helgeson-Kirkham-Flowers equation of state parameters indicates an increased divergence with temperature. Discrepancies in standard partial molal Gibbs energies range between ∼ 1 − 12 kJ/mol and result in a predicted La hydroxide solubility differing by up to 3 orders of magnitude at 250 °C. Speciation calculations indicate a higher stability of La3+ and LaOH2+ over the other La hydroxyl species in the studied pH range of 3.4 to 6. The optimized thermodynamic properties for La aqueous species have important implications for modeling the solubility of REE minerals such as monazite and the mobility of REE in hydrothermal systems.

中文翻译：

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氢氧化物 La 的溶解度以及 La3+ 和 La 羟基络合物在 25 °C 至 250 °C 酸性至弱酸性 pH 条件下的稳定性


稀土元素 （REE） 在天然热液系统中的迁移率可以使用地球化学建模进行评估，这需要相关水物质的可靠热力学数据。在这项研究中，我们评估了 pH 值和温度对 La 形态的控制以及羟基络合物在热液条件下 REE 转运中的作用。使用合成氢氧化铉粉末进行间歇式水热溶解度实验，该粉末在温度为 150 至 250 °C 且起始 pH 值为 2 至 5 的高氯酸基水溶液中平衡。氢氧化物 La 的溶解度随温度逆行，并表现出很强的 pH 依赖性，随着 La 浓度从酸性降低到弱酸性 pH 值，范围在 3 到 5 个数量级之间（e.g. log 250 °C 时 La 摩尔浓度为 -2.5 至 -7.2）。使用 GEMSFITS 的热力学优化允许在 25 至 250 °C 之间检索 La3+ 水离子的标准偏摩尔吉布斯能量和 La 羟基物质（即 LaOH2+、La（OH）2+、La（OH）30）的形成常数。 将实验得出的热力学性质与 Helgeson-Kirkham-Flowers 状态参数方程的计算值进行比较，表明随温度的变化，发散增加。标准偏摩尔吉布斯能量的差异范围在 ∼ 1 − 12 kJ/mol 之间，导致预测的氢氧化物 La 溶解度在 250 °C 时相差多达 3 个数量级。 形态计算表明，在所研究的 pH 值为 3.4 至 6 的范围内，La3+ 和 LaOH2+ 的稳定性高于其他 La 羟基物质。 La 水物质的优化热力学性质对于模拟 REE 矿物（如独居石）的溶解度和 REE 在热液系统中的流动性具有重要意义。