Han Hannah Juan, Gysi Alexander P
New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA.
Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA.
Dalton Trans. 2024 Aug 6;53(31):13129-13141. doi: 10.1039/d4dt01515k.
The speciation and mobility of rare earth elements (REE) strongly depends on pH, which controls the formation of charged aqueous hydroxyl species. The latter potentially play an important role in controlling heavy REE adsorption on clay minerals in near-neutral to alkaline waters such as in regolith-hosted REE mineral deposits. However, accurate REE hydrolysis constants are needed for developing geochemical models that can predict the role of these charged species in natural systems. Here, we develop a robust experimental UV-Vis spectrophotometric method using -cresol purple to determine pH from 25 to 75 °C. This method is used to derive the average ligand number and hydrolysis constants of erbium (Er) at 25 °C in aqueous solutions with low ionic strength (≤0.001 mol L) at pH from ∼7 to 9.5 and in the presence of Er concentrations from 0 to 0.057 mM. The average ligand number ranges between 1 and 3 indicating that Er(OH), Er(OH) and Er(OH)03 control speciation in the experiments. The logarithm of the Er hydrolysis constants (, = 1 to 3) derived at infinite dilution for the reaction Er + HO = Er(OH) + H are: , , . Implementation of these experimental data into a geochemical model indicates that the Er(OH) and Er(OH)03 species are both stable in a much wider pH range than previously predicted. Consequently, the positively charged REE hydroxyl complexes can potentially control the fractionation of light heavy REE adsorption as observed in the formation of certain regolith-hosted REE deposits.
稀土元素（REE）的形态和迁移率很大程度上取决于pH值，pH值控制着带电荷的水合羟基物种的形成。后者可能在控制重稀土元素在近中性至碱性水体（如风化壳型稀土矿床中的水体）中对粘土矿物的吸附方面发挥重要作用。然而，要建立能够预测这些带电物种在自然系统中作用的地球化学模型，就需要准确的稀土元素水解常数。在此，我们开发了一种稳健的实验紫外可见分光光度法，使用间甲酚紫在25至75°C范围内测定pH值。该方法用于推导在25°C、低离子强度（≤0.001 mol/L）、pH值约为7至9.5且铒（Er）浓度为0至0.057 mM的水溶液中铒的平均配位数和水解常数。平均配位数在1到3之间，表明在实验中Er(OH)⁺、Er(OH)₂⁰和Er(OH)₃⁻控制着物种形态。在无限稀释条件下，反应Er³⁺ + H₂O = Er(OH)²⁺ + H⁺的铒水解常数（β₁、β₂、β₃）的对数分别为：β₁ = 10.68，β₂ = 8.75，β₃ = 6.58。将这些实验数据应用到地球化学模型中表明，Er(OH)²⁺和Er(OH)₃⁻物种在比之前预测的更宽的pH范围内都是稳定的。因此，带正电荷的稀土元素羟基络合物可能控制轻、重稀土元素吸附的分馏，正如在某些风化壳型稀土矿床的形成过程中所观察到的那样。
