X-Ray Spectroscopic Studies of the Structure of Rare Earth Element Aqueous Ion Complexes under Hydrothermal Conditions

目前还缺乏对地壳流体中常见的配体（H2O、Cl-、F-、OH-等）与稀土元素（REE）离子相互作用的详细了解，包括热液条件下的一系列压力和温度。这些知识对于全面评估配体与稀土离子的结合程度以及由此产生的水离子络合物的稳定性至关重要，而水离子络合物在热液矿床中稀土的运输、分馏和矿化中起着直接作用。我们对这种地质过程缺乏了解，在很大程度上是由于水热溶液中稀土水络合物的结构和键合性质的实验数据不足，涵盖了广泛的温度和压力范围。本项目采用同步x射线吸收精细结构（XAFS）技术和热液金刚石砧池对水热条件下含稀土水溶液的结构特性进行了实验研究。稀土水热溶液在温度高达500℃，压力高达1 GPa下测量的XAFS光谱将通过详细分析确定稀土水离子和氯配合物的结构和相关性质。系统地研究了均匀三价电荷、随着原子序数的增加而减小的离子半径，以及可能在系列的很大一部分上填充4f电子壳层对稀土水、氯配合物结构的影响。该结果将用于测试稀土水离子络合物稳定性的既定热力学计算，并作为未来水热溶液中稀土模型的基础。由于化学性质相似，预计该项目的结果将用于解决放射性废物贮存库中锕系元素的命运和运输问题。该项目包括在同步加速器设施内对学生进行科学训练。

A detailed understanding of the interaction of rare earth element (REE) ions with ligands common in crustal fluids (H2O, Cl-, F-, OH- and others), covering a range of pressures and temperatures representative of hydrothermal conditions, is presently lacking. Such knowledge is critical for a full assessment of the degree of association of ligands with REE cations and of the stability of the resultant aqueous ion complexes, which play a direct role in the transport, fractionation, and mineralization of REE in deposits of hydrothermal origin. Our lack of understanding of such geologic processes is in large measure due to insufficient experimental data on the structure and bonding properties of aqueous REE complexes in hydrothermal solutions, covering a broad range of temperatures and pressures. This project involves experimental investigations of the structure properties of REE bearing aqueous solutions under hydrothermal conditions using the synchrotron x-ray absorption fine structure (XAFS) technique and the hydrothermal diamond anvil cell. The structure and related properties of REE aquo ion and chloro complexes will be determined from a detailed analysis of the XAFS spectra measured from REE bearing hydrothermal solutions at temperatures up to 500 degrees C and pressures up to 1 GPa. A systematic study of the effects of uniform trivalent charge, decreasing ionic radius with increasing atomic number, and possibly of 4f electron shell filling across a significant portion of the series on the structure of REE aquo and chloro complexes will be accomplished. The results will serve to test established thermodynamic calculations of the stability of REE aqueous ion complexes and as a basis for future modeling of REE in hydrothermal solutions. Because of similar chemistry, it is anticipated that the results from this project will be used to address issues of fate and transport of actinides in radioactive waste repositories. This project includes the scientific training of students within synchrotron facilities.