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

目前还缺乏对稀土元素离子与地壳流体中常见的配体(H2O、Cl-、F-、OH-等)相互作用的详细了解，这些配体涵盖代表热液条件的各种压力和温度范围。这些知识对于全面评估配体与稀土离子的缔合程度和生成的水溶液离子络合物的稳定性至关重要，这些离子络合物在热液成因的矿床中对稀土的迁移、分馏和矿化起着直接作用。我们对这种地质过程缺乏了解，在很大程度上是因为在涵盖广泛温度和压力范围的热液中，关于稀土水合物的结构和成键性质的实验数据不足。本项目利用同步辐射X射线吸收精细结构(XAFS)技术和热液金刚石顶压室，对水热条件下含稀土水溶液的结构性质进行了实验研究。稀土水离子和氯络合物的结构和相关性质将通过详细分析在温度高达500摄氏度和压力高达1 Gpa的含稀土热液中测量的XAFS光谱来确定。系统地研究了均匀的三价电荷、随原子序数增加而减小的离子半径、可能的4f电子壳层填充对稀土水合和氯络合物结构的影响。这些结果将用于检验已建立的稀土水合离子络合物稳定性的热力学计算，并作为未来热液中稀土元素模拟的基础。由于化学成分相似，预计这一项目的成果将用于解决放射性废物储存库中鳗系元素的去向和运输问题。该项目包括在同步加速器设施内对学生进行科学培训。

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.