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

Missouri State University, Springfield MO

Investigators

Abstract

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.