Collaborative Research: Crystal Chemistry of U, Th and Other Radionuclides in Apatite: Environmental and Geochemical Implications

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Apatite, Ca10(PO4)6(F,OH,Cl)2, can accommodate numerous substituents, including many radionuclides of environmental concern (90Sr, 90Y, REE, U and Th). Scores of studies have focused on the crystal chemistry of substituents in apatite; surprisingly, however, little is known about the mechanisms of incorporation and the structural response of apatite to U and Th, despite the fact that the presence of these elements in this mineral has been used in geochronologic and petrogenetic studies for decades. This two-part study focuses on fundamental mineralogical and geochemical aspects of U and Th incorporation in apatite and implications for apatite as a solid nuclear-waste form and a metal sequestration agent.Interest in apatite as a potential solid waste form for radionuclides is based on: 1) its high affinity for elements of environmental concern; 2) its thermal annealing properties; and 3) its relatively low solubility in most surface environments. Fundamental to our understanding of radionuclide retention and release are crystal chemical parameters such as site preference, oxidation state, and structural distortions/symmetry-breaking created by substituents such as U and Th; surprisingly, the crystal chemistry of their substitution in apatite is unknown. Part I of this study is to determine these structural parameters in a variety of natural and synthetic apatites with substituent U and Th. This will be accomplished through complementary use of crystal synthesis, single crystal X-ray diffraction, and X-ray absorption spectroscopy.Part II of this study focuses on the role of precursor calcium phosphate phases on the uptake of U, Th and other metals and the fate of these contaminants through structural transformations in these sequestered states. Use of apatite formation in contaminated sediments (sometimes called phosphate-induced metal stabilization, PIMS) is a new and promising method for metal sequestration (including radionuclides) and environmental remediation. Numerous experimental studies of apatite formation under the temperature and pH conditions found in sediments and soils indicate the formation of precursor phases such as OCP (octacalcium phosphate) and brushite is essential to the process. This fundamental aspect of apatite formation under surface conditions has not been addressed in the context of metal sequestration and fate. This has critical bearing on the effective use of apatite for metal immobilization and our understanding of the role of phosphate in the global geochemical cycling of heavy metals. Part II of the study will be accomplished through a combination of 1) low-temperature synthesis in the presence of U, Th and other metals with in situ (time-resolved synchrotron X-ray diffraction) and 2) ex situ chemical and structural analyses including Rietveld structure analysis and X-ray absorption spectroscopy.This work has broad environmental implications (heavy-metal and radionuclide sequestration) that are of immense importance to a society that generates radioactive waste. The research will include the participation and education of undergraduate and graduate students, as well as support the ongoing research of a Postdoctoral Fellow at Miami University.