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

磷灰石，Ca10(PO4)6(F，OH，Cl)2，可以容纳许多取代基，包括许多与环境有关的放射性核素(90Sr，90Y，REE，U和Th)。数十项研究集中在磷灰石中取代基的晶体化学；然而，令人惊讶的是，人们对磷灰石的掺入机制和对U和Th的结构响应知之甚少，尽管这种矿物中这些元素的存在已经被用于地质年代学和岩石成因研究数十年。这项由两部分组成的研究集中在磷灰石中U和Th的基本矿物学和地球化学方面，以及磷灰石作为固体核废料和金属固着剂的意义。人们对磷灰石作为潜在的放射性核素固体废料的兴趣基于：1)它对环境关注的元素的高亲和力；2)它的热退火特性；3)它在大多数表面环境中的相对较低的溶解度。我们理解放射性核素保留和释放的基础是晶体化学参数，如位置偏好、氧化态和取代基如U和Th造成的结构扭曲/对称性破坏；令人惊讶的是，它们在磷灰石中取代的晶体化学尚不清楚。本研究的第一部分是测定各种天然和合成的具有U和Th取代基的磷灰石的结构参数。这将通过晶体合成、单晶X射线衍射和X射线吸收光谱的互补使用来实现。本研究的第二部分重点讨论了前驱体钙磷酸盐相在吸收U、Th和其他金属方面的作用，以及这些污染物在这些隔离状态下通过结构转换的去向。利用污染沉积物中的磷灰石形成(有时称为磷酸盐诱导的金属稳定化，PIMS)是一种新的、有前途的金属封存(包括放射性核素)和环境修复方法。在沉积物和土壤中发现的在温度和pH条件下形成磷灰石的大量实验研究表明，OCP(八钙磷酸盐)和刷石等前体相的形成是这一过程的关键。表面条件下磷灰石形成的这一基本方面还没有在金属隔离和命运的背景下讨论。这对有效利用磷灰石进行金属固定化和理解磷酸盐在全球重金属地球化学循环中的作用具有重要意义。研究的第二部分将通过1)U、Th和其他金属存在下的低温原位合成(时间分辨同步X射线衍射)和2)非原位化学和结构分析(包括Rietveld结构分析和X射线吸收光谱)相结合来完成。这项工作具有广泛的环境影响(重金属和放射性核素隔离)，对产生放射性废物的社会非常重要。这项研究将包括本科生和研究生的参与和教育，以及支持迈阿密大学一名博士后研究员正在进行的研究。

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.