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

合作研究：磷灰石中 U、Th 和其他放射性核素的晶体化学：环境和地球化学影响

• 批准号: 0409435
• 负责人: John Rakovan
• 金额: $ 20万
• 依托单位: Miami University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2004
• 资助国家: 美国
• 起止时间: 2004-07-01 至 2008-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0409435&HistoricalAwards=false
• 关键词: Collaborative; Research; Crystal; Chemistry; Th

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.

磷灰石，Ca 10（PO 4）6（F，OH，Cl）2，可以容纳许多取代基，包括许多环境关注的放射性核素（90 Sr，90 Y，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，很少有人知道磷灰石对U和Th的掺入机制和结构反应，尽管这些元素在这种矿物中的存在已经被用于地质年代学和岩石成因研究几十年。本研究分两部分，主要研究了磷灰石中U和Th掺入的基本矿物学和地球化学特征，以及磷灰石作为固体核废物和金属螯合剂的意义。和3）其在大多数表面环境中的相对低的溶解度。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）产生的结构扭曲/破坏;令人惊讶的是，磷灰石中它们取代的晶体化学是未知的。本研究的第一部分是确定这些结构参数在各种天然和合成磷灰石与取代基U和Th。这将是通过补充使用晶体合成，单晶X射线衍射，和X射线吸收spectroscopy.Part II的本研究的重点是前体磷酸钙相的作用上的U，Th和其他金属的吸收和这些污染物的命运，通过在这些螯合状态的结构转变。在污染沉积物中形成磷灰石（有时称为磷酸盐诱导金属稳定化，PIMS）是一种新的和有前途的金属螯合（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温度和pH条件下磷灰石形成的大量实验研究表明，OCP（磷酸八钙）和透钙磷石等前体相的形成对该过程至关重要。在表面条件下磷灰石形成的这一基本方面尚未在金属螯合和归宿的背景下得到解决。这对磷灰石金属固定的有效利用和我们对磷酸盐在全球重金属地球化学循环中的作用的理解具有重要意义。研究的第二部分将通过1）在U存在下的低温合成，Th和其他金属与原位（时间分辨同步辐射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。这项工作具有广泛的环境意义（重金属和放射性核素螯合），这对产生放射性废物的社会极为重要。该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博士后研究员正在进行的研究。