Evidence of a Stable Uranyl Site in Ancient Organic-Rich Calcite

详细信息    查看全文

• 作者：Shelly D. Kelly ; E. Troy Rasbury ; Soma Chattopadhyay ; A. Jeremy Kropf ; and Kenneth M. Kemner
• 刊名：Environmental Science & Technology
• 出版年：2006
• 出版时间：April 1, 2006
• 年：2006
• 卷：40
• 期：7
• 页码：2262 - 2268
• 全文大小：360K
• 年卷期：v.40,no.7(April 1, 2006)
• ISSN：1520-5851

文摘

The mechanism of uranium (U) incorporation into calcite(calcium carbonate) is of fundamental importance to the fateand transport of U at the surface and in the shallowsubsurface and has implications for (a) the accuracy ofU-Pb and U-series isotope ratio methods used to determinethe ages of ancient deposits and (b) potential remediationstrategies based on sequestration of U in the subsurface.Extended X-ray absorption fine structure (EXAFS)spectroscopy is uniquely suited to the study of U-calcitesystems. The sensitivity of the EXAFS spectrum to thelocal atomic Ca coordination about U(VI) in the calcitestructure results in an increase in the number and amplitudeof Ca signals as the U(VI) becomes more ordered withinthe crystal structure. Our X-ray microprobe (10-m)measurements of an ancient 298 million-year-old organic-rich calcite (calcrete) clearly revealed three coordinationshells of Ca atoms, defining a well-ordered calcite structureabout uranyl to a distance of ~6.5 Å. These resultsindicate that uranyl is incorporated at the Ca²⁺ site incalcite and that the uranyl environment may evolve overlong time scales, becoming more calcite-like and more stablefor long-term sequestration of uranium. These resultstherefore validate U-related dating methods and show thatcalcite can be effective at sequestering U in vadosezone sediments.