A Joint Experimental and Computational Investigations of U(VI) Excited States in Uranyl Compounds Relevant to U(VI) Remediation At DOE Sites
EMSL Project ID
40074
Abstract
Actinides widely exist in natural resources such as mines, atmosphere, and underground water, and in man-made radioactive nuclear wastes from nuclear power plants and weapons production. Particularly, uranium is a major radioactive contaminant at many Department of Energy weapons production sites. Much effort has been made to understand the speciation and transport of uranium in subsurface environment in order to characterize, predict, and remediate uranium contamination at these sites. It is therefore essential to understand, at the molecular level, the interactions of actinides and various ligands to provide fundamental information on developing effective ways to handle the nuclear wastes and processing of the nuclear fuels. One challenge in the experimental investigation of actinide complexes is to determine their structures and stabilities in solutions. Fluorescence spectroscopy is particularly useful in providing that information because of its high spectral sensitivity and low requirement on the concentration of the sample. Our recent work on cryogenic laser-induced time-resolved U(VI) fluorescence spectroscopic studies has shown that under cryogenic conditions, the weak, poorly-resolved fluorescence spectra of many U(VI) species in contaminated natural sediments become highly-resolved. Furthermore, the spectral intensity is enhanced many orders of magnitude, allowing identification of U(VI) species in subsurface sediments by spectral comparison with those of pure minerals and/or aqueous U(VI) complexes. However, such spectral comparisons are solely based on the measured spectra of pure samples, which are often difficult to come by. There has been no theoretical prediction of U(VI) fluorescence spectra based on the compositions and structures to be compared against. To date, neither a theoretical basis nor a predictive framework exists to define the dependence of the fluorescence spectrum of a uranyl compound on its chemical or crystal structure. It is hence highly desirable to develop computational approaches that can model structures and stabilities of such complexes and simulate the luminescent spectra to offer insight into the correlation between the spectroscopic properties and its structural characteristics. In this project, we propose to use a joint experimental and computational investigations to explore U(VI) excited states and its fluorescence spectroscopic characteristics in uranyl compounds relevant to U(VI) remediation effort at US DOE sites including dicalcium uranyl-tricarbonate, triuranyl-diphosphate, boltwoodite, and uranyl difluoride. We'll collect high-resolution laser-induced time-resolved fluorescence spectra and absorption spectra (both UV-visible and infra-red regions) of the minerals; perform computational modeling of structures, stabilities and luminescent spectroscopic properties of the selected U(VI) minerals using relativistic quantum chemistry and the time-dependent spectroscopic theory to provide theoretical interpretation of the properties of the observed excited states and computational modeling and simulation of the experimental fluorescence spectra; understand how the bonding characteristics of the ground and excited states affect the spectroscopic properties of the excited states; and provide computational information and theoretical guidance on tuning the properties of the excited states of uranyl compounds by varying or optimizing the geometric structures of the uranyl compounds.
Project Details
Project type
Large-Scale EMSL Research
Start Date
2010-10-06
End Date
2013-09-30
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members
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