Heavy Element Photophysics and Photochemistry

Speciation of Electronically Excited Uranyl Fluorides

Observation of unusually long-lived luminescence in studies of hydrolyzed UF₆gas prompted us to begin an effort to speciate electronically excited uranyl in appreciably acid solutions containing high-total-fluoride concentration. Our work has shown that aqueous HF solutions of uranyl fluoride can be appreciably and visibly luminescent. The figure shows the characteristic yellow-green luminescent glow of uranyl fluoride complexes when a sample cell containing uranyl in hydrofluoric acid is illuminated by ultraviolet light. (Click on the image for more information.)

Speciation consists of determining the number and nature of chemical complexes of metal ions in solution. Our investigation has provided the first detailed speciation of electronically excited uranyl fluoride complexes. Over a broad range of acidity and fluoride ion concentration, equilibrium among these complexes is achieved on the luminescence time scale such that most of the observed luminescence arises from a single uranyl fluoride complex that has been identified as UO₂F₂ in modeling studies. This is a surprising conclusion because it is known from work by others that UO₂F₂in its ground electronic state is the predominant uranyl species over only a small portion of the free fluoride ion concentration, [F ^-], that we studied. For ground-state complexes, at lower [F ^-], UO₂F ⁺is the dominant complex and, at higher [F ^-], UO₂F₃^-and then UO₂F₄^2-are the major uranyl species. Our luminescence decay rate data as a function of added sodium fluoride (to vary the free fluoride ion concentration) are shown in the image. (Click on the image for additional experimental details.)

Based on our work, most luminescence in acidic HF solutions of uranyl fluorides arises from electronically excited UO₂F₂and the luminescence lifetime of UO₂F₂is nearly 100-fold longer than that of aquated UO₂²⁺. For these reasons, luminescence detection of uranyl fluorides is potentially a rapid, high-sensitivity, signature method for early detection of leaking UF₆cylinders. We have received Laboratory Directed Research and Development funding to assess the feasibility of this novel approach to monitoring the condition of stored UF₆cylinders. The DOE currently stores ~560,000 metric tons of UF₆in ~46,500 cylinders, seven of which are known to have leaked. See "Uranyl Fluoride Luminescence and DUF₆" for additional information about stored UF₆.

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