Spectroscopy Technique Studies Uranium
Developing effective hazard control and remediation programs to deal with uranium in the environment requires an understanding of how the substrate interacts with surface water, groundwater, soil and minerals in the subsurface zone. To this end, researchers at Pacific Northwest Nuclear Laboratory in Richland, Wash., and at the University of Washington in Seattle are using time-resolved laser-induced fluorescence spectroscopy.
The cooling water used in nuclear power plants contains low-level radioactive waste, including traces of uranium. Other major sources of uranium in the environment include nuclear waste, depleted uranium ammunition and the element’s naturally occurring form leaching into water.
Although its radioactivity does not pose the concerns of that from some nuclear waste products and from naturally occurring radionuclides such as radon, uranium is toxic, and the uranyl complexes that it forms do present a chemical and radiological hazard. Uranyl may be tightly bound to the core of a mineral host or loosely bound to its surface. The character of the adsorbed state has implications for remediation treatment.
Gregory V. Korshin, an associate professor in the department of civil and environmental engineering at the university, noted that the low molar absorbance coefficients of uranyl species make traditional absorption spectroscopy unsuited for the work. In contrast, time-resolved laser-induced fluorescence spectroscopy is very sensitive to the presence of uranyl, particularly in cryogenic conditions, and thus provides uniquely detailed information about its environmental chemistry.
In recent experiments, the scientists explored the method’s sensitivity to the structure of the chemical complexes that uranyl and its mineral hosts form. Uranyl samples were adsorbed into gibbsite, which is representative of minerals that control the subsurface mobility of uranium. Suspensions of uranyl-containing gibbsite in NaClO4 were prepared at various pH levels and NaClO4 ionic strengths to represent the variety of conditions present in a cooling pond environment.
The scientists excited the first strong absorption band of the uranyl ion at 415 nm with a Spectra-Physics Mopo-730 optical parametric oscillator. An Acton Research SpectraPro 300i double-monochromator spectrograph collected the resulting fluorescence at 90° to the excitation beam, and a Princeton Instruments PI-Max time-gated CCD camera, which was protected by a 460-nm high-pass cutoff filter, collected the spectra.
Using a 100-µs acquisition gate with delay times of 205 to 1205 µs, the researchers produced a series of spectra illustrative of the time behavior of the vibronic excitation of the linear uranyl molecule, which is strongly influenced by the environment. They maintained the samples at a temperature of 5 K during the measurement runs.
They analyzed the spectra using evolving factor analysis to determine the contributions of dissimilar emitting complexes. The changes in the time-resolved laser-induced fluorescence spectroscopy spectra could be explained by the presence of four uranyl surface complexes, each characterized by a distinct vibronic sequence and specific response to variations in pH and ionic strength.
The insights gained from the investigation will help adjust the models of transport of radionuclides generated in nuclear waste. Now that the technique has been proved valid in a relatively simple chemical system, it can be applied to other situations representative of the waste environment.
Environmental Science & Technology, online Jan. 14, 2006, doi:10.1021/ es051714i.
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