Heavy Element Photophysics and Photochemistry

Assessing the Degree of Photocatalytic Destruction of Organic Complexants using Laser-Induced Fluorescence

Our work has shown that europium can play a dual role by acting both as a photocatalyst for complexant destruction and, using laser-induced fluorescence, a basis for assessing the degree of such destruction achieved by photolysis. The complexant used in this case was tetrahydrofurantetracarboxylic acid (THFTCA). THFTCA possess novel properties that would be useful inseparating nuclear waste streams, such as those likely to be generated when high level waste is processed at U. S. Department of Energy production facilities. However, use of THFTCA in nuclear waste separation requires that a means be found to convert it into benign products, such as carbon dioxide and water, to avoid generation of additional hazardous waste. The difficulty in destroying THFTCA arises from its extremely oxidation-resistant character. For example, it resists chemical attack by boiling nitric acid. We have shown that europium can photocatalytically to destroy the complexing power of THFTCA. Europium is a heavy element ion often present in nuclear waste because it is a fission product. Because europium acts as a photocatalyst, only a small amount of it, relative to THFTCA, is required. The figure illustrates the use laser-induced fluorescence on europium to assess the degree of photocatalytic destruction of THFTCA.

The data shown in the figure provide an example of assessing the degree of photocatalytic destruction of THFTCA via europium fluorescence monitoring. The blue symbols in section a of the figure are observed data from a solution that contained Eu³⁺ ions and THFTCA (note the logarithmic intensity scale). In each of the three cases shown (a, b, and c), the solid gray line is the result of fitting a single exponential decay model to the observed data.

The red symbols in section b of the figure are data recorded after a few hours of near-ultraviolet photolysis of the solution that had been used to record the data shown in section a. This faster, but still single exponential, observed fluorescence decay following photolysis, along with excitation and emission spectra, provide evidence that a brief ultraviolet photolysis period has converted essentially all of the THFTCA into compounds that are much weaker complexants than is THFTCA. This degree of destruction is likely to be sufficient to enable use of THFTCA in processing nuclear waste streams. Complete destruction of THFTCA to carbon dioxide and water would result in observed fluorescence decays comparable to that shown by the green symbols in section c of the figure. This data shows the fluorescence decay of Eu³⁺ ions whose inner coordination sphere contains only water molecules.

Data shown in the figure were recorded using a pulsed laser whose wavelength was tuned to 395 nm as the fluorescence excitation source. Both complexed and uncomplexed Eu³⁺ ions absorb light at 395 nm. Following absorption of laser light, excited Eu³⁺ ions lose energy ("relax") within a few millionths of a second to the lower-lying, visible emitting ⁵D₀ state. Optical filters were used to select characteristic reddish-orange emission from this state. The ⁵D₀ state is an electronically excited 4f electron state of Eu³⁺ whose purely radiative decay rate is on the order of 100 s^-1 because emission from it to all lower lying states is parity forbidden. Because the radiative decay rate of this state is so slow, many nonradiative decay processes, such as conversion of electronic energy into vibrational excitation of surrounding ligands, can be quantified via fluorescence decay rate measurements. We used a cooled photomultiplier and a signal averaging digital storage oscilloscope to measure fluorescence decay data. The data shown are in "arbitrary" units which means, in this case, that multiplying the data by a constant would convert it into units such as "photons emitted per unit time." While it is, in principle, possible to accurately determine the value of this constant, for our purposes it is neither necessary nor of value to do so.

In summary, in the case of THFTCA, we have shown that photocatalytic destruction of organic complexants in aqueous solution is readily achieved using europium ions and near-ultraviolet photolysis. Our work has provided a new option for processing nuclear waste and expands fundamental knowledge of the excited state properties of heavy element complexes in solution.

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