Catherine Beaucaire

Time-Resolved Laser-Induced Fluorescence Studies of Uranium/Sodium Dodecyl Sulfate Interactions

Time-Resolved Laser-Induced Spectrofluorometry (TRLIS) is a very sensitive and selective technique for uranium(VI) (UO22+) determination based on laser excitation followed by temporal resolution of the fluorescence signal. This analytical technique has been used for several years for uranium analysis in various matrices in the nuclear fuel cycle, in the environment, and in the medical field. Limits of detection (LODs) for uranium are now lower than the ng/L range, and trends are leading towards direct analysis for on-line analysis via fiber optics and optode in the above fields. However, aside from elementary analysis, TRLIS possesses an interesting characteristic, namely, its capability of giving both spectral information with the fluorescence spectrum (wavelength shifts, peaks ratio modifications) and temporal information with the fluorescence lifetime (characterization of the molecular environment). These features have been used to study the interaction (complexation, quenching, energy transfer) between (UO22+) and ions(Fe2+, Ag+, Cl−, etc.) or organic molecules, but very few investigations have been carried out with surfactants and uranium, Surfactants are organic compounds which possess both a polar or ionic head group and a nonpolar tail. Depending upon the nature of the head group, they can be classified as cationic, anionic, or nonionic surfactants. When the surfactant is present at concentrations greater than its critical micellar concentration (CMC), aggregates called micelles are formed, into which organic compounds are solubilized. Micelles composed of ionic surfactants exhibit a high surface charge, and counter ions (ions of opposite charge to the surfactant) will bind to the surface of the aggregate.

Determination of Uranium source term in a Polluted Site: a multitechnique study

This study aims at understanding a potential migration of uranium in a polluted calcareous peat-land. Aqueous uranium monitoring indicates a seasonal variation of uranium content. During winter, aqueous uranium was found to be mostly under the +VI oxidation state, which is potentially mobile, while it was reduced into less soluble U(IV) species in summer. These observations were correlated with changes of the redox potential due to the activity of sulphate-reducing bacteria. A companion study (Phrommavanh et al., 2008 in this congress) focuses on the determination of the uranium aqueous speciation. Concurrently, one has to know the origin of the source term which is a part of the understanding of the uranium mobility. Several techniques were used in order to discriminate the chemical form of uranium: gamma spectrometry and aqua regia extractions on soil samples, porous PTFE quartz cells in order to sample the aqueous fraction, SEM observations with automatic U particles detection and XANES to assess the U redox state. The association of these different techniques gave consistent results with an increasing U content at a depth of 0.8 m. Such a localisation is explained by a redox front where anoxic and reducing conditions prevail.

Speciation of Uranium in a Polluted Site: a TRLFS study

The uranium speciation in a polluted soil was investigated to assess the possible impact on geo- and biosphere. During winter, uranium was found to be mostly under the +VI oxidation state, which is potentially mobile, while it was reduced into less soluble U(IV) species in summer. These observations were correlated with changes of the redox potential due to the activity of sulphate-reducing bacteria. The potential migration of uranium is also highly dependent on its aqueous speciation, particularly concerning U(VI). In this work, the speciation of U(VI) was investigated in real samples by using time-resolved laser-induced fluorescence spectroscopy (TRLFS) and speciation calculations. We show that U(VI) speciation can be determined in carbonate-rich solutions and at ambient temperature by using TRLFS on chemically-treated samples, to remove U(VI)-fluorescence-quenching molecules such as organics.