Evidence of Uranium and Associated Trace Element Mobilization and Retention Processes at Oklo (Gabon), a Naturally Radioactive Site

Abstract

The processes that affect the mobility of uranium and other radionuclides in the environment have been largely studied at both the laboratory and the field scales. The natural reactors found at the Oklo uranium mine in Gabon constitute a unique investigation setting as spontaneous fission reactions occurred two billion years ago. Oklo uraninites contain a large amount of other radionuclides as a result of the fission process. We have investigated the dissolution behavior of four uraninite samples from Oklo as a function of temperature (25 and 60 degrees C) and bicarbonate concentration (2.7-30 mmol/L). We have also investigated the dissolution behavior of minor components of the uraninites (i.e., Nd, Cs, Mo, Yb, and Sb) in relation to the dissolution of uranium. The results of the reported work are in good agreement with the kinetic rate laws derived from other uranium(IV) dioxide studies. Some of the minor components are found to be congruently released from the uraninite phase, while it is postulated that dissolution from segregated phases might affect the final concentrations of some of the rare earth elements, i.e., Nd and Yb. In addition, we have performed dissolution studies at 60 degrees C with two uraninites representative of different geochemical environments at Oklo, to study the uranium dissolution rates as a function of the temperature. This has allowed derivation of apparent activation energies for the bicarbonate-promoted oxidative dissolution of the Oklo uraninites. The dissolution behavior of the minor components of the uraninites at 60 degrees C was found to closely follow the behavior observed at 25 degrees C. This indicates that similar codissolution mechanisms operate in the temperature range studied. The implications for the mobility of uranium and other radionuclides in natural and anthropogenic environments are discussed.

Oxidation and dissolution of UO2 in bicarbonate media: Implications for the spent nuclear fuel oxidative dissolution mechanism

“…This mechanism explains most of the overall dissolution rates found in the literature, including some obtained using spent fuel [5] and natural uraninites [13,14].…”

Sorption of Uranyl Cations on a Rutile (001) Single Crystal Monitored by Surface Second-Harmonic Generation

“…Migration is a dynamic process that involves radionuclide partitioning between the mobile aqueous phase and solid phase through, for example, adsorption/absorption, solubility, or percolation steps . Retention time is influenced by the physicochemical properties and the chemical speciation of radionuclides in the aqueous phase as well as the mineral and organic composition of soils and sediments. − By adopting reductionism, a part of this complex problem can be addressed by choosing a representative radionuclide and studying its physicochemical behavior at the interface between the aqueous phase and a reference mineral surface. Information should be collected at both the macroscopic and the molecular levels to understand the factors that impact interfacial reaction efficiency, for instance, pH, redox potential and ionic strength of the aqueous phase, speciation of the radionuclide, and acidity of the sorption sites and their distribution on exposed crystallographic faces.…”

“…In the present study, uranium was selected as a radionuclide of environmental interest: it has a long half-life and remains stable in the ecosphere at various oxidation states, including uranyl species UO 2 2+ . , To understand uranyl adsorption behavior toward a mineral surface, rutile TiO 2 was selected as a reference oxide with surface chemistry that has been extensively studied and high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), X-ray standing wave (XSW) spectroscopy, scanning tunneling microscopy (STM), and atomic force microscopy (AFM).…”

Uranium and other trace elements’ distribution in Korean granite: implications for the influence of iron oxides on uranium migration

“…The strong association of U with secondary iron oxide minerals has been investigated at numerous U ore deposit sites (Koons et al 1980;Edghill 1991;Koppi et al 1996;Casas et al 2004;Noseck et al 2004;Payne and Airey 2006). An understanding of the interaction between U and iron oxides is very important for assessing the retardation capacity of geological systems into which U may be disposed.…”