title: Study of uranium oxidation states in geological material.
abstract: A wet chemical method to determine uranium (U) oxidation states in geological material has been developed and tested. The problem faced in oxidation state determinations with wet chemical methods is that U redox state may change when extracted from the sample material, thereby leading to erroneous results. In order to quantify and monitor U redox behavior during the acidic extraction in the procedure, an analysis of added isotopic redox tracers, (236)U(VI) and (232)U(IV), and of variations in natural uranium isotope ratio ((234)U/(238)U) of indigenous U(IV) and U(VI) fractions was performed. Two sample materials with varying redox activity, U bearing rock and U-rich clayey lignite sediment, were used for the tests. The Fe(II)/Fe(III) redox-pair of the mineral phases was postulated as a potentially disturbing redox agent. The impact of Fe(III) on U was studied by reducing Fe(III) with ascorbic acid, which was added to the extraction solution. We observed that ascorbic acid protected most of the U from oxidation. The measured (234)U/(238)U ratio in U(IV) and U(VI) fractions in the sediment samples provided a unique tool to quantify U oxidation caused by Fe(III). Annealing (sample heating) to temperatures above 500 °C was supposed to heal ionizing radiation induced defects in the material that can disturb U redox state during extraction. Good agreement between two independent methods was obtained for DL-1a material: an average 38% of U(IV) determined by redox tracer corrected wet chemistry and 45% for XANES.
authors.affiliation: Saint-Petersburg State University, Department of Chemistry, Laboratory of Radiochemistry (RU), Russian Federation. ivan.pidchenko@gmail.com
authors.forename: I
authors.initials: I
authors.lastname: Pidchenko
authors.forename: S
authors.initials: S
authors.lastname: Salminen-Paatero
authors.forename: J
authors.initials: J
authors.lastname: Rothe
authors.forename: J
authors.initials: J
authors.lastname: Suksi
mesh_terms: D019015:Geologic Sediments / Q000737:chemistry*; D006358:Hot Temperature; D007501:Iron / Q000737:chemistry; D008903:Minerals / Q000737:chemistry; D010084:Oxidation-Reduction; D011839:Radiation, Ionizing; D014501:Uranium / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy
chemical_list: D008903:Minerals; D014501:Uranium; D007501:Iron
keywords: Extraction; U oxidation states; Wet chemistry; XANES