The Effect of Redox Mechanisms on the Fractionation of Uranium `Stable' Isotopes
Abstract
Uranium is the heaviest naturally occurring element. It consists of three natural isotopes, 238U, 235U and 234U and has four oxidation states, U(III)-U(VI), of which only U(IV) and U(VI) are common. In the oxidized, hexavalent condition, U(VI) exists as uranyl ion (UO22+), which forms soluble, non-reactive complexes with carbonates. In the reduced condition, tetravalent U(IV) forms immobile minerals with hydroxides, fluorides and phosphates that are removed from the water column. In the recent years, isotopic fractionation between 235U and 238U has been detected in a range of terrestrial environments. Changes to U concentrations in natural waters occur due to biological uptake, adsorption/desorption to/from particulates and surfaces, diffusion into sediments, and chemically and biologically induced redox mechanisms (Swarzenski et al., 1999, Mar. Chem. 67, 181). The largest isotopic shifts evidently occur during the reduction of U(VI) to U(IV) in waters, during which precipitation changes the dissolved U concentration. However, the exact mechanisms controlling 235U/238U fractionation remain unclear. Some results implicate mass-dependent zero-point energy effects (preferential removal of 235U over 238U) (Rademacher et al., 2006, Environ. Sci. Technol. 40, 6943) as the main cause for the isotopic variations measured, while others suggest volume-dependent nuclear field shift effects (preferential removal of 238U over 235U) (Weyer et al., 2008, GCA 72, 345, Bopp et al., 2010, Environ. Sci. Technol. 44, 5927), which are predicted to be of the opposite sign and up to three times larger than mass-dependent effects. In this study, profiles of two different water masses were examined for their uranium concentration and their 235U/238U isotopic composition to determine the magnitude of the natural isotopic shifts as well as their origins. One set of samples was collected from a 160 m depth profile in the Framvaren Fjord, an anoxic basin. In this basin, the biogeochemical cycling of U is strongly controlled by U(VI)-U(IV) redox transformations, and our measurements allow the investigation of the potential of U as a tracer of paleo-anoxic events. A second set of samples was collected along a flow path of the regional aquifer in the Mojave Desert. Variations in these samples are expected due to adsorption of U onto solids, affecting U mobility, which can be used to predict the migration of actinides polluting groundwater. A suite of samples from each profile was analysed using a Nu Instruments Nu Plasma multiple-collector ICPMS (MC-ICPMS) using a 233U/236U double spike to monitor instrumental mass fractionation. The 235U/238U variability shown by the analyzed samples is ca. 0.1 %, and is more than a factor of 10 larger than the analytical precision of ±0.006 % (2σ ).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.V21B2335K
- Keywords:
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- 1040 GEOCHEMISTRY / Radiogenic isotope geochemistry;
- 4802 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Anoxic environments;
- 4870 OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL / Stable isotopes