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Redox reactivity of selenium(VI) in the presence of Fe(II) and S(-II) bearing mineral phases under the conditions of Callovo-Oxfordian pore water
With regards to CO2 emission, nuclear power plants are one of the promising sources of electricity production. However, the question of the safe long-term storage of the generated high-level nuclear waste (HLW) is still not resolved. Among the radionuclides present in HLW 79Se with a half-life period of 2.95 × 105 a [1] is presently considered as one key mobile fission product for the disposal of spent fuel (SF) and HLW and also the core of my research project. Selenium is present in five oxidation states in nature from -II, -I, 0, +IV and +VI. Solubility of selenium is largely controlled by its oxidation state, hence depends on the redox conditions present in soils, sediments and aquifers. The -II, -I and 0 oxidation states are commonly predominant in “reducing” anoxic environments, while the +IV and +VI states predominate in “oxidizing” environments [2].
Waste management agencies are therefore especially interested in the redox reactions that could lead to the reductive immobilization of oxidized (mobile) species. My work is framed within this context: I focus on the study of selenate [Se(VI)], which is an oxyanion that is typically adsorbed via the formation of outer-sphere complexes, and for which kinetic barriers for electron transfer could be high. The interactions with Fe(II) and S(–II)-bearing redox active solids- mediate the oxido-reduction kinetics of selenium oxyanions, playing an important role in the control of Se speciation [3]. Regarding Se(VI), it was found to be metastable (far from thermodynamic equilibrium) and to co-exist in different oxidation states in the Callovo-Oxfordian pore waters. The reduction of Se(VI) by magnetite, much slower than for Se(IV), has been described to include different steps (adsorption, reduction to Se(IV), and further reduction to less soluble Se phases), each of them imposing a kinetic barrier for the whole reduction process. At present it is not clear whether the Se(VI) initial adsorption or its reduction to Se(IV) are concomitant or not [5], and extra work needs to be done in this direction to establish the reduction pathway. Though it has been shown that Se(VI) can be reduced by Fe(II)-bearing solids [4,5] such as magnetite or pyrite, little is known about the potential competition with other environmental ions such as carbonate or sulfate. They have been shown to adsorb forming both outer and inner-sphere complexes, therefore potentially limiting the contact of Se(VI) molecules with the Fe(II)-bearing solids, and thus potentially inhibiting the electron transfer.
Hence, my research will mainly focus on the redox reactions and coupled precipitation of Se(VI) and Se(-II) under the conditions of the Callovo-Oxfordian pore water with the three minerals: i.e. magnetite, pyrite, and siderite under relevant conditions (neutral pH) and in the presence of other competitor ions like sulfates and carbonates.
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- Fernandez-Martinez, A. and Charlet, L., Rev. Environ. Sci. Biotechnol. (2009), 8, 81–110.
- Scheinost, A. C. et al., J Contam Hydrol (2008), 102, 228-45.
- Savoye, S. et al., Appl. Geochemistry (2021), 128, 104932.
- Poulain, A. et al., Environ. Sci. & Technol. (2022), 56(20), 14817-14827.
