Investigations in the Fe – S – Se system

N. Finck^1),*), E. Alekseenko²⁾ and D. Bosbach^3),*)

¹⁾ Karlsruhe Research Centre (FZK), Institute for Nuclear Waste Disposal (INE), P.O. Box 3640, D-76021 Karlsruhe, Germany (Nicolas.finck@ine.fzk.de).

²⁾ MSU, Faculty of Materials Science, GSP-3, Leninskiye Gory, Moscow, 119899, Russian Federation.

³⁾ Forschungszentrum Jülich, Institute für Energieforschung 6, D-52425 Jülich, Germany.

^*) Helmholtz Virtual Institute „Advanced Solid-Aqueous RadioGeochemistry“, Germany.

Motivation

The long-lived fission product ⁷⁹Se (half-life >10⁵ yrs) is one of the elements of concern for the safe storage of High-Level nuclear Waste (HLW). The chemistry of Se resembles that of sulfur and the Se solubility is largely controlled by its oxidation state. The +VI and +IV oxidation states prevail as mobile aqueous oxyanions, while Se in the oxidation states 0, -I and –II prevail as solids with low solubility [1]. The geochemistry of Se is largely controlled by that of Fe, with which Se is closely affiliated [2]. The redox potential (reducing conditions) of most rock formations currently considered for the HLW disposal is controlled by pyrite (FeS₂). FeS₂ is the thermodynamically stable end product of iron compounds under reducing conditions, and can be produced from iron monosulfide precursor (FeS). This work presents the first results of the interaction between Se and FeS.

Experimental - results

Se(IV) ions were added to a suspension of FeS_am (disordered mackinawite) and allowed to react for seven days. In a separate experiment, a Fe(II) solution was added to a solution containing S(-II) and Se(-II) ions and allowed to react overnight. These experiments aimed at simulating Se retention on pre-existing mackinawite and mackinawite formation in the presence of selenide ions, respectively. Both samples were characterized using X-ray absorption spectroscopy (XAS) at the Se K-edge. The XANES region provided information on the Se oxidation state and the EXAFS region on short-range structure.

In the XANES region, both the edge position and the relative intensity of the white line [3] suggest a lower formal Se oxidation state in the co-precipitation experiment (-II) than in the adsorption experiment (-I or 0). Analysis of the EXAFS data indicates the presence of Se neighbors at 2.34(2) Å in the adsorption experiment. XANES results combined with EXAFS data strongly suggests a reduction and precipitation of a solid Se(0) phase upon Se(IV) adsorption on FeS_am, corroborating previous findings [3]. In contrast, the presence of one Fe (interatomic Se-Fe distance = 2.37(1) Å) and two S shells surrounding Se upon co-precipitation are found in EXAFS data modeling. The detection of Fe and S backscatterers associated with the low Se oxidation state may indicate that Se is located in a mackinawite-like environment. Upcoming work will focus on the Se co-precipitation with pyrite. These results will provide valuable information of the Se retention by iron sulfides.

Acknowledgements

N. F. acknowledges the Helmholtz Virtual Institute “Advanced Solid-Aqueous Radiogeochemistry” (Germany) for funding. E. A. thanks the Helmholtz-Russia Joint Research Group 011 for funding. ANKA is thanked for provision of beamtime and the INE-beamline staff for assistance during the experiments.

References

[1] Elrashidi M.A., Adriano D.C., Workman S.M. and Lindsay W.L. (1987) Soil Sci. 144, 141-152.

[2] Hatten Howard III J. (1977) Geochim. Cosmochim. Acta 41, 1665-1678.

[3] Scheinost A.C. and Charlet L. (2008) Environ. Sci. Technol. 42, 1984-1989.