Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Environmental and health hazards of the trace element Se are mainly related to the presence of highly mobile Se oxyanion species (oxidation states +4 and +6). In this study, we investigate the immobilization of dissolved Se oxyanions during the formation process of magnetite by the progressive oxidation of an alkaline, anoxic Fe2+ system (pH 9.2). Up to initial concentrations of c(Se)0 = 10-3 mol/L (m/V ratio = 3.4 g/L), logRd values of xxxx demonstrate a strong retention of Se oxyanions during this mineral formation process. This Se immobilization is due to the reduction of Se(IV) or Se(VI), resulting in the precipitation of sparingly soluble Se compounds. By XRD analysis, these Se compounds were identified as crystalline elemental Se(0) that occurred in all coprecipitation products after the completed magnetite formation. The time-resolved analysis of the Se retention during the magnetite formation and detailed spectroscopic analyses (XPS, XAS) of the involved solid phases showed that the reduction takes place under the anoxic conditions in the early phase of the coprecipitation process by the interaction with iron(II) hydroxide and green rust. Both minerals represent the primary Fe(II)-containing precipitation products in the aquatic Fe2+ system and the precursor phases of the later formed magnetite. Spectroscopic and electron microscopic analysis prove that this early Se interaction leads to the formation of a nanoparticulate iron selenide phase [FeSe], which is oxidized and transformed into trigonal gray elemental Se during the progressive oxidation of the system. Regarding the retention behavior of Se, it is irrelevant whether the oxidation of the meta-stable iron oxide phases leads to the formation of magnetite only or also to other iron oxide phases like goethite. This reductive precipitation of Se induced by an interaction with metastable Fe(II)-containing iron oxide minerals should affect the mobility of Se oxyanions in contaminated environments, including the behavior of 79Se in the near-field of HLW repositories.