Selenium uptake onto natural pyrite

Abstract

79 Se is a major dose-determining redox-sensitive nuclide in safety analysis of radioactive waste disposal sites. In aqueous solutions, selenium forms soluble anionic species (Se IV O 3 2and Se VI O 4 2-) that hardly sorb on negatively charged surfaces of common host-rock minerals. However, Se is known to have a strong affinity with sulphides and interacts with pyrite, a common minor mineral of argillaceous rocks being considered as host formations for radioactive waste repositories. In this study, we present micro-and bulk X-ray spectroscopy data (l-XRF, l-XANES, and EXAFS) showing that, under nearly anoxic conditions, dissolved SeO 3 2and SeO 4 2sorb directly onto the pyrite surface and are subsequently reduced to Se 0 with increasing ageing time (up to 8 months). These results suggest that the mobility of 79 Se IV released from radioactive waste could greatly decrease through uptake on the pyrite surface followed by transformation into a sparingly soluble reduced form.

Se(IV) uptake by Äspö diorite: Micro-scale distribution

“…Selenium sorption to magnetite, pyrite or Fe-oxyhydroxides can be also expected, since they are present as accessory minerals (Widestrand et al, 2001), but cannot be distinguished by PIXE analyses. For example, Se anions are considered to have a strong affinity with sulphides and to interact with pyrite (Curti et al, 2013) as well as with magnetite (Missana et al, 2009b) or clays (Missana et al, 2009a). On Ca areas not associated to other elements (i.e.…”

Sorption behavior of hydroselenide (HSe−) onto iron-containing minerals

“…The stokes radii, conductivity and softness of HSe and 0.65, respectively [78]. In addition, the incorporation of selenide into iron sulfides, such as pyrite (FeS 2 ) and mackinawite (FeS), can be seen [54][55][56]77]. Therefore, the sorption behavior of HSe − onto ironcontaining minerals is expected to be similar to that of HS − .…”

“…In contrast to Se(IV) and Se(VI), the data of Se(−II) are scarce [9,13,52] and the sorption mechanisms of Se(−II) are poorly understood. Although the exact speciation of selenium in the vitrified HLW waste is unclear, it is estimated as Se(IV) or Se(VI) because selenium is exposed to oxidizing conditions during the reprocessing of spent fuel, and SeO 3 2− and SeO 4 2− can substitute trigonal borate or tetrahedral silicate entities, respectively, in the glass network [53][54][55][56]. Selenium had been considered to migrate as soluble SeO 3 2− /SeO 4 2− because of its slow reduction kinetics [53][54][55][56], in spite of thermodynamic stability of Se(−II) under reducing disposal conditions.…”

“…Although the exact speciation of selenium in the vitrified HLW waste is unclear, it is estimated as Se(IV) or Se(VI) because selenium is exposed to oxidizing conditions during the reprocessing of spent fuel, and SeO 3 2− and SeO 4 2− can substitute trigonal borate or tetrahedral silicate entities, respectively, in the glass network [53][54][55][56]. Selenium had been considered to migrate as soluble SeO 3 2− /SeO 4 2− because of its slow reduction kinetics [53][54][55][56], in spite of thermodynamic stability of Se(−II) under reducing disposal conditions. Recent works have demonstrated the abiotic reduction of Se(IV) and Se(VI) to Se(0) by several Fe(II)-bearing minerals [24,[55][56][57][58][59][60][61][62] and to Se(−II) by Fe(0) nanoparticles [63].…”

“…Selenium had been considered to migrate as soluble SeO 3 2− /SeO 4 2− because of its slow reduction kinetics [53][54][55][56], in spite of thermodynamic stability of Se(−II) under reducing disposal conditions. Recent works have demonstrated the abiotic reduction of Se(IV) and Se(VI) to Se(0) by several Fe(II)-bearing minerals [24,[55][56][57][58][59][60][61][62] and to Se(−II) by Fe(0) nanoparticles [63]. Understanding the sorption behavior of Se(−II) species is important; however, the sorption data of Se(−II) are scarce because of the experimental difficulties of maintaining the reducing state of Se.…”