E N V I R O N M E N T , E A R T H A N D P L A N E T A R Y S C I E N C E S

S C I E N T I F I C H I G H L I G H T S

1 4 0 H I G H L I G H T S 2 0 2 3 I

Probing selenium nanowire formation using X-ray absorption spectroscopy The mobility of 79Se, a fission product of 235U and a long-lived radioisotope, is a crucial parameter in the safety assessment of radioactive nuclear waste disposal systems. X-ray absorption experiments have revealed that, upon interactions with the corrosion products of steel canisters, selenium can be transformed into a non-mobile form with low environmental risk.

Selenium is an essential micronutrient often called a double-edged sword element or essential toxin , due to one of the narrowest human tolerance limits in the periodic table (40 to 400 µg/day) [1]. The 79Se radioisotope is also an important 235U fission product, with long half-life and high activity, present in the radioactive wastes stored in underground repositories. Aqueous species of selenium show a variety of oxidation states, the distribution of which depends strongly on the environmental conditions. Selenate (Se(VI)O42-) and selenite (Se(IV)O32-) are water- soluble and thus very mobile species. On the other hand, elemental selenium [Se(0)], metal selenides [Se(-II) and Se(-I)], and selenium sulfides are essentially insoluble and are therefore considered immobile in soil and geologic systems. Quantifying the rates and thermodynamic conditions of selenium reduction is therefore essential for assessing the mobility of selenium in the environment, in particular in the eventual release of waste repositories to the biosphere, or the eventual breach of oxygenated waters into the repository.

Nuclear wastes are stored in steel canisters, which are susceptible to corrode, leading to the formation of different corrosion products. Among them, magnetite (Fe3O4) nanoparticles are important reactive phases, due to their redox reactivity towards selenium species [2,3]. Magnetite nanoparticles have been shown to reduce Se(IV) and Se(VI) to elemental selenium and iron selenides [2,4], even in the presence of oxidised layers or maghemite (γ-Fe2O3) and coatings [3]. The product of these redox processes is a partially oxidised magnetite (i.e., a magnetite particle containing some proportion of maghemite a Fe(III) mineral isostructural to magnetite), a phase that is also able to adsorb selenium oxyanions.

Despite the high number of studies dealing with selenium redox reactivity in the presence of Fe(II) redox active phases, some important questions remain concerning the mechanism of selenium reduction by magnetite and the growth of the reduced selenium phases. For example: is electron transfer possible when selenium oxyanions are adsorbed forming an outer-sphere complex (i.e., an adsorbate with no direct covalent ligand shared with the magnetite)? Indeed, selenate [Se(VI)] oxyanions adsorb forming outer-sphere complexes, and the efficiency of the electron transfer process from the magnetite to selenium remains an open question.

This has been addressed by long-term experiments of selenate adsorption at the magnetite-water interface, coupled with X-ray absorption spectroscopy (XAS) experiments at beamline BM20. The results show that selenate oxyanions are effectively reduced, in particular

Fig. 114: a) Magnetite nanoparticles are exposed to a solution containing selenate [Se(VI)] for several months. b) High-angle annular dark-field imaging (top left panel) and scanning transmission electron microscopy (top right panel) reveal the formation of anisotropic

crystalline precipitates of selenium in the form of nanowires. Bottom panels show diffraction patterns along the [100] (left) and [211] (right) directions of the P3121 Se(0) structure.