[en] Document available in extended abstract form only. The long-lived fission product ⁷⁹Se (half-life > 10⁵ years) is one of the elements of concern for the safe disposal of High-Level nuclear Wastes (HLW). The Se speciation resembles that of S and is highly dependent on pH - Eh conditions. The +VI and +IV oxidation states prevail as mobile oxyanions, whereas Se at 0, -I and -II oxidation states prevail as solids with low solubility. Furthermore, the geochemistry of Se is largely controlled by that of Fe, with which Se is closely affiliated. The in situ redox potential of geological formation considered by several countries, such as Belgium (Boom Clay) and France (argillite), is considered to be mainly controlled by pyrite (FeS₂) and / or siderite (FeCO₃). Pyrite is the thermodynamically stable end product of iron compounds and is often present as inclusions in temperate soils. At ambient temperature, FeS₂ can be produced via different pathways, with disordered iron(II) monosulfide (FeSam) often being the initial precipitate. Both FeS₂ and FeSam are scavenger of trace elements from solution in anoxic environment. Specifically, sulfide (rVIS(-II) = 1.84 A) and selenide (rVISe(-II) = 1.98 A) ions have very similar ionic radii, suggesting that Se can substitute for S in both FeSam and FeS₂. Actually, according to Coleman and Deleveaux, about 3 weight-percent can enter the FeS₂ structure under low temperature conditions. Se adsorption on and co-precipitation with iron sulfide phases were investigated. The ultimate goal is to achieve the S substitution by Se in the pyrite structure for effective long term immobilization. To this end, Se was introduced in the synthesis protocol from the early stage. Sorption experiments were carried out and the samples analyzed to unambiguously discriminate a surface from a structurally incorporated Se species. The structure of the reaction products were primarily characterized by means of X-ray diffraction (XRD). Information on the Se oxidation state as well as from the crystal-chemical environment was obtained form X-ray Absorption Spectroscopy (XAS) experiments (XANES and EXAFS regions, respectively). In the first step, selenide ions were co-precipitated with iron monosulfide. XANES data indicated a low Se oxidation state (-II). EXAFS data modeling suggested a first Fe shell surrounding Se at d(Se-Fe) 2.37(1) A. Compared to pure FeSam, the increase in bond length (d(S-Fe) = 2.23 A) matches the difference in atomic radii. This result, together with the next-nearest S shells may suggest a FeSam-like environment. In the second step, this precursor phase was oxidized by hydrogen sulfide to form pyrite. The bulk reaction product was identified as FeS₂ from its XRD pattern. XAS analysis provided information on the Se oxidation state and on the short-range structure. Separately, selenite ions were adsorbed on FeSam. XRD pattern and XAS data strongly suggested a reduction and precipitation of a solid Se(0) phase. The same reaction mechanism was observed for pyrite. In the adsorption experiments, selenite was very likely reduced by sulfide released from FeS dissolution and from the oxidative dissolution of FeS₂. Whether incorporated in the iron sulfide bulk structure or precipitated as solid phase on the surface, Se mobility and availability will be dramatically lowered in the geological barrier surrounding HLW in geological formations. (authors)