X-ray absorption and photoelectron spectroscopy investigation of selenite reduction by FeII-bearing minerals

International audienceThe long-lived radionuclide 79Se is one of the elements of concern for the safe storage of high-level nuclear waste, since clay minerals in engineered barriers and natural aquifer sediments strongly adsorb cationic species, but to lesser extent anions like selenate (SeVIO42−) and selenite (SeIVO32−). Previous investigations have demonstrated, however, that SeIV and SeVI are reduced by surface-associated FeII, thereby forming insoluble Se0 and Fe selenides. Here we show that the mixed FeII/III (hydr)oxides green rust and magnetite, and the FeII sulfide mackinawite reduce selenite rapidly (< 1 day) to FeSe, while the slightly slower reduction by the FeII carbonate siderite produces elemental Se. In the case of mackinawite, both S−II and FeII surface atoms are oxidized at a ratio of one to four by producing a defective mackinawite surface. Comparison of these spectroscopic results with thermodynamic equilibrium modeling provides evidence that the nature of reduction end product in these FeII systems is controlled by the concentration of HSe−; Se0 forms only at lower HSe− concentrations related to slower HSeO3− reduction kinetics. Even under thermodynamically unstable conditions, the initially formed Se solid phases may remain stable for longer periods since their low solubility prevents the dissolution required for a phase transformation into more stable solids. The reduction by Fe2+-montmorillonite is generally much slower and restricted to a pH range, where selenite is adsorbed (pH < 7), stressing the importance of a heterogeneous, surface-enhanced electron transfer reaction. Although the solids precipitated by the redox reaction are nanocrystalline, their solubility remains below 6.3 × 10− 8 M. No evidence for aqueous metal selenide colloids nor for Se sorption to colloidal phases was found. Since FeII phases like the ones investigated here should be ubiquitous in the near field of nuclear waste disposals as well as in the surrounding aquifers, mobility of the fission product 79Se may be much lower than previously assumed

Intrinsic formation of nanocrystalline neptunium dioxide under neutral aqueous conditions relevant to deep geological repositories

International audienceThe dilution of aqueous neptunium carbonate complexes induces the intrinsic formation of nanocrystalline neptunium dioxide (NpO2) particles, which are characterised by UV/Vis and X-ray absorption spectroscopies and transmission electron microscopy. This new route of nanocrystalline NpO2 formation could be a potential scenario for the environmental transport of radionuclides from the waste repository (i.e. under near-field alkaline conditions) to the geological environment (i.e. under far-field neutral conditions