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

The long-lived radionuclide Se-79 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 ((SeO42-)-O-VI) and selenite ((SeO32-)-O-VI). Previous investigations have demonstrated, however, that Se-IV and Se-VI are reduced by surface-associated Fe-II, thereby forming insoluble Sea and Fe selenides. Here we show that the mixed Fe-II/III (hydr)oxides green rust and magnetite, and the Fe-II sulfide mackinawite reduce selenite rapidly (< 1 day) to FeSe, while the slightly slower reduction by the Fe-II carbonate siderite produces elemental Se. In the case of mackinawite, both S-II and Fe-II 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 Fe-II systems is controlled by the concentration of HSe-: Se-0 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 x 10(-8) M. No evidence for aqueous metal selenide colloids nor for Se sorption to colloidal phases was found. Since Fe-II phases like the ones investigated here should be ubiquitous ill the near field of nuclear waste disposals as well as in the surrounding aquifers, mobility of the fission product Se-79 may be much lower than previously assumed. (c) 2008 Elsevier B.V. All rights reserved.