X-ray Absorption And Photoelectron Spectroscopy Investigation Of Selenite Reduction By Fe~Ⅱ-bearing Minerals

摘要

The long-lived radionuclide ~(79)Se 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 (Se~ⅥO_4~(2-)) and selenite (Se~ⅣO_3~(2-)). Previous investigations have demonstrated, however, that Se~Ⅳ and Se~Ⅵ are reduced by surface-associated Fe~Ⅱ, thereby forming insoluble Se~0 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 HSeO_3~- 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 Fe~(2+)-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 Fe~Ⅱ 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 ~(79)Se may be much lower than previously assumed.