Probing redox reactions at the mineral/water interface by X-ray absorption spectroscopy: Reduction of Se, Sn, Sb and Pu by Fe(II)-bearing minerals

Fe(II)-bearing phases are naturally occuring in most anoxic aquifers, and form also at the surface of corroding steel containers under typical nuclear waste repository conditions. Due to their ability to reduce metal and metalloid contaminants, they are expected to play a key role for the migration behaviour of a wide range of radionuclides, including actinides and fission products. Using X-ray absorption spectroscopy as main tool, we have studied reaction end products, mechanisms and kinetics of redox processes at a range of water/mineral interfaces, including magnetite, green rust, mackinawite, siderite and Fe2+-sorbed clays. Our results show that the electron transport within mineral structures and at the surface is controlling the extent and the kinetics of multi-electron redox reactions. Examples that I will show include: (1) The fast reduction of Se(IV) to Se and Se(-II) nanoparticles, a reaction which was previously thought to be extremely slow. (2) The reduction of Pu(V) to Pu(III), which then forms a highly ordered inner-sphere sorption complex at the 111 face of magnetite, instead of the expected precipitation of PuO2 clusters. (3) The oxidation of Sn(II) to Sn(IV), which then forms inner-sphere sorption complexes, instead of the expected precipitation of SnO2. The results highlight the need for direct spectroscopic investigation of such processes, which are difficult to predict by thermodynamic methods, in order to provide reliable risk assessments.