Perrhenate and Pertechnetate Behavior on Iron and Sulfur-Bearing Compounds
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Perrhenate and Pertechnetate Behavior on Iron and Sulfur-Bearing Compounds B. E. Anderson1, U. Becker1, K. B. Helean2, and R. C. Ewing3 1 Geological Sciences, University of Michigan, 2534 C. C. Little Building, 1100 North University Ave, Ann Arbor, MI, 48109-1005 2 Sandia National Laboratories, Albuquerque, NM, 87123 3 Geological Sciences, Material Science, and Nuclear Engineering, University of Michigan, Ann Arbor, MI, 49109 ABSTRACT Investigations of the behavior of the element 99Tc frequently use a stable isotope of rhenium as an analogue. This is based on the elementsí similar radii, major oxidation states of +7 and +4, and analogous eH-pH diagrams. However, recent studies [1] have shown this analogy to be imperfect. Therefore, one goal of this study is to compare the behavior of these elements, with an emphasis on the adsorption of perrhenate and pertechnetate (the major forms of Re and Tc in natural waters) onto mineral surfaces. Quantum mechanical calculations were performed for the adsorption of these two anions onto relaxed clusters of the well-characterized sulfide galena (PbS). With these calculations, we have gained insight into differences between the anionsí adsorption behavior, including geometry, adsorption energies, and electronic structure. Differences between interactions on terraces and step edges, the effects of co-adsorbates such as Na+ and Cl-, and chloride complexation were also explored. The influence of water was calculated using homogeneous dielectric fluids.As a complement to the calculations, batch sorption tests are in progress involving ReO4-/TcO4- solution in contact with Fe metal, 10% Fe-doped hydroxyapatite, goethite, hematite, magnetite, pyrite, galena, pyrrhotite, and sphalerite. INTRODUCTION The element Tc is of interest because its radioactivity (99Tc, the most stable isotope, has a half-life of 213,000 years) and high mobility suggest that it will be a major contributor to longterm dose on release from a nuclear waste repository. When Tc is in a reduced oxidation state of 4+, the element tends to form relatively insoluble solids such as TcO2 and TcS2. However, oxidized Tc7+ forms the pertechnetate ion TcO4-, which is highly mobile in natural waters because most potentially adsorbent mineral surfaces are also negatively charged. Recent interest has focused on TcO4- interactions with Fe- and S-bearing materials that may sorb the ion and possibly reduce the Tc to a less soluble form. Unfortunately, these experiments are often difficult to perform because Tc has no stable isotopes. Therefore, Re, which is directly below Tc in the periodic table, is frequently used as a chemical analogue for Tc. This analogy is justified primarily based on the elementsí similar radii and major oxidation states of +7 and +4. Other arguments note the dominance of pertechnetate and the analogous perrhenate (ReO4-) over similar ranges of Eh-pH space ([2, 3] despite the fact that the same Eh-pH diagrams used in these arguments show the elements forming different solids under reducing condit
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