XAFS Study of the Chemical and Structural States of Technetium in Fe(III) Oxide Co-precipitates
- PNNL
XAFS has been used to study the chemical state and structural environment of technetium in Fe(III) oxide co-precipitates. {sup 99}Technetium is an abundant fission product which poses a significant environmental hazard due to its long half-life, abundance in nuclear wastes, and environmental mobility as the pertechnetate ion [Tc(VII)O{sub 4}{sup -}] under oxidizing conditions. Tetravalent Tc [Tc(IV)] is the stable valence state under reducing or anoxic conditions where its environmental mobility is significantly lowered by formation of a sparingly soluble, hydrated amorphous oxide precipitate [Tc(IV)O{sub 2} {center_dot} nH{sub 2}O(s)]. We have been studying the kinetics and solid products resulting from abiotic reduction of Tc(VII)O{sub 4}{sup -} by aqueous, adsorbed, and structural Fe(II) to provide insights on Tc migration in microaerophilic groundwaters. The reduction reaction yields Fe/Tc precipitates of variable structures that have not been previously studied. For the homogeneous reaction with aqueous Fe(II) at relatively high Tc:Fe concentrations, the predominant redox product is a solid containing Tc(IV) dimers attached in a bidentate edge-sharing configuration to FeO6 octahedra on the surface or unoccupied interior sites of a ferrihydrite-like precipitate. A similar ferrihydrite-type solid is formed on the surface of Fe oxide minerals such a hematite and goethite following the heterogeneous reaction of Tc(VII)O{sub 4}{sup -} with surface-complexed Fe(II). These co-precipitates greatly slow the oxidation rate of Tc(IV) relative to amorphous Tc(IV)O{sub 2} {center_dot} nH{sub 2}O(s), possibly allowing for the long term sequestration of {sup 99}Tc in stable (bio)geochemical mineral forms that may reduce the long term environmental risk of {sup 99}Tc subsurface contamination.
- Research Organization:
- Argonne National Lab. (ANL), Argonne, IL (United States). Advanced Photon Source (APS)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 1009008
- Resource Relation:
- Conference: 13th International Conference on X-ray Absorption Fine Structure-XAFS13;9-14 July 2006;Stanford, California, USA
- Country of Publication:
- United States
- Language:
- ENGLISH
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