Reduction of Pertechnetate [Tc(VII)] by Aqueous Fe(II) and the Nature of Solid Phase Redox Products
The subsurface behaviour of 99Tc, a contaminant resulting from nuclear fuels reprocessing, is strongly dependent on its valence (e.g., IV or VII). Abiotic reduction of soluble Tc(VII) by Fe(II)(aq) in pH 6-8 solutions was investigated under strictly anoxic conditions using an oxygen trap (<7.5 10-9atm O2) in the absence of atmospheric or aqueous carbonate. The reduction kinetics were strongly pH dependent. Complete and rapid reduction of Tc(VII) to a precipitated Tc(IV)/Fe form was observed when 11 µmol/L of Tc(VII) was reacted with 0.4 mmol/L Fe(II) at pH 7.0 and 8.0, while no significant reduction was observed over 1 month at pH 6.0. Experiments conducted at pH 7.0 with Fe(II)(aq) = 0.05-0.8 mmol/L further revealed that Tc(VII) reduction was a combination of homogeneous and heterogeneous reaction. The heterogeneous reaction was more rapid, but not quantified. The kinetics of homogeneous reduction were slow at pH 7, but increased dramatically at pH 8, and correlated with the concentration of Fe(OH)+ and Fe(OH)2o(aq). Wet chemical and Fe-x-ray absorption near edge spectroscopy measurements (XANES) indicated that both Fe(II) and Fe(III) were present in the Fe/Tc(IV) redox reaction products. 57Fe- Mössbauer, extended x-ray adsorption fine structure (EXAFS), and transmission electron microscopy (TEM) measurements on the solid phase redox products that contained 13-15% Tc indicated that they were poorly ordered and dominated by Fe(II)-containing ferrihydrite with minor magnetite. Tc(IV) exhibited homogeneous spatial distribution within the precipitates. According to Tc-EXAFS measurements and structural modeling, its molecular environment was consistent with a Tc2O10 octahedral dimer bound in bidentate edge-sharing mode to an octahedral FeO6 surface or vacancy site in ferrihydrite. The precipitate maintained Tc(IV)aq concentrations that were slightly below those in equilibrium with amorphous Tc(IV)O2•nH2O(s). The oxidation rate of sorbed Tc(IV) in the Fe/Tc precipitate was considerably slower than Tc(IV)O2•nH2O(s) as a result of its intraparticle/intragrain residence. It is suggested that precipitates of this nature may form in anoxic sediments or groundwaters, and that the intraparticle residence of sorbed/precipitated Tc(IV) may limit 99Tc remobilization upon the return of oxidizing conditions.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 912984
- Report Number(s):
- PNNL-SA-50399; GCACAK; 4594; KP1302000; TRN: US0800621
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 71, Issue 9; ISSN 0016-7037
- Publisher:
- The Geochemical Society; The Meteoritical Society
- Country of Publication:
- United States
- Language:
- English
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