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Title: Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products

Authors:
; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Argonne National Lab. (ANL), Argonne, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC); Environmental Remediation Sciences Program
OSTI Identifier:
919354
Report Number(s):
ANL/XSD/JA-60449
DOE Contract Number:
DE-AC02-06CH11357
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochim. et Cosmochim. Acta; Journal Volume: 71; Journal Issue: 9 ; May 2007
Country of Publication:
United States
Language:
ENGLISH

Citation Formats

Zachara, J. M., Heald, S. M., Jeon, B.-H., Kukkadapu, R. K., Liu, C., McKinley, J. P., Dohnalkova, A. C., Moore, D. A., X-Ray Science Division, PNNL, and Yonsei Univ. Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products. United States: N. p., 2007. Web. doi:10.1016/j.gca.2006.10.025.
Zachara, J. M., Heald, S. M., Jeon, B.-H., Kukkadapu, R. K., Liu, C., McKinley, J. P., Dohnalkova, A. C., Moore, D. A., X-Ray Science Division, PNNL, & Yonsei Univ. Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products. United States. doi:10.1016/j.gca.2006.10.025.
Zachara, J. M., Heald, S. M., Jeon, B.-H., Kukkadapu, R. K., Liu, C., McKinley, J. P., Dohnalkova, A. C., Moore, D. A., X-Ray Science Division, PNNL, and Yonsei Univ. Tue . "Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products". United States. doi:10.1016/j.gca.2006.10.025.
@article{osti_919354,
title = {Reduction of pertechnetate [Tc(VII)] by aqueous Fe(II) and the nature of solid phase redox products},
author = {Zachara, J. M. and Heald, S. M. and Jeon, B.-H. and Kukkadapu, R. K. and Liu, C. and McKinley, J. P. and Dohnalkova, A. C. and Moore, D. A. and X-Ray Science Division and PNNL and Yonsei Univ.},
abstractNote = {},
doi = {10.1016/j.gca.2006.10.025},
journal = {Geochim. et Cosmochim. Acta},
number = 9 ; May 2007,
volume = 71,
place = {United States},
year = {Tue May 01 00:00:00 EDT 2007},
month = {Tue May 01 00:00:00 EDT 2007}
}
  • 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 atmore » 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.« less
  • Technetium-99 is a byproduct of uranium fission. It exists in two stable valence states (IV/VII) and exhibits a half-cell potential of intermediate value (Eo = 0.748 V). The oxidized form of 99Tc [pertechnetate, TcO4-] is an oxyanion that sorbs poorly to amphoteric surfaces and forms few solid phases with geochemically relevant cations. It is consequently highly mobile in oxic water-rock systems. The reduced valence state [Tc(IV)] is relatively insoluble (<10-8 mol L-1), and, hence immobile as an oxyhydroxide precipitate [TcO2•nH2O(s)]. In spite of favorable thermodynamics, Tc(VII) reacts slowly with Fe2+(aq) under anoxic conditions. Experiments were performed herein to investigate themore » rates and products of heterogeneous reduction of Tc(VII) by Fe(II) sorbed to hematite and goethite, and by structural Fe(II) in a dithionite-citrate-bicarbonate (DCB) reduced natural phyllosilicate mixture containing vermiculite, illite, and muscovite. The heterogeneous reduction of Tc(VII) by Fe(II) sorbed to the Fe(III) oxides increased with increasing pH and was coincident with a second event of Fe2+(aq) adsorption. The reaction was almost instantaneous above pH 7. In contrast, the reduction rates of Tc(VII) by structural Fe(II) in the DCB-reduced phyllollsilicates, were not sensitive to pH or the concentration of ion-exchangeable Fe(II), and were orders of magnitude slower than observed for the Fe(III) oxides. Tc-EXAFS spectroscopy revealed that the reduction products were virtually identical on hematite and goethite that were comprised primarily of sorbed octahedral TcO2 monomers and dimers with significant Fe(III) in the second coordination shell. The nature of heterogeneous Fe(III) resulting from the redox reaction was ambiguous as probed by Tc-EXAFS spectroscopy, although Mössbauer spectroscopy applied to an experiment with 56Fe-goethite with adsorbed 57Fe(II) implied that redox product Fe(III) was goethite-like. The Tc(IV) reduction product formed on the DCB-reduced phyllosilicates was different from the Fe(III) oxides, and was more similar to Tc(IV) oxyhydroxide in its second coordination shell. The heterogeneous reduction of Tc(VII) to less soluble forms by sorbed and structural Fe(II) in anoxic environments may be a very important geochemical process that will proceed at very different rates and that will yield different surface species depending subsurface pH and mineralogy.« less
  • The effects of the structure of a carboxylic acid on the processes of oxidation and reduction of iron ions in irradiated aqueous solutions were directly observed during irradiation. The solutions contained the mono- or dicarboxylic acid, H/sub 2/SO/sub 4/, Fe(NH/sub 4/)/sub 2/(SO/sub 4/)/sub 2/ and O/sub 2/. Th e ferrous oxidation was rather independent of the acid and its concentration, but the reduction of ferric was possible in the presence of formic, oxalic, or valeric acid only. The radical formed from the valeric acid acts similar to those formed from hydrocarbons. The G-values of oxidation and reduction of the ironmore » ions were established. The molar extinction coefficients for oxalic and malonic complexes of ferric ions were determined. (auth)« less
  • Under oxic conditions, Tc exists as the soluble, weakly sorbing pertechnetate [TcO{sub 4}{sup -}] anion. The reduced form of technetium, Tc(IV), is stable in anoxic environments and is sparingly soluble as TcO{sub 2} {center_dot} nH{sub 2}O{sub (S)}. Here we investigate the heterogeneous reduction of Tc(VII) by Fe(II) adsorbed on Al (hydr)oxides [diaspore ({alpha}-AlOOH) and corundum ({alpha}-Al{sub 2}O{sub 3})]. Experiments were performed to study the kinetics of Tc(VII) reduction, examine changes in Fe surface speciation during Tc(VII) reduction (Moessbauer spectroscopy), and identify the nature of Tc(IV)-containing reaction products (X-ray absorption spectroscopy). We found that Tc(VII) was completely reduced by adsorbed Fe(II)more » within 11 (diaspore suspension) and 4 days (corundum suspension). Moessbauer measurements revealed that the Fe(II) signal became less intense with Tc(VII) reduction and was accompanied by an increase in the intensity of the Fe(III) doublet and magnetically ordered Fe(III) sextet signals. Tc-EXAFS spectroscopy revealed that the final heterogeneous redox product on corundum was similar to Tc(IV) oxyhydroxide, TcO{sub 2} {center_dot} nH{sub 2}O.« less