skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Heterogeneous Reduction of Tc(VII) by Fe(II) at the Solid-Water Interface

Abstract

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 the 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 formore » 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

Authors:
; ; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
928271
Report Number(s):
PNNL-SA-56929
Journal ID: ISSN 0016-7037; GCACAK; TRN: US0804294
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Geochimica et Cosmochimica Acta, 72(6):1521-1539
Additional Journal Information:
Journal Volume: 72; Journal Issue: 6; Journal ID: ISSN 0016-7037
Country of Publication:
United States
Language:
English
Subject:
38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEAR CHEMISTRY; 58 GEOSCIENCES; TECHNETIUM 99; REDUCTION; ADSORPTION; CHEMICAL REACTION KINETICS; GOETHITE; HEMATITE; THERMODYNAMICS; SORPTIVE PROPERTIES; SILICATE MINERALS; GEOCHEMISTRY

Citation Formats

Peretyazhko, Tetyana, Zachara, John M, Heald, Steve M, Jeon, Byong Hun, Kukkadapu, Ravi K, Liu, Chongxuan, Moore, Dean A, and Resch, Charles T. Heterogeneous Reduction of Tc(VII) by Fe(II) at the Solid-Water Interface. United States: N. p., 2008. Web. doi:10.1016/j.gca.2008.01.004.
Peretyazhko, Tetyana, Zachara, John M, Heald, Steve M, Jeon, Byong Hun, Kukkadapu, Ravi K, Liu, Chongxuan, Moore, Dean A, & Resch, Charles T. Heterogeneous Reduction of Tc(VII) by Fe(II) at the Solid-Water Interface. United States. https://doi.org/10.1016/j.gca.2008.01.004
Peretyazhko, Tetyana, Zachara, John M, Heald, Steve M, Jeon, Byong Hun, Kukkadapu, Ravi K, Liu, Chongxuan, Moore, Dean A, and Resch, Charles T. Sat . "Heterogeneous Reduction of Tc(VII) by Fe(II) at the Solid-Water Interface". United States. https://doi.org/10.1016/j.gca.2008.01.004.
@article{osti_928271,
title = {Heterogeneous Reduction of Tc(VII) by Fe(II) at the Solid-Water Interface},
author = {Peretyazhko, Tetyana and Zachara, John M and Heald, Steve M and Jeon, Byong Hun and Kukkadapu, Ravi K and Liu, Chongxuan and Moore, Dean A and Resch, Charles T},
abstractNote = {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 the 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.},
doi = {10.1016/j.gca.2008.01.004},
url = {https://www.osti.gov/biblio/928271}, journal = {Geochimica et Cosmochimica Acta, 72(6):1521-1539},
issn = {0016-7037},
number = 6,
volume = 72,
place = {United States},
year = {2008},
month = {3}
}