Heterogeneous reduction of Tc(VII) by Fe(II) at the solid–water interface

Abstract

Experiments were performed herein to investigate the rates and products of heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to hematite and goethite, and by Fe(II) associated with a dithionite-citrate-bicarbonate (DCB) reduced natural phyllosilicate mixture [structural, ion-exchangeable, and edge-complexed Fe(II)] containing vermiculite, illite, and muscovite. The heterogeneous reduction of Tc(VII) by Fe(II) adsorbed to the Fe(III) oxides increased with increasing pH and was coincident with a second event of Fe 2þ ðaqÞ adsorption. The reaction was almost instantaneous above pH 7. In contrast, the reduction rates of Tc(VII) by DCB-reduced phyllosilicates were not sensitive to pH or to added Fe 2þ ðaqÞ that adsorbed to the clay. The reduction kinetics were orders of magnitude slower than observed for the Fe(III) oxides, and appeared to be controlled by structural Fe(II). The following affinity series for heterogeneous Tc(VII) reduction by Fe(II) was suggested by the experimental results: aqueous Fe(II) $ adsorbed Fe(II) in phyllosilicates [ion-exchangeable and some edge-complexed Fe(II)] ( structural Fe(II) in phyllosilicates ( Fe(II) adsorbed on 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 TcO 2 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 Mö ssbauer spectroscopy applied to an experiment with 56 Fe-goethite with adsorbed 57 Fe(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 Fe(III) oxideadsorbed Fe(II) and structural Fe(II) in phyllosilicates may be an important geochemical process that will proceed at very different rates and that will yield different surface species depending on subsurface pH and mineralogy.

The Geochemistry of Technetium: A Summary of the Behavior of an Artificial Element in the Natural Environment

“…In contrast, when Fe(II) is sorbed onto other mineral phases, especially iron oxyhydroxides, surface-mediated heterogeneous catalysis becomes important and reduction of Tc(VII) to Tc(IV) takes place rapidly above pH 6 (Peretyazhko et al 2008a;Peretyazhko et al 2008b;Zachara et al 2007). …”

Chemical Differences Between Sludge Solids at the F and H Area Tank Farms

“…28 In SRS Tank Farm waste, it is assumed that the vast majority of technetium existing in sludge solids is present as Tc(IV) and relatively insoluble, in the form of hydrated technetium dioxide (TcO 2 xH 2 O) 29 and/or co-precipitated with iron(II) sorbed onto iron(III) oxide and/or oxyhydroxide particles. 30,31 In contrast, the majority of technetium existing in salt waste is assumed to be present as Tc(VII) and relatively soluble, in the form of sodium pertechnetate (NaTcO 4 ). 29 Some portion of the soluble technetium could also be present as a Tc(IV)-chelant complex 32 or as a Tc(VII)-solvent complex, 33 but these portions are expected to be minor in most SRS waste tanks due to the very low organic content.…”

Review of Potential Candidate Stabilization Technologies for Liquid and Solid Secondary Waste Streams

“…However, when Fe(II) is adsorbed or surface precipitated on solid phases, especially oxyhydroxides of iron, a surface-mediated catalysis of Tc(VII) reduction is more rapid (Cui and Eriksen 1996a, Cui and Eriksen 1996b, Peretyazhko et al 2008a, Peretyazhko et al 2008b, Zachara et al 2007. Other experiments aimed at assessing the interaction of pertechnetate with iron sulfide minerals, such as mackinawite (FeS), pyrrhotite (Fe 7 S 8 ), and 2.17 greigite (Fe 3 S 4 ), showed that Tc(IV) has an affinity for the surface of sulfide minerals and, upon oxidation to form goethite and sulfate minerals, the…”

“…The atomic structure of goethite, recently determined by Yang et al (2006), is shown in Figure 2.5. On the other hand, examining the reduced run products with X-ray absorption spectroscopy (XAS) methods (briefly described in Appendix A) has revealed that the 99 Tc-bearing phase is a separate, reduced Tc(IV) oxide phase and may not be substituting for Fe(III)-O in the host ferric oxyhydroxide phase (Burke et al 2006, Fredrickson et al 2004, Fredrickson et al 2009, Morris et al 2008, Peretyazhko et al 2008a, Watson et al 2001, Wharton et al 2000, Zachara et al 2007). In the most detailed study, Fredrickson and coworkers (Fredrickson et al 2009) found that Tc(IV)O 2 -like compounds could exist in a number of states, including physically separate phases or those that are inter-grown with Fe(III)-O compounds.…”