Reoxidation Behavior of Technetium, Iron, and Sulfur in Estuarine Sediments

Abstract

Technetium is a redox active radionuclide, which is present as a contaminant at a number of sites where nuclear fuel cycle operations have been carried out. Recent studies suggest that Tc(VII), which is soluble under oxic conditions, will be retained in sediments as Fe(III)-reducing conditions develop, due to reductive scavenging as hydrous TcO2. However, the behavior of technetium during subsequent reoxidation of sediments remains poorly characterized. Here, we describe a microcosm-based approach to investigate the reoxidation behavior of reduced, technetium-contaminated sediments. In reoxidation experiments, the behavior of Tc was strongly dependent on the nature of the oxidant. With air, reoxidation of Fe(II) and, in sulfate-reducing sediments, sulfide occurred accompanied by approximately 50% remobilization of Tc to solution as TcO4-. With nitrate, reoxidation of Fe(II) and, in sulfate-reducing sediments, sulfide only occurred in microbially active experiments where Fe(II) and sulfide oxidation coupled to nitrate reduction was occurring. Here, Tc was recalcitrant to remobilization with <10% Tc remobilized to solution even when extensive Fe(II) and sulfide reoxidation had occurred. X-ray absorption spectroscopy on reoxidized sediments suggested that 15-50% of Tc bound to sediments was present as Tc(VII). Overall, these results suggest that Tc reoxidation behavior is not directly coupled to Fe or S oxidation and that the extent of Tc remobilization is dependent on the nature of the oxidant.

Photoreduction of99Tc Pertechnetate by Nanometer-Sized Metal Oxides: New Strategies for Formation and Sequestration of Low-Valent Technetium

“…11 Bioimmobilization efforts have also been employed in the efforts to reduce and stabilize TcO 4 -in soil by natural organic matter. 8,12 In these studies chemical reduction 7,13 and microbial reduction 5,14 led to the formation of TcO 2 2 O. However, the reduced 99 Tc IV in the form of the amorphous oxides also reoxidizes back to the mobile Tc VII O 4 -anion.…”

Redox interactions of technetium with iron-bearing minerals

“…In addition, when considering Tc behaviour in the environment, it is also necessary to explore the potential for oxidative remobilization of Tc(IV). Reoxidation of immobilized radionuclides in contaminated aquifers could occur via transport of oxygenated waters through the sediments, water table fluctuations, or disturbance of the geosphere by construction or erosion (Burke et al, 2006;Wu et al, 2007). Furthermore, reoxidation caused by nitrate, a common pollutant found at nuclear facilities, is also a potential route to radionuclide reoxidation via, for example, biologically mediated Fe(II)-oxidation coupled to NO 3 À reduction (Burke et al, 2006;Geissler et al, 2011;Morris et al, 2008;Senko et al, 2005;Wu et al, 2010;Law et al, 2010b;Law et al, 2011).…”

“…Reoxidation of immobilized radionuclides in contaminated aquifers could occur via transport of oxygenated waters through the sediments, water table fluctuations, or disturbance of the geosphere by construction or erosion (Burke et al, 2006;Wu et al, 2007). Furthermore, reoxidation caused by nitrate, a common pollutant found at nuclear facilities, is also a potential route to radionuclide reoxidation via, for example, biologically mediated Fe(II)-oxidation coupled to NO 3 À reduction (Burke et al, 2006;Geissler et al, 2011;Morris et al, 2008;Senko et al, 2005;Wu et al, 2010;Law et al, 2010b;Law et al, 2011). Recent experimental work on sediments suggests that Tc associated with Fe(II)-bearing sediments is recalcitrant to reoxidation even though significant reoxidation of Fe(II) to Fe(III) occurs during both air and nitrate reoxidation (Burke et al, 2006;Fredrickson et al, 2009;Geissler et al, in press;Jaisi et al, 2009 .…”

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

“…A number of experiments that examined the reduction and retention of 99 Tc in natural sediments whose oxidation states have been artificially altered (Burke et al 2006, Burke et al 2005, Fredrickson et al 2004, Morris et al 2008, Zachara et al 2007) yielded some interesting and useful results for secondary 99 Tc sequestration. Reduction of Tc(VII) to Tc(IV) by ferrous iron (Fe(II)) in solution is sluggish (homogeneous kinetics), even though the reaction…”

“…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.…”