Probing the Biogeochemical Behavior of Technetium Using a Novel Nuclear Imaging Approach (original) (raw)
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The fate of technetium in reduced estuarine sediments: Combining direct and indirect analyses
Applied Geochemistry, 2010
Technetium -99 is an important fission product in radioactive wastes. As Tc(VII)O4-, Tc is highly mobile in oxic environments but, under reducing conditions, Tc becomes strongly associated with sediments as hydrous Tc(IV)O2 like phases. In order to further examine the behaviour of Tc over a range of concentrations in estuarine sediments, anoxic incubation experiments were combined with a range of direct (transmission electron microscopy and gamma camera imaging) and indirect (incubation experiments and chemical extractions) experimental techniques. When TcO4- was incubated in sediment microcosms at micro-molar (10-6 mol L-1) concentrations, > 99 % TcO4- was removed from solution over the course of 36 days in systems undergoing active microbial Fe(III)-reduction. By contrast, when spiked into pre-reduced estuarine sediments that were predominantly Fe(III)-reducing (incubated for 60 days) or sulfate-reducing (incubated for 270 days), > 99 % TcO4- was removed from solution in under 10 minutes in both microbially active and heat sterilised systems. Chemical extraction techniques showed that 70 ± 3 % of Tc bound to sediments was remobilised when sediments were exposed to the first strong oxidant (H2O2) in the extraction scheme. At higher Tc concentrations (~ 0.05 mol kg-1 of sediment) scanning transmission electron microscopy, combined with energy dispersive X-ray mapping, was used to examine the associations of Tc in sediments. At these concentrations, Tc was localised and co-associated with nano-metre size Fe(II)-rich particles, consistent with the hypothesis that removal of Tc may be controlled by reduction of Tc(VII) to Tc(IV) by biogenic Fe(II) in sediments. In addition, gamma camera imaging with the gamma emitting 99mTcO4- (half-life 6 hours) at pico-molar (10-12 mol L-1) concentrations, was used to visualise the interaction of Tc in sediments at very low concentrations. Here, over the course of 24 hours the scavenging of Tc to sulfate-reducing sediments was observed. As the Tc concentrations used in the 99mTc experiments were below the solubility limits for hydrous Tc(IV)O2 (ca 10-9 mol L-1 at pH 7 - 9), sorption of Tc(IV) species is likely to be a significant control on Tc behaviour in these sediments even at very low concentrations. Overall, the results of this study show that multiple approaches are essential to understanding Tc speciation in complex heterogeneous sediments over the wide range of concentrations relevant to contaminated natural and engineered environments.
Technetium Reduction and Reoxidation in Aquifer Sediments
Geomicrobiology Journal, 2007
This study describes the biogeochemical behaviour of the radionuclide technetium (99Tc) in background area sediments from the US Department of Energy Field Research Center (FRC) in Oak Ridge, TN, USA. Microcosm experiments with trace levels of 99Tc(VII) were used to examine Tc reduction and reoxidation. Efficient removal of 0.5 μM Tc(VII) from solution was seen under Fe(III)-reducing conditions, and was attributed to a lower valence insoluble form of the radionuclide. Molecular and cultivation-dependent analysis confirmed the presence of known Fe(III)-reducing bacteria (Geothrix and Geobacter species) in these sediments. Extended X-ray Absorption Fine Structure (EXAFS) spectroscopic analysis of analogous microcosm experiments, challenged with higher (550 μM) concentrations of Tc(VII), confirmed the presence of reduced insoluble Tc(IV) as hydrous TcO2 in the Fe(II)-bearing sediments. Reoxidation experiments of pre-reduced microcosms challenged with 0.5 μM99Tc showed very limited (<3 %) remobilization of the reduced 99Tc with 100 mM nitrate but significant (ca 80%) remobilization of 99Tc under air reoxidation conditions. Fe(II) oxidation was, however, significant in all oxidation treatments. EXAFS analyses of Fe(II)-bearing sediments challenged with higher (550 μM) concentrations of Tc(VII) and then reoxidized with 100mMnitrate contained both Tc(IV) and Tc(VII) immobile phases. These results suggest that under anaerobic oxidation conditions, Tc(IV) will not remobilize rapidly, even in the presence of high concentrations of nitrate. This has implications for the biogeochemical cycling of technetium in contaminated environments, including those where bioreduction has been stimulated to minimize transport of the radionuclide.
Microbially mediated Fe(II) oxidation in reduced technetium-containing sediments.
Anoxic Tc(IV)-containing sediments representative of the UK Sellafield reprocessing facility were exposed to either air or NO3− to investigate redox cycling of technetium and iron. With air, oxidation of Fe(II) in the reduced sediments was accompanied by ∼75% mobilization of Tc to solution, as soluble Tc(VII). Nitrate additions resulted in the bio-oxidation of Fe(II), coupled to microbially mediated NO−3 reduction but was accompanied by only very limited (<5%) mobilization of the reduced, sediment-bound Tc, which remained as Tc(IV). PCR-based 16S rRNA and narG gene analyzes were used to investigate changes in the microbial community during sediment oxidation by air and nitrate. Contrasting microbial communities developed in the different treatments and were dominated by Betaproteobacteria (including Herbaspirillum and Janthinobacterium spp.) in the presence of high NO− 3 concentrations. This suggests that the Betaproteobacteria are involved in the redox cycling of Fe and N in these systems, but are unable to mediate NO3 −-dependent Tc(IV) oxidation. These microorganisms may play a previously unrecognized yet pivotal role in nfluencingcontaminant fate and transport in these environments which can have implications to the long-term stewardship of radionuclidecontaminated sediments.
Effects of Progressive Anoxia on the Solubility of Technetium in Sediments
Environmental Science & Technology, 2005
Technetium is a fission product that is highly mobile in its oxic form (as Tc(VII)O4-), but is scavenged to sediments in its reduced forms (predominantly as Tc(IV)). In progressive microcosms, a cascade of stable element terminal electron accepting processes developed as a result of indigenous microbial activity. TcO4- removal from solution occurred during microbial Fe(III) reduction, and was essentially complete (>99%) by the onset of SO42- reduction. Microbial community analysis revealed a similar and complex microbial population at all three sample sites. At the intermediate salinity site, Paull, a broad range of NO3--, Mn(IV)-, Fe(III)- and SO42-- reducers were present in sediments including microbes with the potential to reduce Fe(III) to Fe(II). When sterilised sediments were incubated with pure cultures of NO3--, Fe(III)- and SO42--reducing bacteria, TcO4- removal occurred only during active Fe(III) reduction. X-ray absorption spectroscopy confirmed that TcO4- removal in these sediments was due to reduction to hydrous Tc(IV)O2 in both Fe(III)- and SO42--reducing sediments.
Applied Geochemistry, 2008
Technetium is a long lived (2.13 x 105 y), beta emitting radionuclide which is a groundwater contaminant at a number of nuclear facilities throughout the world. Its environmental behaviour is primarily governed by its redox state: Under oxic conditions it forms the highly soluble pertechnetate (TcO4-) ion; under reducing conditions it forms the poorly soluble, reduced forms of Tc, particularly the Tc(IV) ion which, above its solubility limit (10-9 mol l-1 at ~ pH 7), is expected to precipitate as hydrous TcO2. Thus the redox cycling behaviour of technetium is predicted to be key to its environmental behaviour in the natural and engineered environment. Here we present the results of a series of X-ray absorption spectroscopy (XAS) experiments which examine the oxidation state and coordination environment of technetium in a range of estuarine and freshwater sediment suspensions, and in an environmentally relevant Fe(II)- mineral suspension under both reduced and reoxidised biogeochemical conditions. In both reduced sediments and Fe(II) containing minerals prior to reoxidation, XAS results show that Tc was retained as a hydrous TcO2 like phase which was remarkably common across all samples. Under air reoxidation, experiments showed significant (up to 80 %) remobilisation of Tc to solution as TcO4-, and XAS indicated that in pre-reduced freshwater sediments and Fe(II) minerals oxidised with air, Tc remained associated with solid phases as a hydrous TcO2 like phase. By contrast, in air reoxidised estuarine sediment XAS analysis suggested that both hydrous TcO2 and TcO4- were retained within the sediment phase. Finally, when microbially mediated nitrate reoxidation occurred in estuarine and freshwater sediment slurries, experiments showed low (< 8%) remobilisation of Tc from solids over similar timescales to air reoxidation experiments, whilst XAS again showed that both hydrous TcO2 and TcO4- were retained within the sediment phase. These results are discussed in the context of the redox cycling behaviour of Tc in the natural and engineered environment.