Technetium is the lightest element whose isotopes are all radioactive. Among them, 99Tc (hereafter simply referred as technetium or Tc) is the most abundant and raises great environmental concern due to its relatively long half-life of 2.14×105 years and the high mobility of pertechnetate, Tc(VII)O4, its most stable form under aerobic conditions. The reduction from Tc(VII) to Tc(IV) is one of the most successful strategies for Tc immobilization; however, the mechanism of this redox reaction is not yet fully understood. This presents a large gap in the general knowledge of technetium chemistry and a significant obstacle for the modeling of its reactivity in contexts like a nuclear waste repository. This thesis was developed in the frame of the BMWi funded VESPA II project, and it studies the surface-mediated reduction of 99Tc(VII) using a combination of fundamental chemistry and its application for remediation and nuclear waste management.
First, spectro-electrochemical methods (cyclic voltammetry, rotating disk electrode, chronoamperometry coupled with UV-vis, Raman microscopy and nuclear magnetic resonance) were employed to study the reduction mechanism of 0.5 mM KTcO4 in non-complexing media (2 M NaClO4) in the pH range from 2.0 to 10.0. It was found that the mechanism depends on the pH. At pH 2.0 it splits into two steps: Tc(VII) gains 2.1 ± 0.3 electrons and becomes Tc(V) that rapidly reduces to Tc(IV) with the transfer of further 1.3 ± 0.3 electrons. In contrast, at pH ≥ 4.0 there is a direct transfer of 3.2 ± 0.3 electrons. The complete reduction of Tc(VII) yielded a black solid that was successfully characterized by NMR and Raman microscopy as Tc(IV) regardless of the initial pH at which the reaction occurred. Unfortunately, it was not possible to observe the Tc(V) species at pH 2.0 by the spectroscopic tools used.
Second, the reductive immobilization of Tc(VII) by pure pyrite and a synthetic mixture marcasite-pyrite 60:40 (synthetic FeS2, with both minerals being polymorphs) was studied by a combination of batch sorption experiments (Tc-removal was studied varying pH, contact time, ionic strength and Tc concentration) and several spectroscopies and microscopies such as Raman microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy and
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X-ray absorption spectroscopy. It was found that both pyrite and the synthetic FeS2 promote the reduction of Tc(VII) to Tc(IV). In the case of pure pyrite, the Tc-removal is complete after one day in contact at pH ≥ 5.5. The spectroscopic analysis showed at pH 6.0 an inner-sphere complex between Tc(IV) dimers and hematite formed as secondary mineral on the pyrite surface. In contrast, at pH 10.0 Tc(IV) gets incorporated into surficial magnetite by replacing Fe3+ in octahedral position, with Fe2+ providing reasonable charge compensation for Tc4+.
The presence of marcasite made the process slower and less efficient since the synthetic FeS2 was capable to remove 100% Tc from solution only after seven days in contact at 6.0 < pH ≤ 9.0 while the Tc-removal at pH 10.0 was only around 80%. At pH 6.0 the formation of hematite was also observed, suggesting that the formed Tc(IV) species at the surface is the same as with pure pyrite. However, at pH 10.0 the formation of sulfate minerals evidences a change of redox active species: S2- instead of Fe2+. This, combined with the fact that in both solids the formation of TcSx species was detected by XPS at pH 10.0, shows the potential of sulfur as another reducing agent for Tc(VII). The effect of polymorphism on the Tc removal is remarkable and this work shows the relevance of more studies on the interaction of radionuclei with other mineral polymorphs.
Regardless of the kinetics of the Tc removal, both pyrite and synthetic FeS2 hindered the re-oxidation of Tc(IV) when exposed to ambient atmosphere for two months. This feature makes them good candidates for the remediation of technetium from contaminated waters. Moreover, natural attenuation effects can be expected for technetium in the near and far field of nuclear waste repositories wherever iron sulfide is present.
The results presented in this work contribute to a better understanding of the fundamental aqueous chemistry of technetium and confirm pyrite, a ubiquitous mineral, as a very good candidate for technetium scavenging even in the presence of marcasite. These results close important gaps in thermodynamic databases that are needed for the safety assessment, i.e. modeling of fission products.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:75399 |
| Date | 08 July 2021 |
| Creators | Rodríguez Hernandez, Diana Marcela |
| Contributors | Stumpf, Thorsten, Frisch, Gero, Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/publishedVersion, doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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