Qian, Yanting (2024). Coupled adsorption and redox interface reactions governing the retention of Se and Tc on Fe(II)/Fe(III)-bearing clay minerals. (Thesis). Universität Bern, Bern
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Abstract
⁷⁹Se and ⁹⁹Tc are two long-lived fission productions with high levels of toxicity. Se(IV) and Tc(VII) are present in vitrified high-level radioactive waste (HLW) in highly mobile forms. The potential release of these elements from the repository poses a significant threat to the biosphere. Therefore, it is imperative to employ the clay engineered backfill materials to impede the migration of Se(IV) and Tc(VII) towards the biosphere. Considering the extremely low solubility of reduced Se and Tc, reduction of Se(IV) and Tc(VII) stands out as one of the most effective approaches to slow down their migration. Clay minerals, primary components of bentonite, the chosen engineered backfill material, naturally contain structural Fe(III) which readily undergoes reduction to Fe(II) in the reducing environment of the repository. This transformation facilitates its role as a reductant for Se(IV) and Tc(VII). As such, this thesis primarily aims to investigate the pivotal role of Fe within the clay mineral, and to quantitatively assess its impact on the retention of Se and Tc. In the first step of the study, it is essential to reduce the structural Fe(III) present in the native clay mineral to Fe(II), thus establishing a reducing potential. This thesis employed three distinct types of 2:1 dioctahedral clay minerals, namely Texas montmorillonite STx-1, Wyoming montmorillonite SWy-2, and nontronite clay NAu-2. These clays were chosen due to their varying Fe content, which leads to different ratios of Fe(II)/Fe(III) and diverse Fe distributions within the octahedral sheet. Consequently, the reduced clay exhibits varying redox potentials. Following reduction, comprehensive characterization of the modified clay minerals was performed, including assessments of their structure, morphology, cation exchange capacity, and the accessibility of sorption edge sites. The results of this characterization demonstrated negligible alterations in structural integrity and sorption properties of the reduced clay minerals when compared to their native counterparts. In the second step, batch sorption experiments were systematically conducted, varying parameters including pH levels, Se and Tc concentrations, reaction durations, and the use of reduced clay samples with different Fe(II) content and redox potential. Following these sorption experiments, spectroscopic techniques, X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS), were applied to discern the reduction products and ascertain the local structure of Se and Tc. The sorption experiments revealed that all reduced clay minerals possess the capacity to effectively impede the migration of Se and Tc. Se(IV) exhibited sorption via an inner-sphere surface complex, followed by reduction, resulting in the formation of red Se(0) initially, which gradually transformed into grey Se(0) as insoluble solids. In contrast, the retention mechanism for Tc(VII) involved its reduction to Tc(IV), leading to the formation of TcO₂∙nH₂O precipitate, with no detectable adsorbed Tc species observed. Despite the differences in the retention mechanisms, both Se and Tc retention efficiency were found to be directly proportional to the structural Fe(II) content. Furthermore, the presence of surface adsorbed Fe²⁺aq was found to enhance the retention of Tc. The findings presented in this thesis provide compelling evidence that Fe(II)/Fe(III)-bearing clay minerals hold significant promise for immobilizing Se and Tc within the engineered barriers of a repository. Moreover, the sorption data generated enriches the geochemical sorption modeling database, further enhancing our understanding of these retention processes. The insights gained from this thesis make a substantial contribution to the risk assessment of repository systems.
| Item Type: | Thesis |
|---|---|
| Dissertation Type: | Cumulative |
| Date of Defense: | 16 January 2024 |
| Subjects: | 500 Science > 550 Earth sciences & geology |
| Institute / Center: | 08 Faculty of Science > Institute of Geological Sciences |
| Depositing User: | Hammer Igor |
| Date Deposited: | 16 Jul 2025 09:24 |
| Last Modified: | 16 Jul 2025 09:30 |
| URI: | https://boristheses.unibe.ch/id/eprint/6395 |
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