title: Technetium retention and migration in clay systems in presence of iron
Creators: name: Chen, Ping
Publisher: name: Universität Bern
Year: 2024
Subjects: subject: 550 Earth sciences &amp; geology

description: Technetium is a redox sensitive radionuclide. Due to its high fission yield in nuclear reactors, its long half-life and radiotoxicity, and its high mobility, retention and migration of 99Tc in environment is of great importance for the safety assessment of radioactive waste disposal. Significant mobility difference has been measured for technetium in different oxidation states. Clay rich formations are considered as favorable host rocks in the repository concept for the deep geological disposal of High Level radioactive Waste (HLW). Montmorillonite, illite, kaolinite and chlorite are the major components of clay rocks. Therefore, a mechanistic understanding of Tc interaction with clay minerals is important for increasing the confidence in the safety assessment of nuclear waste disposal. The repository concept is based on the multi-barrier system, in which HLW and/or spent fuel are solidified, contained in iron casks and finally emplaced in tunnels in a deep underground geological formation. The omnipresence of iron and its strong redox activity make iron being an important redox agent in the environment. In addition to the iron present in host rocks, the corrosion of engineered barriers, e.g. steel disposal casks, is another considerable source of iron in the repository nearfield. Therefore, the presence of iron in clay minerals may affect Tc migration in clay minerals. In this study, a through-diffusion technique, developed by Luc Van Loon (Van Loon et al., 2003), was used to investigate Tc migration in compacted illite, in which the diffusion set-up was a compacted illite sandwiched between two stainless steel filters. The impact of two types of Fe(II) on Tc migration was studied, Fe(II) sorbed on the surface of illite and Fe(II) added as pyrite. In order to make a comprehensive interpretation of Tc diffusion in compacted illite in the presence of Fe(II), the Fe(II) sorption behavior on illite was investigated in detail. The diffusion of TcO4- in natural illite was also studied, together with its behaviour in the filters used in the sandwich set-up. The latter is necessary to properly model the diffusion data. With this knowledge, a reactive transport modeling of Tc diffusion in Opalinus Clay was performed. In a first step, a simple model was used in which only diffusion and redox reactions with solid surface complexed Fe(II) were considered. This model was further improved by including the reactivity of Fe-bearing phases.
Aqueous Fe, Fe-minerals, surface complexed Fe and structural Fe show a different activity toward redox sensitive nuclides, especially Tc. Thus, a quantitative description of Fe(II) sorption on clay minerals such as illite, is important to understand its subsequent influence on reactions with redox sensitive radionuclides. In this study, the sorption of Fe(II) on illite was investigated in batch experiments, including sorption as a function of pH (sorption edge) and sorption as a function of concentration (sorption isotherms) under anoxic condition. A 2 SPNE SC/CE (2 sites protolysis non-electrostatic surface complexation/cation exchange) sorption model extended to account for surface oxidation of sorbed Fe(II) was used to model the sorption data with the geochemical modeling code PHREEQC. Three models (A, B, C) with an increasing level of complexity were tested. The model A, in which only Fe(II) interacts with illite, does not satisfactorily fit the experimental data. This model was modified by including oxidation of surface complexes (Model B) and precipitation of iron bearing phases (Model C). The modified models showed that most of the sorbed Fe(II) was oxidized at pH below 6.5. At pH above 6.5, the oxidized surface complexes either react with water, forming surface iron hydroxides or precipitating as hematite, although the presence of these mineral phases could not be confirmed by X-ray diffraction measurements. These sorption processes might be coupled with the behavior of redox sensitive radionuclides in future reactive transport research. 
In order to study the role of iron on technetium migration, a through-diffusion technique was employed to investigate the diffusion of Tc in compacted illite in presence of Fe(II) under oxic and anoxic conditions. The sorption studies described above were used to help pre-loading the illite with Fe(II) by sorption. The other Fe was introduced to the system by mixing illite with pyrite grains. Because the stainless filters are necessary to confine clay in diffusion experiments, the diffusion parameters of Tc in stainless filter were measured for the first time. The COMSOL Multiphysics software was used to simulate the diffusion process. Under oxidizing conditions, Tc is transported in its anionic form, i.e. as TcO4-, and a similar effective diffusion coefficient was obtained for Tc diffusion in the filters and in illite. Fe(II) has a significant effect on the Tc diffusion behavior. The effective diffusion coefficient of Tc in Fe-loaded illite was approximately one order of magnitude smaller than in “Fe-free” illite. Analysis of the Tc distribution along the diffusion path suggested that most Tc was retarded in the filters, and only a small part reaches the clay phase. A delocalized redox reaction was proposed to explain this observation, in which means that electrons from the oxidation of the preloaded Fe(II) in the central part of the cell are transferred to the filter via the walls of the steel diffusion cell. Tc reacts with the electrons in the filter at a relative slow rate, which results in most of the Tc retarded in the filter, while some small amounts of it may diffuse through the clay.
A reactive transport modeling of Tc diffusion in Opalinus Clay (OPA) was performed in cooperation with the University of Mainz. Diffusion of 99Tc in OPA was studied under an inert argon and under air atmosphere. In a first step, a model was proposed that couple pyrite dissolution, diffusion, and redox reactions in PFLOTRAN. The model results showed that the final immobilized Tc is affected by the dissolution rate of pyrite, the equilibrium constant of redox reactions of Tc with surface complexed Fe, and the diffusion coefficient of TcO4-, which means that all these processes play an important role on the migration and immobilization of Tc. Under argon atmosphere, TcO4- diffuses into the OPA clay column and is partially reduced into surface complexed Tc (IV) by pyrite or dissolved pyrite. Under air atmosphere, the presence of O2 competitively consume Fe2+ and pyrite, result in no Tc retarded in the beginning zone, where O2 is available.
This study provide us with a better understanding of Tc transport in clay rich rocks. Repository evolution scenarios assume that iron casks may corrode and fail after exposure to ground water after a long time. The corroded iron diffuses in the ground water and sorbs on surface sites of clay minerals such as illite and montmorillonite. It is expected that Fe will occupy so called strong and weak sorption sites on the edges of clay minerals, and on planar sites by cation exchange. Some of the adsorbed iron at edge sites may be oxidized by structural iron and influences its redox activity towards redox sensitive nuclides, for instance Tc in our study, which will migrate in pore water after iron casks failure. Opalinus Clay is the selected host rock for a repository for radioactive waste in Switzerland. It is a reducing sediment that contains substantial amounts of Fe(II), mainly present as siderite and pyrite. Based on the observed migration behavior of Tc in Fe-loaded illite and in Opalinus Clay, it is expected that 99Tc released from the waste as TcO4- will migrate in the OPA but will be quickly reduced by the Fe(II) present in OPA and therefore Tc will be immobilized.