The objective of the work performed at FZJ is molecular-level understanding of 129I retention on layered double hydroxides (LDH), which are proven to form under disposal relevant conditions. In a long-term safety analysis 129I, as a dose-determining isotope, is of high concern due to its long half-time (1.57∙104 a) and high mobility in the geosphere. That is why the aspects, like efficiency of iodine retention, have to be systematically addressed.
In geosphere iodine is stabilized as I-. Formation of IO3- is possible on strong oxidizing conditions, and is therefore of secondary importance regarding to most European disposal concepts. As monovalent negatively charged ion, I- reveals poor interaction with most mineral phases, except for LDH, which contribute the most to the retention of I- and can significantly influence its mobility in the geosphere.
The work of FZJ is focused on detailed investigation of mechanisms of 129I retention by (Mg,Ni,Al)-LDH through anion exchange and co-precipitation (e.g. retention simultaneously with (Mg,Ni,Al)-LDH formation). Along with that, the retention of 129I by corresponding calcined mixed oxide phases due to restoration of LDH-structure will be investigated. Results from previous VESPA project indicated higher I- retention by synthesized Fe-, Co-, and Ni-doped (Mg,Al)-LDH compared to “pure" (Mg,Al)-LDH. The choice of metal cations and their stoichiometry seems to exert a decisive influence on the retention of I-. For this reason, a complete series of mixed phases, including the end-members (Mg,Al)-LDH and (Ni,Al)-LDH, will be synthesized in chloride-form and subsequently structurally and thermodynamically characterized. An extension of the sparse existing thermodynamic dataset on LDH should be made, in particular to clarify the influence of the cation stoichiometry on the stability of LDH and 129I retention (e.g. through anion exchange, co-precipitation or reformation route). Model experiments on simplified conditions will provide for understanding of individual phenomena, their contribution into the complex process of I- retention and allow derivation of respective distribution coefficients, Kd, which are used in long-term safety analyses. This knowledge will be subsequently transferred to the repository relevant systems by investigating I- behavior in repository-relevant groundwaters containing different anions (e.g. Cl-, ClO4-, NO3- or HCO3-).
Systematic study of I- behavior on the above specified conditions combined with the application of modern analytical techniques, like vibrational (infrared and raman) spectroscopy, PXRD, and XAS will provide for reliable thermodynamic data and contribute to understanding of 129I transport in generic radioactive waste repositories in clay stone and rock salt formations.
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