Project

Discipline

Fe(II) sorption; montmorillonite; illite; redox reactions; competitive sorption; sorption modelling; linear free energy relationship; 2SPNE SC/CE sorption model; EXAFS; Fe(II)-phyllosilicates; Fe(II); dioctahedral clays; sorption; reducing conditions; competitiveness; radionuclides

Lead
Lay summary
Abstract: The influence of strongly reducing conditions on the state of structural iron in clay minerals and whether this influences the characteristics (especially sorption) of the clay will be investigated. The sorption behaviour of ferrous iron on dioctahedral clay minerals will be studied with the aim of elucidating the uptake mechanism. This is particularly relevant to predict quantitatively the influence of high Fe(II) concentrations on the retention behaviour of other radionuclides in the system, since this is currently unknown. Quantifying the competitive effect of Fe is of importance for the prediction of the fate of metals in the environment. X-ray absorption spectroscopy will be applied to determine the nature of surface complexes located at clay edges sites and to study the formation of potentially newly formed phases such as Fe-phyllosilicates.Objectives: Relatively high concentrations of ferrous iron are expected to be present in the strongly reducing near- and far fields of a radioactive waste repository and will not be depleted to any significant extent by sorption/diffusion since they are maintained by solubilities of iron phases present and by corrosion products/secondary mineral formation at the canister/bentonite interface. The overall objective is to characterise the clay mineral's ability to retain radionuclides under reducing conditions similar to those induced by corroding iron canisters in bentonite buffers by investigating the influence of high Fe(II) concentrations on the sorption behaviour of key radionuclides.Relevance: Bentonite and argillaceous rocks have been widely proposed as backfill materials and host rocks respectively in deep geological disposal concepts. Sorption of radionuclides on the main clay minerals contained in the backfills and the host rock has a decisive influence on their mobility and is a major consideration in the safety analysis. In order to assess the long-term safety of repositories, sorption values of radionuclides over a large range of realistic geochemical conditions (Eh, pH, ionic strength, T) are needed. A molecular level understanding of the sorption mechanisms increases the confidence in these values. The outcome of the proposed doctoral study is of direct relevance to the assessment of the long-term impact of the interaction of Fe(II) on the retention/retardation of key radionuclides. The results will fill gaps in the existing sorption databases and will contribute to the further development of thermodynamic based sorption databases which will be used in performance assessment, and, will strengthen the scientific basis for the safety case.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

Title	Year

Number	Title	Start	Funding scheme

SummaryIn the safety case for high-level radioactive waste repositories, redox phenomena play an important role in radionuclide retention. Virtually all deep underground repository concepts contain large amounts of iron, and reducing conditions will prevail in the long-term. In the near-field, the corrosion processes of metallic iron and production of H2 will control the redox potential. Through the corrosion of steel canisters large amounts of ferrous iron are produced and released into the engineered barrier system (bentonite) as well as into the argillaceous host rocks. The presence of high Fe(II) concentrations in the interstitial porewaters in the near- and far-fields could have a significant influence on the sorption behaviour of radionuclides and the consequences of this need to be quantified.The overall objective of the proposed PhD project is to investigate, in a combined macroscopic (wet chemistry) and microscopic (surface analysis) approach, the influence of reducing conditions on the characteristics of representative clay materials, particularly with respect to radionuclide retention in the presence of high aqueous Fe(II) concentrations. One of the key questions regarding strongly reducing conditions is whether the state of the structural Fe(III) changes and whether this influences the characteristics (especially sorption) of the clay mineral. This will be an important part of the investigations. Subsequently, the sorption behaviour of Fe(II) on dioctahedral clay minerals (montmorillonite and illite) will be studied with the aim of elucidating the uptake mechanism. This is particularly relevant to predict quantitatively the influence of high Fe(II) concentrations on the retention behaviour of other radionuclides in the system, since this is currently unknown. The competitiveness of Fe(II) on the sorption of radionuclides with valence states between (II) and (IV) will be investigated. Quantifying the competitive effect of Fe(II) is of importance in repository safety analyses and for the prediction of the fate of metals in the environment. X-ray absorption spectroscopy (EXAFS, XANES) will be applied at low metal loadings to determine the nature of surface complexes located at clay edges sites and to study the formation of newly formed phases such as Fe-phyllosilicates at elevated metal concentrations. The sorption of metals onto clay mineral surface and the possible formation of neoformed phyllosilicates can significantly decrease their ability for migration into the geosphere and influences thus the long-term behaviour of radionuclides. This work will contribute significantly to an improved knowledge of the Fe(II)/clay interface chemistry and will be beneficial for developing the safety cases for the disposal of chemically toxic and radioactive wastes.