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.