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
Soltermann Daniela, Marques Fernandes Maria, Baeyens Bart, Dähn Rainer, Miehé-Brendlé Jocelyne, Wehrli Bernhard, Bradbury Michael H. (2013), Fe(II) Sorption on a Synthetic Montmorillonite. A combined Macroscopic and Spectroscopic Study, in Environmental Science & Technology
, 47(13), 6978-6986.
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.