Fe distribution, redox state and electrochemical activity in Boom Clay

摘要

In the context of radioactive waste disposal in clay formations, the sorption of a number of redox sensitive radionuclides (e.g. U, Se, Tc) is controlled by the presence of Fe-containing minerals, mainly smectite (Guimaraes et al., 2016), illite (Bruggeman and Maes, 2010), and pyrite (Breynaert et al., 2010). In the repository lifetime, oxidizing conditions will prevail during the open drift phase enhanced by ventilation followed by recovery of reducing conditions after back-filling. Thus Fe-containing minerals will be subjected to redox cycles in the course of the repository operation. In order to evaluate the sensitivity of different Fe containing minerals in the Boom Clay to changing redox conditions, Fe2+/Fe3+ partitioning was determined in various size fractions (<0.2, 0.2-2, 2-8 mu m) and bulk samples by complexation with phenanthroline and XANES spectroscopy. The integration of the Fe speciation with quantitative mineralogical data allowed for a calculation of Fe redox activities. These values were then compared with the empirical electron accepting capacity (EAC) and electron donating capacity (EDC) values deduced from the mediated electrochemical reduction (MER) and mediated electrochemical oxidation (MEO) experiments. The results indicate that Fe3+ present in the smectite has a major control over the measured EACs in the redox cycled (oxidized, reduced and re-oxidized) Boom Clay samples. Fe2+ in illite and chlorite explain well the measured EDC values in the various size fractions, whereas in bulk samples pyrite and organic matter are the major contributors to EDC. The theoretical electrochemical activities (both EACs and EDCs) are about factor two higher than the experimental values. It is therefore concluded that the electrochemical activities of pure minerals do not necessarily reflect their electrochemical activities in the natural redox cycled samples. The methodological approach presented here may serve as a basis for further electrochemical investigations of natural, redox cycled sediments.