Redox-active iron in different types of clay minerals – mediated electrochemical characterization and reactivity towards Se(IV)

Clay minerals are ubiquitous in the environment and are an important reactant to pollutants including radionuclides. Besides their adsorptive capacity, clay minerals can also participate in redox reactions due to the presence of iron (Fe) in their structure and can thereby influence the mobility of redox-active compounds such as technetium, selenium and uranium. However, not all Fe(II) or Fe(III) in the structure of clay minerals may be accessible for redox reactions. The redox activity of Fe can depend on the quantity and coordination of Fe in the clay mineral structure. Research on the redox activity of structural Fe in clay minerals has mainly focused on smectites whereas other types of clay minerals have received less attention. In this study we measured redox-activity of a variety of clay minerals, i.e. illites, chlorites, glauconites and mixed-layered illite-smectites. The redox-activity of the clay minerals was investigated by mediated electrochemistry. To examine the relation between electrochemically active structural Fe(II) in these clay minerals and their reactivity towards radionuclides, batch experiments were performed with selenite, a redox-active, mobile, long-lived radionuclide in radioactive waste. Both pristine (oxidized) clay minerals and chemically reduced clay minerals were tested. Results from mediated electrochemical oxidation and reduction show that all clay minerals have electrochemically active Fe to some extent, but there is a large variation in the fractions of electrochemically-active Fe between the different clay minerals, e.g. around 100% redox-active Fe in smectites, <20% redox-active Fe for the tested illites. K-edge XAFS spectroscopy of these batch experiment samples reveals that these different types of chemically reduced clay minerals reduce selenite to elemental selenium.