Redox capacities of different types of clay minerals and their potential to immobilize redox-active radionuclides

Clay minerals play an important role as barrier in radioactive waste storage. In addition to their function as hydraulic barrier, clay minerals can also immobilize radionuclides by adsorption and redox reactions with radionuclides. Reduction of redox-active radionuclides such as  technetium and selenium by smectites has already been shown. Many clay minerals contain iron (Fe) in their structure, however, not all of this structural Fe is accessible for redox reactions. This can depend on multiple factors such as the quantity and coordination of Fe in the clay mineral structure. While the redox-activity of Fe in smectites has received quite some attention, other common types of clay minerals present in clay host rocks have received less attention. In this study we investigated redox capacities of a variety of common clay minerals using mediated electrochemistry. Results show that most 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. Smectites had the highest redox-active Fe fraction (~100%), followed by mixed illite-smectite (~40%), glauconite (~15%), illite (~10%) and chlorite (~1%). Within the same clay mineral type there was also variation in the redox-active fraction. Batch experiments showed that even for clay minerals in which only a small fraction of Fe was redox-active, reduction of selenite to elemental selenium took place.