Characterizing Reactive Iron Mineral Coatings in Redox Transition Zones

Reactive iron mineral coatings in redox transition zones play an important role in contaminant attenuation. These mineral coatings include poorly crystalline to crystalline iron sulfides, carbonates, and oxyhydroxides, and are a signature of the biogeochemical processes occurring. To better understand these processes, reactive iron mineral coatings were characterized in an anaerobic 18 m (60 ft) core collected from a contaminated industrial site. This study targets two redox transition zones uncovered in the core. A suite of complementary analyses was applied to distinguish the surface coating mineralogy using X-ray diffraction, X-ray fluorescence, and field-emission scanning electron microscopy with energy dispersive X-ray analysis. In the shallowest transition zone running through an aquifer with clay lenses, framboidal pyrite and greigite were observed in the clay lenses, while iron (III) phases in the aquifer included goethite, ferrihydrite, lepidocrocite, and hematite. In a deeper aquitard transition zone, iron sulfides were found as flaky aggregates of mackinawite, pyrite, and pyrrhotite. In addition, the iron (II)/(III) mineral magnetite was also observed in this same area. Moving deeper into this zone, the most abundant coatings were found to shift to the iron (III) oxyhydroxide minerals. Overall, reactive mineral coatings observed are important surfaces contributing to the natural attenuation processes in redox transition zones.