TRACING REDOX CYCLES DURING MICROBE-CLAY INTERACTIONS USING STABLE IRON ISOTOPES

Clay minerals influence the behavior of soils and the fate of contaminants through high cation exchange capacity, high swelling capacity, and large surface area. In particular, redox reactions involving iron-bearing clay minerals are responsible for many changes in the physical and chemical properties of soils and sediments. Microorganisms have been shown to dissolve, precipitate, and transform iron-containing clay minerals, altering properties of clays and producing distinct mineral assemblages in the rock record. The development of iron isotope research over the last decade has demonstrated that stable iron isotope is a valuable tool to study biogeochemical cycling of iron throughout Earth's history. This work aims to determine iron isotope fractionation factor during microbial reduction of iron-bearing clay minerals. Cell suspensions of Shewanella oneidensis MR-1 has been used for reduction of nontronite NAu-1, which has been pre-purified to remove iron oxide impurities. Sequential extraction was performed on solid samples upon separation of the aqueous component. Specifically, calcium chloride was used to extract Fe(II) sorbed to clay mineral basal planes, followed by sodium phosphate extraction targeting Fe(II) sorbed to clay mineral edge OH-groups, and 0.5 M HCl extraction for iron in secondary minerals. Results show that a total of ~10% reduction occurred in biotic reactors after 8 days, with ~70% Fe(II) removal by sodium phosphate extraction. Iron isotope compositions will be determined for aqueous and sequential extractions. Iron isotope fractionation factor between aqueous Fe(II), produced by microbial reduction, and Fe(III) in nontronite will be determined.