Nontronite (NAu-1) Structure Associated with Microbial Fe(III) Reduction in Various Redox Conditions

Shewanella oneidensis MR-1 respires the structural Fe(III) of smectite and promotes illite formation in O2-free environment (Kostka et al., 1996, Kim et al., 2004). Since S. oneidensis is a facultative iron reducing bacterium, it is crucial to understand the structural changes induced by bio-reduction of structural Fe(III) in various redox conditions. Furthermore, the changes in cation exchange capacity (CEC) of bio-reduced nontronite upon the modification of mineral structure has not been extensively studied in terms of Fe-cycling. In this present study, we reported the evolution of nontronite structure at various time points in various redox conditions and corresponding CEC upon reduction and re-oxidation. S. oneidensis MR-1 was incubated in M1 medium with Na-lactate as the electron donor and Fe in nontronite (NAu-1) as the sole electron acceptor at pH 7 in anaerobic chamber for 3 hrs, 12 hrs, 1 day, 2 days, 4 days, 7 days, 14 days, and 21 days. O2 gas bubbling was then applied to the sample at each time point for 24 hours for re-oxidation. The triplet samples at each time point for both reduction and re-oxidation experiments were prepared. The extent of Fe(III) reduction measured by 1,10-phenanthroline method (Stucki and Anderson, 1981) indicated that the structural Fe(III) was reduced up to 8.8% of total Fe(III) within 21 days. XRD data with various treatments such as air dried, glycolated and lithium-saturated showed that K-nontronite may be formed because no discrete 10-Å illite peak was observed in Li-saturated sample upon glycolation. The CEC increased from 747 meg/kg to 1145 meg/kg during Fe(III) reduction and decreased to 954 meg/kg upon re-oxidation, supporting the possible formation of K-nontronite. The direct observation by electron microscopy verified the structural changes in nontonite in various redox conditions. The long-term experiment for 6 months, is in progress in anaerobic chamber, and results will be discussed. Kim, J. W., Dong, H., Seabaugh, J., Newell, S. W.and Eberl, D. D. (2004) Role of microbes in the smectite-to-illite reaction. Science, 303, 830-832. Kostka, J. E., Stucki, J. W., Nealson, K. H. and Wu, J. (1996). Reduction of structural Fe(III) in smectite by a pure culture of Shewanella putrefaciens strain MR-1. Clays and Clay Minerals, 44, 522-529. Stucki, J.W. and Anderson, W. L. (1981) The quantitative assay of minerals for Fe2+ and Fe3+ using 1,10-phenanthroline: I. Souves of Variability. Soil Scinec Society of America journal, 45, 633-637. Key Words : Illite, K-nontronite, Fe(III) reduction/re-oxidation, S. oneidensis MR-1