B21A-0418
Iron Isotope Fractionation Reveals Structural Change upon Microbial and Chemical Reduction of Nontronite NAu-1

Tuesday, 15 December 2015
Poster Hall (Moscone South)
Kai Liu1, Lingling Wu2, Bingjie Shi1, Christina Marie Smeaton3, Weiqiang Li4, Brian L Beard5, Clark Johnson5, Eric E Roden6 and Philippe Van Cappellen1, (1)University of Waterloo, Waterloo, ON, Canada, (2)University of Waterloo, Department of Earth and Environmental Sciences, Waterloo, ON, Canada, (3)University of Waterloo, Ecohydrology Research Group, Waterloo, ON, Canada, (4)Nanjing University, School of Earth Sciences and Engineering, Nanjing, China, (5)University of Wisconsin Madison, Department of Geoscience, Madison, WI, United States, (6)University of Wisconsin Madison, Geoscience, Madison, WI, United States
Abstract:
Iron (Fe) isotope fractionations were determined during reduction of structural Fe(III) in nontronite NAu-1 biologically by Shewanella oneidensis MR-1 and Geobacter sulfurreducens PCA and chemically by dithionite. ~10% reduction was achieved in biological reactors, with similar reduction extents obtained by dithionite. We hypothesize that two stages occurred in our reactors. Firstly, reduction started from edge sites of clays and the produced Fe(II) partially remained in situ and partially was released into solution. Next aqueous Fe(II) adsorbed onto basal planes. The basal sorbed Fe(II) then undergoes electron transfer and atom exchange (ETAE) with octahedral Fe(III) in clays, with the most negative fractionation factor Δ56Febasal Fe(II)-structural Fe(III)of −1.7‰ when basal sorption reached a threshold value. Secondly, when the most reactive Fe(III) was exhausted, bioreduction significantly slowed down and chemical reduction was able to achieve 24% due to diffusion of small size dithionite. Importantly, no ETAE occurred between basal Fe(II) and structural Fe(III) due to blockage of pathways by collapsed clay layers.

This two-stage process in our reduction experiments is distinctive from abiotic exchange experiments by mixing aqueous Fe(II) and NAu-1, where no structural change of clay would block ETAE between basal Fe(II) and structural Fe(III). The separation of reduction sites (clay edges) and sorption sites (basal planes) is unique to clay minerals with layered structure. In contrast, reduction and sorption occur on the same sites on the surfaces of Fe oxyhydroxides, where reduction does not induce structure change. Thus, the Fe isotope fractionations are the same for reduction and abiotic exchange experiments for Fe oxides. Our study reveals important changes in electron transfer and atom exchange pathways upon reduction of clay minerals by dissimilatory Fe reducing bacteria, which is prevalent in anoxic soils and sediments.

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