Iron Isotope Fractionations Reveal a Finite Bioavailable Fe Pool for Structural Fe(III) Reduction in Nontronite
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Date
2017-08-16
Authors
Shi, Bingjie
Liu, Kai
Wu, Lingling
Li, Weiqiang
Smeaton, Christina M.
Beard, Brian L.
Johnson, Clark M.
Roden, Eric E.
Van Cappellen, Philippe
Journal Title
Journal ISSN
Volume Title
Publisher
American Chemical Society
Abstract
We report on stable Fe isotope fractionation during microbial and chemical reduction of structural Fe(III) in nontronite NAu-1. Fe-56/Fe-54 fractionation factors between aqueous Fe(II) and structural Fe(III) ranged from -1.2 to +0.8 parts per thousand. Microbial (Shewanella oneidensis and Geobacter sulfurreducens) and chemical (dithionite) reduction experiments revealed a two-stage process. Stage 1 was characterized by rapid reduction of a finite Fe(III) pool along the edges of the clay particles, accompanied by a limited release to solution of Fe(II), which partially adsorbed onto basal planes. Stable Fe isotope compositions revealed that electron transfer and atom exchange (ETAE) occurred between edge-bound Fe(II) and octahedral (structural) Fe(III) within the clay lattice, as well as between aqueous Fe(II) and structural Fe(III) via a transient sorbed phase. The isotopic fractionation factors decreased with increasing extent of reduction as a result of the depletion of the finite bioavailable Fe(III) pool. During stage 2, microbial reduction was inhibited while chemical reduction continued. However, further ETAE between aqueous Fe(II) and structural Fe(III) was not observed. Our results imply that the pool of bioavailable Fe(III) is restricted to structural Fe sites located near the edges of the clay particles. Blockage of ETAE distinguishes Fe(III) reduction of layered clay minerals from that of Fe oxyhydroxides, where accumulation of structural Fe(II) is much more limited.
Description
© 2016 American Chemical Society. This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. The definitive version is available via: http://dx.doi.org/10.1021/acs.est.6b02019
Keywords
Smectite Clay-Minerals, Sole Electron-Acceptor, Microbial Reduction, Oxide Reduction, Aqueous Fe(Ii), Geobacter-Sulfurreducens, Fe(II)-Fe(III) Electron, Oxidation-State, Atom Exchange, Ancient Earth