Iron Isotope Fractionations Reveal a Finite Bioavailable Fe Pool for Structural Fe(III) Reduction in Nontronite

Abstract

We report on stable Fe isotope fractionation during microbial and chemical reduction of structural Fe(III) in nontronite NAu-1. (56)Fe/(54)Fe fractionation factors between aqueous Fe(II) and structural Fe(III) ranged from -1.2 to +0.8‰. 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.

Fe(II) Interactions with Smectites: Temporal Changes in Redox Reactivity and the Formation of Green Rust

“…7,11,25 For example, previous studies have indicated that biologically reduced NAu-1 results in the reduction of edge-site Fe(III) species and the formation of, mostly, sorbed Fe(II) with limited structural Fe(III) reduction. 23,25,26 When smectites are chemically reduced, however, a greater amount of structural Fe(III) reduction can occur. 23,25,26 Spectroscopic evidence has linked this to the ability of chemical reductants to penetrate the clay surface via the basal plane combined with their very low reduction potential.…”

“…23,25,26 When smectites are chemically reduced, however, a greater amount of structural Fe(III) reduction can occur. 23,25,26 Spectroscopic evidence has linked this to the ability of chemical reductants to penetrate the clay surface via the basal plane combined with their very low reduction potential. 27,28 In this study we have examined the ability of Fe(III)-smectite clay mineral suspensions to reduce contaminants in the presence of added aqueous Fe(II) at pH 7.8, as these conditions favor Fe(II) sorption to edge-sites 22 which reflects the clay surface that microbes target for Fe(III)-smectite reduction.…”

“…27,28 In this study we have examined the ability of Fe(III)-smectite clay mineral suspensions to reduce contaminants in the presence of added aqueous Fe(II) at pH 7.8, as these conditions favor Fe(II) sorption to edge-sites 22 which reflects the clay surface that microbes target for Fe(III)-smectite reduction. 23,25,26 Smectite clay minerals can undergo a variety of structural changes during reduction as the clay lattice restructures itself in order to neutralize the associated change in charge, 22,24,29 although structural changes have only been observed at very high reduction extents in Fe-rich clay minerals. 24,25 These structural changes within the clay have been inferred from spectroscopic studies with findings suggesting that the rearrangement of these clays can be permanent, 30−32 thereby making the redox history of the clay a major factor in determining its reduction potential as transformation may limit access to redox active sites.…”

“…Iron Extractions and Chemical Analyses. The Fe(II) extraction regime employed by Neumann et al 22 and Shi et al 23 was used to extract Fe(II) from the suspensions at each tested time period. Although the same extraction solutions were employed, the extractions times were dramatically reduced from those previously employed as any additional aging could obscure the time-dependent results.…”

Phenazine-1-Carboxylic Acid-Producing Bacteria Enhance the Reactivity of Iron Minerals in Dryland and Irrigated Wheat Rhizospheres

“…However, micro-XANES spectra from the dryland PCA + and PCA – rhizospheres (groups 1, 3, 4, and 5 in Figures , , and S3) did not differ consistently, indicating that the PCA + strain did not alter the identity of Fe-bearing minerals in the dryland rhizospheres. Prior work has demonstrated that extensive electron transfer and atom exchange can occur at the surfaces of Fe­(III)-bearing minerals without causing changes in aqueous Fe­(II) concentrations or mineral identity. ,,,,,− In particular, the formation of secondary minerals during DIR is inhibited by the complexation and coprecipitation of Fe with organic matter, including the extracellular polymeric substances found in biofilm matrices. ,,,,− …”