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Edited by François M. M. Morel, Princeton University, Princeton, NJ, and approved January 3, 2019 (received for review September 25, 2018)
Significance
The bioavailability of iron in the environment, and coupled metals, nutrients, and contaminants, depends on the stability of common Fe(III) minerals such as goethite (FeOOH) and hematite (Fe2O3). At redox boundaries, iron isotopic tracer studies suggest that interaction with aqueous Fe(II) creates dynamic conditions of atom exchange (AE). However, mechanistic models have not advanced beyond speculation, because of the challenges of mapping AE fronts recorded in isotopic distributions in individual nanoscale crystallites. Here we demonstrate successful use of 3D atom probe tomography for this purpose. The penetration depth, spatial heterogeneity, and ties to mineral defects are visualized, helping constrain mechanistic models and setting a precedent for detailed interrogation of iron redox cycling in the environment.
Abstract
The autocatalytic redox interaction between aqueous Fe(II) and Fe(III)-(oxyhydr)oxide minerals such as goethite and hematite leads to rapid recrystallization marked, in principle, by an atom exchange (AE) front, according to bulk iron isotopic tracer studies. However, direct evidence for this AE front has been elusive given the analytical challenges of mass-resolved imaging at the nanoscale on individual crystallites. We report successful isolation and characterization of the AE front in goethite microrods by 3D atom probe tomography (APT). The microrods were reacted with Fe(II) enriched in tracer 57Fe at conditions consistent with prior bulk studies. APT analyses and 3D reconstructions on cross-sections of the microrods reveal an AE front that is spatially heterogeneous, at times penetrating several nanometers into the lattice, in a manner consistent with defect-accelerated exchange. Evidence for exchange along microstructural domain boundaries was also found, suggesting another important link between exchange extent and initial defect content. The findings provide an unprecedented view into the spatial and temporal characteristics of Fe(II)-catalyzed recrystallization at the atomic scale, and substantiate speculation regarding the role of defects controlling the dynamics of electron transfer and AE interaction at this important redox interface.
Footnotes
- ↵1To whom correspondence may be addressed. Email: sandra.taylor{at}pnnl.gov or kevin.rosso{at}pnnl.gov.
↵2Present address: Nanolab Technologies, Milpitas, CA 95035.
Author contributions: S.D.T. and K.M.R. designed research; S.D.T., J.L., X.Z., B.W.A., L.K., D.K.S., and D.E.P. performed research; S.D.T. contributed new reagents/analytic tools; S.D.T., J.L., L.K., D.K.S., and D.E.P. analyzed data; and S.D.T. and K.M.R. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1816620116/-/DCSupplemental.
Published under the PNAS license.
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Visualizing the iron atom exchange front in the Fe(II)-catalyzed recrystallization of goethite by atom probe tomography
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