Environmental Molecular Sciences Laboratory

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Mechanisms of Fe(II)-Catalyzed Transformation of Fe(III)-(Oxyhydr)oxides

Date: 
Saturday, October 1, 2016
Principal Investigator: 
Kevin Rosso
Lead Institution: 
Pacific Northwest National Laboratory
Closed Date: 
Sunday, September 30, 2018
Project ID: 
49381
Abstract: 

Interaction of aqueous Fe(II) with major Fe(III)-oxides and oxyhydroxides yields the enigmatic catalytically reactive "sorbed Fe(II)" species and apparent facile recrystallization of even the most stable forms of these minerals. Such phenomena exemplify a broad category of poorly understood redox-active mineral/fluid interfaces where fundamental understanding remains limited, despite far-reaching implications for interpreting redox geochemistry in most subsurface and aquatic environments. This subtask has established a working model of recrystallization: Fe(II) adsorbs to and transfers electrons across the interface yielding lattice Fe(III) addition, coupled by electron conduction to remote Fe(III) reduction and Fe(II) release elsewhere from the lattice. However, the model is challenged by unfavorable energetics and kinetics of Fe(II)/Fe(III) interfacial electron transfer, limitations in linking macroscopic isotopic tracer observations to microscopic processes, and the fact that the recrystallization process has never been observed in situ. We propose multiscale research on Fe(II)-catalyzed transformations of goethite, hematite, and ferrihydrite, using Fe/O isotope-resolved tomographic mapping of recrystallization fronts in individual crystallites, liquid cell TEM of particle recrystallization in situ, and advanced molecular simulation of up to whole particle recrystallization incorporating the collective electron and atom exchange dynamics. We will unveil the important elementary steps, the operative length scales for conduction-based recrystallization, and relationships between interfacial electron transfer, surface structure, and mineral semiconducting properties. The long-term vision is to translate insights to other redox autocatalytic mineral/fluid interfacial systems.