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Title: Energetics and the Role of Defects in Fe(II)-Catalyzed Goethite Recrystallization from Molecular Simulations

Abstract

Goethite is one of the most stable and common iron(III) minerals at the Earth’s near surface. However, recent isotope tracer studies have suggested that goethite continuously recrystallizes in the presence of aqueous Fe(II) ions. Some of these studies indicate the presence of two regimes of atom-exchange kinetics, a rapid stage assigned to reactive defect sites initially available at particle surfaces, followed by slower continuous exchange. An autocatalytic solid-state electron conduction model coupling Fe(II) oxidative adsorption to its reductive release at spatially distinct sites has been proposed, but the thermodynamic driving force has yet to be pinpointed. Here, using a novel hybrid/reactive molecular simulation method, for goethite (110) surfaces at circumneutral pH, we rigorously tested whether surface free energy minimization, including examining the role of structural defects, is sufficient to overcome the activation energy for interfacial electron transfer and conduction. The simulations quantitatively show that (i) on smooth stable surfaces the available thermal energy at dynamic equilibrium is sufficient to sustain the slow continuous regime of atom-exchange kinetics via short intrasurface electron conduction pathways of 1–2 nm (three to five Fe site hops), (ii) in this slower regime, the model converges to atom-exchange kinetics of 10–5 Fe s–1 cm–2, a ratemore » recently deduced from stochastic modeling of experimental data and linked to the reductive dissolution rate of goethite, and (iii) the driving force for smoothing of initially rough defective goethite surfaces can accelerate atom exchange to an extent quantitatively consistent with that observed in the initial rapid stage, in this case accessing conduction pathways of up to 8 nm. The findings suggest that the interaction of Fe(II) with initially defective goethite surfaces can drive, by the conduction model, atom exchange that is capable of recrystallizing the interiors of nanoscale particles and that, closer to equilibrium on smooth surfaces, slower atom exchange continues in perpetuity but likely involving only the outermost atomic layers.« less

Authors:
ORCiD logo [1]; ORCiD logo [2]
  1. Energy Geosciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720, United States
  2. Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1506695
Report Number(s):
PNNL-SA-142608
Journal ID: ISSN 2472-3452
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
ACS Earth and Space Chemistry
Additional Journal Information:
Journal Volume: 3; Journal Issue: 2; Journal ID: ISSN 2472-3452
Publisher:
American Chemical Society
Country of Publication:
United States
Language:
English

Citation Formats

Zarzycki, Piotr, and Rosso, Kevin M. Energetics and the Role of Defects in Fe(II)-Catalyzed Goethite Recrystallization from Molecular Simulations. United States: N. p., 2019. Web. doi:10.1021/acsearthspacechem.8b00175.
Zarzycki, Piotr, & Rosso, Kevin M. Energetics and the Role of Defects in Fe(II)-Catalyzed Goethite Recrystallization from Molecular Simulations. United States. doi:10.1021/acsearthspacechem.8b00175.
Zarzycki, Piotr, and Rosso, Kevin M. Fri . "Energetics and the Role of Defects in Fe(II)-Catalyzed Goethite Recrystallization from Molecular Simulations". United States. doi:10.1021/acsearthspacechem.8b00175.
@article{osti_1506695,
title = {Energetics and the Role of Defects in Fe(II)-Catalyzed Goethite Recrystallization from Molecular Simulations},
author = {Zarzycki, Piotr and Rosso, Kevin M.},
abstractNote = {Goethite is one of the most stable and common iron(III) minerals at the Earth’s near surface. However, recent isotope tracer studies have suggested that goethite continuously recrystallizes in the presence of aqueous Fe(II) ions. Some of these studies indicate the presence of two regimes of atom-exchange kinetics, a rapid stage assigned to reactive defect sites initially available at particle surfaces, followed by slower continuous exchange. An autocatalytic solid-state electron conduction model coupling Fe(II) oxidative adsorption to its reductive release at spatially distinct sites has been proposed, but the thermodynamic driving force has yet to be pinpointed. Here, using a novel hybrid/reactive molecular simulation method, for goethite (110) surfaces at circumneutral pH, we rigorously tested whether surface free energy minimization, including examining the role of structural defects, is sufficient to overcome the activation energy for interfacial electron transfer and conduction. The simulations quantitatively show that (i) on smooth stable surfaces the available thermal energy at dynamic equilibrium is sufficient to sustain the slow continuous regime of atom-exchange kinetics via short intrasurface electron conduction pathways of 1–2 nm (three to five Fe site hops), (ii) in this slower regime, the model converges to atom-exchange kinetics of 10–5 Fe s–1 cm–2, a rate recently deduced from stochastic modeling of experimental data and linked to the reductive dissolution rate of goethite, and (iii) the driving force for smoothing of initially rough defective goethite surfaces can accelerate atom exchange to an extent quantitatively consistent with that observed in the initial rapid stage, in this case accessing conduction pathways of up to 8 nm. The findings suggest that the interaction of Fe(II) with initially defective goethite surfaces can drive, by the conduction model, atom exchange that is capable of recrystallizing the interiors of nanoscale particles and that, closer to equilibrium on smooth surfaces, slower atom exchange continues in perpetuity but likely involving only the outermost atomic layers.},
doi = {10.1021/acsearthspacechem.8b00175},
journal = {ACS Earth and Space Chemistry},
issn = {2472-3452},
number = 2,
volume = 3,
place = {United States},
year = {2019},
month = {1}
}