Electrochemical Characterization of Magnetite with Agarose-Stabilized Powder Disk Electrodes and Potentiometric Methods

Miranda J. Bradley, Paul Tratnyek

Research output: Contribution to journalArticle

Abstract

The mixed and variable valence of iron in magnetite (Fe(III) tet [Fe(II),Fe(III)] oct O 4 2- ) give this mineral unique properties that make it an important participant in redox reactions in environmental systems. However, the variability in its stoichiometry and other physical properties complicates the determination of its effective redox potential. To address this challenge, a robust method was developed to prepare working electrodes with mineral powders of diverse characteristics and agarose-stabilized pore waters of controlled composition. This second-generation powder-disk electrode (PDEv2) methodology was used to characterize the electrochemical properties of magnetite samples from a wide variety of sources (lab-synthesized, commercial, and magnetically separated from environmental samples) using a sequence of complementary potentiometric methods: chronopotentiometry (CP), linear polarization resistance (LPR), and then linear sweep voltammetry (LSV). The passive method CP gave open-circuit potentials (E OC ) and the active method LPR gave corrosion potentials (E 0,LPR ) that agree closely with each other but vary over a wide range for the magnetite samples tested (ca. 520 mV, from -267 to +253 mV vs SHE). The active method LSV gave values of E 0,LSV that become increasingly more negative than E OC for the samples with more positive potentials (by up to 189 mV). This effect is consistent with the cathodic polarization applied at the beginning of the LSV scan and suggests there is convergence of substoichiometric magnetites to the potential of stoichiometric magnetite after polarization. By all methods, lab-synthesized magnetites gave more negative potentials and smaller polarization resistances (R p ) than magnetite from commercial sources or magnetic separation of environmental samples. This is consistent with the common notion that freshly synthesized minerals are more reactive, but clear correlations were not found between the measured redox potentials and surface area, iron stoichiometry, or magnetic susceptibility. All the measured potentials for magnetite fall in a range between calculated thermodynamic values for redox couples involving relevant iron species, which is consistent with the measured values being mixed potentials. The wide range in effective redox potential of magnetite is likely to influence its role in biogeochemistry and contaminant fate.

Original languageEnglish (US)
JournalACS Earth and Space Chemistry
DOIs
StatePublished - Jan 1 2019

Fingerprint

Ferrosoferric Oxide
magnetite
Powders
Sepharose
electrode
Electrodes
electrodes
polarization
Voltammetry
Polarization
redox potential
linear polarization
Minerals
Iron
stoichiometry
minerals
iron
Stoichiometry
Biogeochemistry
mineral property

Keywords

  • effective redox potential
  • magnetite
  • porous powder disk electrode
  • potentiometry
  • spinel iron oxide
  • voltammetry

ASJC Scopus subject areas

  • Geochemistry and Petrology
  • Atmospheric Science
  • Space and Planetary Science

Cite this

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title = "Electrochemical Characterization of Magnetite with Agarose-Stabilized Powder Disk Electrodes and Potentiometric Methods",
abstract = "The mixed and variable valence of iron in magnetite (Fe(III) tet [Fe(II),Fe(III)] oct O 4 2- ) give this mineral unique properties that make it an important participant in redox reactions in environmental systems. However, the variability in its stoichiometry and other physical properties complicates the determination of its effective redox potential. To address this challenge, a robust method was developed to prepare working electrodes with mineral powders of diverse characteristics and agarose-stabilized pore waters of controlled composition. This second-generation powder-disk electrode (PDEv2) methodology was used to characterize the electrochemical properties of magnetite samples from a wide variety of sources (lab-synthesized, commercial, and magnetically separated from environmental samples) using a sequence of complementary potentiometric methods: chronopotentiometry (CP), linear polarization resistance (LPR), and then linear sweep voltammetry (LSV). The passive method CP gave open-circuit potentials (E OC ) and the active method LPR gave corrosion potentials (E 0,LPR ) that agree closely with each other but vary over a wide range for the magnetite samples tested (ca. 520 mV, from -267 to +253 mV vs SHE). The active method LSV gave values of E 0,LSV that become increasingly more negative than E OC for the samples with more positive potentials (by up to 189 mV). This effect is consistent with the cathodic polarization applied at the beginning of the LSV scan and suggests there is convergence of substoichiometric magnetites to the potential of stoichiometric magnetite after polarization. By all methods, lab-synthesized magnetites gave more negative potentials and smaller polarization resistances (R p ) than magnetite from commercial sources or magnetic separation of environmental samples. This is consistent with the common notion that freshly synthesized minerals are more reactive, but clear correlations were not found between the measured redox potentials and surface area, iron stoichiometry, or magnetic susceptibility. All the measured potentials for magnetite fall in a range between calculated thermodynamic values for redox couples involving relevant iron species, which is consistent with the measured values being mixed potentials. The wide range in effective redox potential of magnetite is likely to influence its role in biogeochemistry and contaminant fate.",
keywords = "effective redox potential, magnetite, porous powder disk electrode, potentiometry, spinel iron oxide, voltammetry",
author = "Bradley, {Miranda J.} and Paul Tratnyek",
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language = "English (US)",
journal = "ACS Earth and Space Chemistry",
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T1 - Electrochemical Characterization of Magnetite with Agarose-Stabilized Powder Disk Electrodes and Potentiometric Methods

AU - Bradley, Miranda J.

AU - Tratnyek, Paul

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N2 - The mixed and variable valence of iron in magnetite (Fe(III) tet [Fe(II),Fe(III)] oct O 4 2- ) give this mineral unique properties that make it an important participant in redox reactions in environmental systems. However, the variability in its stoichiometry and other physical properties complicates the determination of its effective redox potential. To address this challenge, a robust method was developed to prepare working electrodes with mineral powders of diverse characteristics and agarose-stabilized pore waters of controlled composition. This second-generation powder-disk electrode (PDEv2) methodology was used to characterize the electrochemical properties of magnetite samples from a wide variety of sources (lab-synthesized, commercial, and magnetically separated from environmental samples) using a sequence of complementary potentiometric methods: chronopotentiometry (CP), linear polarization resistance (LPR), and then linear sweep voltammetry (LSV). The passive method CP gave open-circuit potentials (E OC ) and the active method LPR gave corrosion potentials (E 0,LPR ) that agree closely with each other but vary over a wide range for the magnetite samples tested (ca. 520 mV, from -267 to +253 mV vs SHE). The active method LSV gave values of E 0,LSV that become increasingly more negative than E OC for the samples with more positive potentials (by up to 189 mV). This effect is consistent with the cathodic polarization applied at the beginning of the LSV scan and suggests there is convergence of substoichiometric magnetites to the potential of stoichiometric magnetite after polarization. By all methods, lab-synthesized magnetites gave more negative potentials and smaller polarization resistances (R p ) than magnetite from commercial sources or magnetic separation of environmental samples. This is consistent with the common notion that freshly synthesized minerals are more reactive, but clear correlations were not found between the measured redox potentials and surface area, iron stoichiometry, or magnetic susceptibility. All the measured potentials for magnetite fall in a range between calculated thermodynamic values for redox couples involving relevant iron species, which is consistent with the measured values being mixed potentials. The wide range in effective redox potential of magnetite is likely to influence its role in biogeochemistry and contaminant fate.

AB - The mixed and variable valence of iron in magnetite (Fe(III) tet [Fe(II),Fe(III)] oct O 4 2- ) give this mineral unique properties that make it an important participant in redox reactions in environmental systems. However, the variability in its stoichiometry and other physical properties complicates the determination of its effective redox potential. To address this challenge, a robust method was developed to prepare working electrodes with mineral powders of diverse characteristics and agarose-stabilized pore waters of controlled composition. This second-generation powder-disk electrode (PDEv2) methodology was used to characterize the electrochemical properties of magnetite samples from a wide variety of sources (lab-synthesized, commercial, and magnetically separated from environmental samples) using a sequence of complementary potentiometric methods: chronopotentiometry (CP), linear polarization resistance (LPR), and then linear sweep voltammetry (LSV). The passive method CP gave open-circuit potentials (E OC ) and the active method LPR gave corrosion potentials (E 0,LPR ) that agree closely with each other but vary over a wide range for the magnetite samples tested (ca. 520 mV, from -267 to +253 mV vs SHE). The active method LSV gave values of E 0,LSV that become increasingly more negative than E OC for the samples with more positive potentials (by up to 189 mV). This effect is consistent with the cathodic polarization applied at the beginning of the LSV scan and suggests there is convergence of substoichiometric magnetites to the potential of stoichiometric magnetite after polarization. By all methods, lab-synthesized magnetites gave more negative potentials and smaller polarization resistances (R p ) than magnetite from commercial sources or magnetic separation of environmental samples. This is consistent with the common notion that freshly synthesized minerals are more reactive, but clear correlations were not found between the measured redox potentials and surface area, iron stoichiometry, or magnetic susceptibility. All the measured potentials for magnetite fall in a range between calculated thermodynamic values for redox couples involving relevant iron species, which is consistent with the measured values being mixed potentials. The wide range in effective redox potential of magnetite is likely to influence its role in biogeochemistry and contaminant fate.

KW - effective redox potential

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KW - potentiometry

KW - spinel iron oxide

KW - voltammetry

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