Reduction of Manganese Oxides

Thermodynamic, Kinetic and Mechanistic Considerations for One- Versus Two-Electron Transfer Steps

George W. Luther, Aubin Thibault de Chanvalon, Véronique E. Oldham, Emily R. Estes, Bradley Tebo, Andrew S. Madison

Research output: Contribution to journalArticle

5 Citations (Scopus)

Abstract

Manganese oxides, typically similar to δ-MnO2, form in the aquatic environment at near neutral pH via bacterially promoted oxidation of Mn(II) species by O2, as the reaction of [Mn(H2O)6]2+ with O2 alone is not thermodynamically favorable below pH of ~ 9. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λmax = 623 nm), their concentration in the aquatic environment can be determined in aqueous environmental samples (e.g., across the oxic–anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary and the Broadkill River estuary surrounded by salt marsh wetlands), and their reaction progress can be followed in kinetic studies. The LBB reaction with oxidized Mn solids can occur via a hydrogen atom transfer (HAT) reaction, which is a one-electron transfer process, but is unfavorable with oxidized Fe solids. HAT thermodynamics are also favorable for nitrite with LBB and MnO2 with ammonia (NH3). Reactions are unfavorable for NH4 + and sulfide with oxidized Fe and Mn solids, and NH3 with oxidized Fe solids. In laboratory studies and aquatic environments, the reduction of manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L] at significant concentrations even when two-electron reductants react with MnO2. Key reductants are hydrogen sulfide, Fe(II) and organic ligands, including the siderophore desferioxamine-B. We present laboratory data on the reaction of colloidal MnO2 solutions (λmax ~ 370 nm) with these reductants. In marine waters, colloidal forms of Mn oxides (< 0.2 µm) have not been detected as Mn oxides are quantitatively trapped on 0.2-µm filters. Thus, the reactivity of Mn oxides with reductants depends on surface reactions and possible surface defects. In the case of MnO2, Mn(IV) is an inert cation in octahedral coordination; thus, an inner-sphere process is likely for electrons to go into the empty eg * conduction band of its orbitals. Using frontier molecular orbital theory and band theory, we discuss aspects of these surface reactions and possible surface defects that may promote MnO2 reduction using laboratory and field data for the reaction of MnO2 with hydrogen sulfide and other reductants.

Original languageEnglish (US)
JournalAquatic Geochemistry
DOIs
StateAccepted/In press - Jan 1 2018

Fingerprint

manganese oxides
manganese oxide
Reducing Agents
electron transfer
thermodynamics
Thermodynamics
aquatic environment
electron
kinetics
Kinetics
Electrons
Oxides
oxide
hydrogen sulfide
Hydrogen Sulfide
ligand
Surface defects
defect
Surface reactions
Estuaries

Keywords

  • Band theory
  • Electron transfer
  • Hydrogen atom transfer
  • Manganese oxide reduction
  • Mn(III) ligands

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

Cite this

Reduction of Manganese Oxides : Thermodynamic, Kinetic and Mechanistic Considerations for One- Versus Two-Electron Transfer Steps. / Luther, George W.; Thibault de Chanvalon, Aubin; Oldham, Véronique E.; Estes, Emily R.; Tebo, Bradley; Madison, Andrew S.

In: Aquatic Geochemistry, 01.01.2018.

Research output: Contribution to journalArticle

Luther, George W. ; Thibault de Chanvalon, Aubin ; Oldham, Véronique E. ; Estes, Emily R. ; Tebo, Bradley ; Madison, Andrew S. / Reduction of Manganese Oxides : Thermodynamic, Kinetic and Mechanistic Considerations for One- Versus Two-Electron Transfer Steps. In: Aquatic Geochemistry. 2018.
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AU - Luther, George W.

AU - Thibault de Chanvalon, Aubin

AU - Oldham, Véronique E.

AU - Estes, Emily R.

AU - Tebo, Bradley

AU - Madison, Andrew S.

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N2 - Manganese oxides, typically similar to δ-MnO2, form in the aquatic environment at near neutral pH via bacterially promoted oxidation of Mn(II) species by O2, as the reaction of [Mn(H2O)6]2+ with O2 alone is not thermodynamically favorable below pH of ~ 9. As manganese oxide species are reduced by the triphenylmethane compound leucoberbelein blue (LBB) to form the colored oxidized form of LBB (λmax = 623 nm), their concentration in the aquatic environment can be determined in aqueous environmental samples (e.g., across the oxic–anoxic interface of the Chesapeake Bay, the hemipelagic St. Lawrence Estuary and the Broadkill River estuary surrounded by salt marsh wetlands), and their reaction progress can be followed in kinetic studies. The LBB reaction with oxidized Mn solids can occur via a hydrogen atom transfer (HAT) reaction, which is a one-electron transfer process, but is unfavorable with oxidized Fe solids. HAT thermodynamics are also favorable for nitrite with LBB and MnO2 with ammonia (NH3). Reactions are unfavorable for NH4 + and sulfide with oxidized Fe and Mn solids, and NH3 with oxidized Fe solids. In laboratory studies and aquatic environments, the reduction of manganese oxides leads to the formation of Mn(III)-ligand complexes [Mn(III)L] at significant concentrations even when two-electron reductants react with MnO2. Key reductants are hydrogen sulfide, Fe(II) and organic ligands, including the siderophore desferioxamine-B. We present laboratory data on the reaction of colloidal MnO2 solutions (λmax ~ 370 nm) with these reductants. In marine waters, colloidal forms of Mn oxides (< 0.2 µm) have not been detected as Mn oxides are quantitatively trapped on 0.2-µm filters. Thus, the reactivity of Mn oxides with reductants depends on surface reactions and possible surface defects. In the case of MnO2, Mn(IV) is an inert cation in octahedral coordination; thus, an inner-sphere process is likely for electrons to go into the empty eg * conduction band of its orbitals. Using frontier molecular orbital theory and band theory, we discuss aspects of these surface reactions and possible surface defects that may promote MnO2 reduction using laboratory and field data for the reaction of MnO2 with hydrogen sulfide and other reductants.

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