Electrochemistry and dissolution kinetics of magnetite and ilmenite
Links
- More information: Publisher Index Page (via DOI)
- Download citation as: RIS | Dublin Core
Abstract
Natural samples of magnetite and ilmenite were experimentally weathered in pH 1–7 anoxic solutions at temperatures of 2–65 °C. Reaction of magnetite is described as [Fe2+Fe23+]O4(magnetite) + 2H+ → γ[Fe23+]O3(maghemite) + Fe2+ + H2O. Dynamic polarization experiments using magnetite electrodes confirmed that this reaction is controlled by two electrochemical half cells, 3[Fe2+Fe23+]O4(magnetite) → 4γ[Fe23+]O3(maghemite) + Fe2+ + 2e− and [Fe2+Fe23+]O4(magnetite) + 8 H+ + 2e− → 3Fe2+ + 4H2O, which result in solid state Fe3+ reduction, formation of an oxidized layer and release of Fe(II) to solution. XPS data revealed that iron is present in the ferric state in the surfaces of reacted magnetite and ilmenite and that the TiFe ratio increased with reaction pH for ilmenite.
Short-term (<36 h) release rates of Fe(II) were linear with time. Between pH 1 and 7, rates varied between 0.3 and 13 × 10−14 mol · cm−2 · s−1 for magnetite and 0.05 and 12.3 × 10−14 mol · cm−2 · s−1 for ilmenite. These rates are two orders of magnitude slower than electrochemical rates determined by Tafel and polarization resistance measurements. Discrepancies are due to both differences in geometric and BET surface area estimates and in the oxidation state of the mineral surface. In long-term closed-system experiments (<120 days), Fe(II) release slowed with time due to the passivation of the surfaces by increasing thicknesses of oxide surface layers. A shrinking core model, coupling surface reaction and diffusion transport, predicted that at neutral pH, the mean residence time for sand-size grains of magnetite and ilmenite will exceed 107 years. This agrees with long-term stability of these oxides in the geologic record.
Publication type | Article |
---|---|
Publication Subtype | Journal Article |
Title | Electrochemistry and dissolution kinetics of magnetite and ilmenite |
Series title | Geochimica et Cosmochimica Acta |
DOI | 10.1016/0016-7037(94)90420-0 |
Volume | 58 |
Issue | 8 |
Year Published | 1994 |
Language | English |
Publisher | Elsevier |
Description | 17 p. |
First page | 1859 |
Last page | 1875 |
Google Analytic Metrics | Metrics page |