Composition and structure of iron oxidation surface layers produced in weak acidic solutions

Montes Atenas, Gonzalo, Mielczarski, Ela and Mielczarski, Jerzy A. (2005) Composition and structure of iron oxidation surface layers produced in weak acidic solutions. Journal of Colloid and Interface Science, 289 1: 157-170. doi:10.1016/j.jcis.2005.03.062

Author Montes Atenas, Gonzalo
Mielczarski, Ela
Mielczarski, Jerzy A.
Title Composition and structure of iron oxidation surface layers produced in weak acidic solutions
Journal name Journal of Colloid and Interface Science   Check publisher's open access policy
ISSN 0021-9797
Publication date 2005-09-01
Sub-type Article (original research)
DOI 10.1016/j.jcis.2005.03.062
Volume 289
Issue 1
Start page 157
End page 170
Total pages 14
Place of publication Amsterdam, The Netherlands
Publisher Elsevier B.V.
Language eng
Subject 0914 Resources Engineering and Extractive Metallurgy
091404 Mineral Processing/Beneficiation
091405 Mining Engineering
Abstract Although oxidation/passivation of iron in basic solution has been extensively investigated, there is very little information on iron corrosion in weak acidic solutions. In this work, iron surface composition and structure, produced in aerobic aqueous solutions ranging from pH 2 to 5, were determined in detail by the use of infrared external reflection spectroscopy, X-ray photoelectron spectroscopy and scanning electron microscopy. The most striking observation is that at pH 2 and 3 almost all oxidized iron is dissolved in solution, whereas at pH 4 and 5 the product of iron oxidation is deposited on the iron surface in the form of lepidocrocite, γ-FeOOH. Detailed iron surface and solution analyses allow the proposition of the following overall oxidation reactions (equations missing) At pH 2 and 3, only a very thin surface layer consisting of FeO andAt pH 2 and 3, only a very thin surface layer consisting of FeO and Fe(OH)2 with polymeric structure is observed on the iron surface. The amounts of these surface species remain almost constant (2–5 nm) from the first minutes to a few hours of reaction, if pH is kept constant. Nevertheless, with time the akaganeite-like, β-FeOOH structure is also detected. At pH 4 and 5, the amount of lepidocrocite deposited on the iron surface increases with reaction time. Detailed quantitative evaluation of the lepidocrocite produced at pH 5 and its surface distribution on iron was performed based on the comparison of infrared spectroscopic data with spectral simulation results of assumed surface structures. At pH 4 and 5 and a temperature of 40–50 °C, in addition to a very large amount of lepidocrocite other oxy-hydroxide surface species such as goethite (α-FeOOH) and feroxyhite (δ-FeOOH), were identified. Addition of Cl− ions to solution at 10−3 M concentration at pH 5 increases the oxidation rate of iron by about 50%, and lepidocrocite remains the only oxidation product. Similarly, an addition of Fe2+ ions to solution at pH 5 very strongly enhances lepidocrocite formation as well as its conductivity. The latter finding is important for the possible application of metallic iron as a catalyst in redox reactions, for example, for decomposition of difficult-to-biodegrade water pollutants.
Keyword Iron surface oxidation
Spectroscopic studies
Reaction pathways
Surface composition and structure
Acidic pH
Q-Index Code C1

Document type: Journal Article
Sub-type: Article (original research)
Collections: Julius Kruttschnitt Mineral Research Centre Publications
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Created: Mon, 18 Jan 2010, 14:55:28 EST by Rosalind Blair on behalf of Sustainable Minerals Institute