Synthetic green rusts, GRs, are prepared by oxidation of Fe(OH)2 incorporating Cl-, SO42-, or CO32- ions. Eh−pH diagrams are drawn, and thermodynamic data are derived. A GR incorporating OH- ions, GR1(OH-), is suspected to exist like similar other M(II)−M(III) compounds. GRs form as corrosion products of steels, implying microbially induced corrosion. Mossbauer and Raman spectroscopies allowed the identification of GR in samples extracted from hydromorphic soils scattered over Brittany, France. This mineral has a varying Fe(III)/Fe(II) ratio. At Fougeres, it increases with depth till the oc currence of more oxidized ferric oxyhydroxide. In the same sites, soil solutions are collected and prevented from any oxidation and photoreduction. In large ranges of pH, pe, and Fe(II) concentration variations, soil solutions are in equilibrium with a Fe(II)−Fe(III) compound, a GR1 mineral with pyroaurite-like structure incorporating OH- ions and having the formula [FeII(1-x)FeIIIx(OH)2]+x·[xOH]-x ≡ Fe(OH)(2+x). Compu...

Thermodynamic Equilibria in Aqueous Suspensions of Synthetic and Natural Fe(II)−Fe(III) Green Rusts: Occurrences of the Mineral in Hydromorphic Soils

Mixed-valent iron [Fe(II)-Fe(III)] minerals such as magnetite and green rust have received a significant amount of attention over recent decades, especially in the environmental sciences. These mineral phases are intrinsic and essential parts of biogeochemical cycling of metals and organic carbon and play an important role regarding the mobility, toxicity, and redox transformation of organic and inorganic pollutants. The formation pathways, mineral properties, and applications of magnetite and green rust are currently active areas of research in geochemistry, environmental mineralogy, geomicrobiology, material sciences, environmental engineering, and environmental remediation. These aspects ultimately dictate the reactivity of magnetite and green rust in the environment, which has important consequences for the application of these mineral phases, for example in remediation strategies. In this review we discuss the properties, occurrence, formation by biotic as well as abiotic pathways, characterization t...

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Magnetite and Green Rust: Synthesis, Properties, and Environmental Applications of Mixed-Valent Iron Minerals

The preparation and thermodynamic properties of Fe(II)-Fe(III) hydroxide-carbonate (green rust 1) ; Pourbaix diagram of iron in carbonate-containing aqueous media

Evaluation of the free energy of formation of Fe(II)-Fe(III) hydroxide-sulphate (green rust) and its reduction of nitrite

The oxidation processes of Fe(II) hydroxo-complexes to α-, β-, γ-, and δ-FeOOH and Fe3O4, which are important atmospheric products of steels, and the effect of Cu2+, PO43− ion on the oxidation of the Fe(II) hydroxo-complexes in aqueous solutions have been investigated. The mechanism of atmospheric rusting deduced from the results obtained in the present investigation has been used to explain the difference in behaviour between ordinary mild steels and low alloy steels during atmospheric exposure. It is concluded that Fe(II) complexes are transformed to amorphous δ-FeOOH by the catalytic effect of Cu and P present in steels. The amorphous δ-FeOOH forms a compact rust layer that enhances corrosion resistance of the steel.

The Mechanism of Atmospheric Rusting and the Effect of Cu and P on the Rust Formation of Low Alloy Steels

The region of stability of green rust II in the electrochemical potential-pH equilibrium diagram of iron in sulphate medium

The shape of DTA endothermic peak, associated with the dehydration of goethite FeOOH into hematite, depends strongly on the size of the goethite particles. With samples of low specific surface, two distinct peaks were observed. The dehydration process of a low-specific surface goethite was followed by X-ray diffraction and lowtemperature nitrogen adsorption. The results are in good agreement with the hypothesis of the formation of an impermeable hematite layer on each grain which temporarily stops the dehydration process of the internal part of the grain.

DTA study of the dehydration of synthetic goethite αFeOOH

Cette etude consacree a l'oxydation d'un gel d'hydroxyde de fer(II), par aeration, en milieu chlorure, et en presence de Fe++ en solution, complete nos travaux anterieurs en milieu sulfate. Nous avons etudie l'influence de differents parametres chimiques et physiques sur la cinetique reactionnelle. Le mecanisme d'oxydation a ete etudie en portant une attention particuliere a l'etape intermediaire. l'ensemble des resultats permet de constater que la produit intermediaire d'oxydation, le green rust I, est un compose chlorure ou l'etat d'oxydation du fer varie entre 2,25 et 2,55. Dans les conditions experimentales usuelles, le produit final d'oxydation en milieu chlorure est uniquement constitue de lepidocrocite. l'apparition de la goethite pour des durees tres longues provient vraisembl‐ablement d'une transformation γ FeOOH → α FeOOH thermodynamiquement possible. Oxidation of Fe (OH)2 by Air in Chloride‐containing Medium We studied the oxidation of an aqueous suspension of Fe(OH)2, by air, in the presence of sodium and iron(II) chloride in solution. The experimental techniques were the same as those used in a previous work on the oxidation of Fe(OH)2 in sulfate medium. We determined the influence of some physico‐chemical factors, such as temperature, concentration, air flow rate, on the oxidation kinetics; we draw a particular attention to the nature and the composition of the intermediate oxidation product, the green rust I. It is a chloride‐containing compound, in which the oxidation state of iron is comprised between 2.25 and 2.55. In the usual experimental conditions, the final oxidation product ist lepidocrocite only. For very long oxidation times, goethite may also be formed, but this is probably due to a subsequent transformation of γ‐into α‐FeOOH, via the aqueous phase. Copyright © 1976 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim

Etude de l'Oxydation par Aeration de Fe(OH)2 en Milieu Chlorure

Behaviour of pyrite during hydrogenation of graphite at atmospheric pressure

Ce travail est consacre a l'etude de l'influence de la temperature et de l'addition des ions etrangers sur la cinetique de formation de la goethite en milieu acide par aeration. La temperature jusqu' a 70°C n'influence pas la cinetique de cette oxydation, ce qui prouve que le transfert d'oxygene se fait par la phase liquide.



Les cations manganese et cobalt ralentissent la formation et la croissance de αFeOOH. Les anions citrate et phosphate qui augmentent la solubilite du fer(III) en solution sont defavorables a la transformation de l'hydroxyde de fer(II) en goethite.



Effect of Temperature and Presence of Foreign Ions on the Kinetics and Mechanism of the Goethite Formation in Liquid Phase



We studied the influence of temperature and addition of some foreign ions on the kinetics of transformation of the goethite in sulfate medium by slow air bubbling. The temperature up to 70°C does not influence the kinetics of oxidation; this proves that the transfer of oxygen taken place through the liquid phase.



Manganese and colbalt cations impede the formation and the growth of αFeOOH crystallites. Citrate and phosphate anions both hinder this transformation.

Mehdi Ghodsi

Papers

The region of stability of green rust II in the electrochemical potential-pH equilibrium diagram of iron in sulphate medium

DTA study of the dehydration of synthetic goethite αFeOOH

Etude de l'Oxydation par Aeration de Fe(OH)2 en Milieu Chlorure

Behaviour of pyrite during hydrogenation of graphite at atmospheric pressure

Influence de la Température et de la Présence des Ions Etrangers sur la Cinétique et le Mécanisme de Formation de la Goethite en Milieu Aqueux