https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1245&context=usdoepub

Biogenic iron mineralization accompanying the dissimilatory reduction of hydrous ferric oxide by a groundwater bacterium

Owing to its high surface area and intrinsic reactivity, ferrihydrite serves as a dominant sink for numerous metals and nutrients in surface environments and is a potentially important terminal electron acceptor for microbial respiration. Introduction of Fe (II), by reductive dissolution of Fe(III) minerals, for example, converts ferrihydrite to Fe phases varying in their retention and reducing capacity. While Fe(II) concentration is the master variable dictating secondary mineralization pathways of ferrihydrite, here we reveal thatthe kinetics of conversion and ultimate mineral assemblage are a function of competing mineralization pathways influenced by pH and stabilizing ligands. Reaction of Fe(II) with ferrihydrite results in the precipitation of goethite, lepidocrocite, and magnetite. The three phases vary in their precipitation extent, rate, and residence time, all of which are primarily a function of Fe(II) concentration and ligand type (Cl, SO4, CO3). While lepidocrocite and goethite precipitate over a large Fe(II) concentration range, magnetite accumulation is only observed at surface loadings greater than 1.0 mmol Fe(II)/g ferrihydrite (in the absence of bicarbonate). Precipitation of magnetite induces the dissolution of lepidocrocite (presence of Cl) or goethite (presence of SO4), allowing for Fe(III)-dependent crystal growth. The rate of magnetite precipitation is a function of the relative proportions of goethite to lepidocrocite; the lower solubility of the former Fe (hydr)oxide slows magnetite precipitation. A one unit pH deviation from 7, however, either impedes (pH 6) or enhances (pH 8) magnetite precipitation. In the absence of magnetite nucleation, lepidocrocite and goethite continue to precipitate at the expense of ferrihydrite with near complete conversion within hours, the relative proportions of the two hydroxides dependent upon the ligand present. Goethite also continues to precipitate at the expense of lepidocrocite in the absence of chloride. In fact, the rate and extent of both goethite and magnetite precipitation are influenced by conditions conducive to the production and stability of lepidocrocite. Thus, predicting the secondary mineralization of ferrihydrite, a process having sweeping influences on contaminant/nutrient dynamics, will need to take into consideration kinetic restraints and transient precursor phases (e.g., lepidocrocite) that influence ensuing reaction pathways.

Competing Fe (II)-induced mineralization pathways of ferrihydrite.

Publisher Summary This chapter provides an overview of the sulfur poisoning of metals. Sulfur apparently bonds so strongly to metal surfaces that marked activity reduction occurs at extremely low gas-phase concentrations of sulfur-containing compounds. In commercial practice, the life of supported metal catalysts may be reduced to only a few months or weeks in the presence of only ppm quantities of sulfur contaminants in the feed. Because of the essentially irreversible adsorption of sulfur compounds on metals, regeneration is usually impossible or impractical. This chapter integrates available information on the interaction of sulfur with metal surfaces with that of poisoning studies to provide a more complete picture of sulfur poisoning and of the mechanism. The chapter stresses the importance of experimental techniques that provide definitive, fundamental information regarding sulfur adsorption and poisoning.

Sulfur Poisoning of Metals

Dissimilatory metal reducing bacteria (DMRB) catalyze the reduction of Fe(III) to Fe(II) in anoxic soils, sediments, and groundwater. Two-line ferrihydrite is a bioavailable Fe(III) oxide form that is exploited by DMRB as a terminal electron acceptor. A wide variety of biomineralization products result from the interaction of DMRB with 2-line ferrihydrite. Here we describe the state of knowledge on the biotransformation of synthetic 2-line ferrihydrite by laboratory cultures of DMRB using select published data and new experimental results. A facultative DMRB is emphasized ( Shewanella putrefaciens ) upon which most of this work has been performed. Key factors controlling the identity of the secondary mineral suite are evaluated including medium composition, electron donor and acceptor concentrations, ferrihydrite aging/recrystallization status, sorbed ions, and co-associated crystalline Fe(III) oxides. It is shown that crystalline ferric (goethite, hematite, lepidocrocite), ferrous (siderite, vivianite), ...

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Biomineralization of Poorly Crystalline Fe(III) Oxides by Dissimilatory Metal Reducing Bacteria (DMRB)

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

Developpement d'une methode de preparation mettant en jeu les anions SO 4 2− , Cl − , Br − et I − , des hydroxydes mixtes qui pourraient avoir la composition Fe (1−x) II Fe x III (OH) 2 •A xy (OH) x(1−y)

Preparation and properties of green rust type substances

The course of the transformation of synthetic γ-FeOOH (lepidocrocite) samples, prepared by different methods was investigated and the effect of a preliminary heat treatment of the initial material on the product phase composition and morphology studied. Both the goethite-tohematite ratio and the particle size of the products depend on the preparation method and on the preliminary heat treatment of the initial lepidocrocite. The results are discussed in terms of the dissolution of the less stable lepidocrocite followed by the crystallization of the more stable phases α-FeOOH and α-Fe2O3 from the solution. The final product phase composition and the particle size are affected considerably by a presence of goethite and/or hematite germs in the initial γ-FeOOH.



Umwandlung von synthetischem γ-FeOOH (Lepidocrocit) in wasrigen Losungen von Eisen(II)-sulfat



Der Verlauf der Umwandlung von synthetischem, nach verschiedenen Methoden dargestelltem γ-FeOOH (Lepidocrocit) in wasrigen Losungen von FeSO4 und der Einflus einer thermischen Vorbehandlung auf die Phasenzusammensetzung und Morphologie der Produktpartikel wurde untersucht. Die Gehalte an α-FeOOH und α-Fe2O3 sowie die Partikelgrosen der Produkte hangen sowohl vom Praparationsweg des Ausgangslepidocrocits wie auch von der thermischen Vorbehandlung ab. Die Ergebnisse werden als Auflosung des weniger stabilen Lepidocrocits und Kristallisation der stabileren Phasen α-FeOOH und α-Fe2O3 aus der Losung diskutiert. Die endliche Zusammensetzung der Produkte sowie ihre Partikelgrosen werden besonders durch die Anwesenheit von Keimen dieser Phasen im Ausgangs-γ-FeOOH beeinflust.

Transformation of synthetic γ‐FeOOH (lepidocrocite) in aqueous solutions of ferrous sulphate

The thermal decomposition of synthetic lepidocrocite,γ-FeOOH, was studied using DTA, infrared and Mossbauer spectroscopy and X-ray diffraction, and the dependence of the phase composition of the dehydration products on the method of preparing the initialγ-FeOOH was described. The results demonstrated thatγ-Fe2O3 formed by dehydration ofγ-FeOOH consists of very small particles with a broad size distribution and that, depending on the method of preparation, the initialγ-FeOOH can contain a large amount of amorphous ferric hydroxide. A preparation method yieldingγ -FeOOH which is not contaminated with other ferric oxide-hydroxides is recommended.

Vladimír Zapletal

Papers

Preparation and properties of green rust type substances

Transformation of synthetic γ‐FeOOH (lepidocrocite) in aqueous solutions of ferrous sulphate

Dehydration of synthetic lepidocrocite (γ-FeOOH)

Liquid distribution in reactors with randomly packed porous beds

Preparation of α-Fe2O3 (Hematite) by oxidation precipitation of aqueous solutions of iron(II) sulphate