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.

Comparative study of the thermal decomposition of iron oxyhydroxides

Understanding the chemistry of nanoparticles is crucial in many applications. Their synthesis in a controlled manner and their characterization at the single particle level is essential to gain deeper insight into chemical mechanisms. In this work, single nanoparticle spectro-microscopy with top-down nanofabrication is demonstrated to study individual iron nanoparticles of nine different lateral dimensions from 80 nm down to 6 nm. The particles are probed simultaneously, under same conditions, during in-situ redox reaction using X-ray photoemission electron microscopy elucidating the size effect during the early stage of oxidation, yielding time-dependent evolution of iron oxides and the mechanism for the inter-conversion of oxides in nanoparticles. Fabrication of well-defined system followed by visualization and investigation of singled-out particles eliminates the ambiguities emerging from dispersed nanoparticles and reveals a significant increase in the initial rate of oxidation with decreasing size, but the reactivity per active site basis and the intrinsic chemical properties in the particles remain the same in the scale of interest. This advance of nanopatterning together with spatially-resolved single nanoparticle X-ray absorption spectroscopy will guide future discourse in understanding the impact of confinement of metal nanoparticles and pave way to solve fundamental questions in material science, chemical physics, magnetism, nanomedicine and nanocatalysis.

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Size-dependent redox behavior of iron observed by in-situ single nanoparticle spectro-microscopy on well-defined model systems.

Uniform particles with a large surface area formed by hydrolysis of Fe2(SO4)3 with urea

Textural evolution of synthetic γ-FeOOH during thermal treatment by differential scanning calorimetry

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

Amorphous FeIII hydroxides prepared at pH = 6 differ from analogous precipitates obtained in alkaline media in that they have an increased content of anions bound in their structures, and crystallize less readily both in dry thermal treatment and under hydrothermal conditions. The shapes of DTA curves obtained for freshly prepared gels and for products of ageing in mother liquor are discussed, and the effect of the anions present on the course of thermal decomposition is assessed.



Bildung und Kristallisation von FeIII-Hydroxiden. 2. Einflus der Fallungstemperatur und Natur des Ausgangs–FeIII-Salzes auf die Eigenschaften des in schwach saurem Medium dargestellten FeIII-Hydroxids



Amorphe, bei pH = 6 dargestellte FeIII-Hydroxide unterscheiden sich von entsprechenden Niederschlagen aus alkalischen Medien durch einen zunehmenden Gehalt an Anionen in ihren Strukturen und kristallisieren weniger leicht sowohl bei trockner, thermischer Behandlung wie unter hydrothermalen Bedingungen. Die DTA-Kurven, die von frisch praparierten Gelen sowie von in Mutterlauge gealterten Produkten erhalten wurden und der Einflus der vorhandenen Anionen auf den Verlauf der thermischen Zersetzung werden diskutiert.

Formation and Crystallization of FeIII Hydroxides. Effects of precipitation temperature and the nature of starting FeIII salt on the properties of FeIII hydroxides prepared in weakly acidic medium

The thermal decomposition of pureγ-FeOOH in air flow has been studied at a water vapour partial pressure of 1 X 10−3 to 20 mm Hg. The specific surface areas of the decomposition products (γ-Fe2O3 andα-Fe2O3) have been found to decrease with increasing partial pressure. It has been also established that the increase in partial pressure of the water vapour accelerates the transformation ofγ-Fe2O3 intoα-Fe2O3.

Effect of the partial pressure of water vapour on the thermal decomposition of γ-FeOOH and on the dispersity of the solid decomposition products

Etude cinetique de la reduction dont la temperature varie de 240 a 330°C. Energie d'activation

Mössbauer study of the reduction ofγ-Fe2O3 in hydrogen

Effects of precipitation temperature and the nature of starting iron(iii) salt on the properties of iron(iii) hydroxides prepared in weakly acidic medium

K. Bechine

Papers

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

Formation and Crystallization of FeIII Hydroxides. Effects of precipitation temperature and the nature of starting FeIII salt on the properties of FeIII hydroxides prepared in weakly acidic medium

Effect of the partial pressure of water vapour on the thermal decomposition of γ-FeOOH and on the dispersity of the solid decomposition products

Mössbauer study of the reduction ofγ-Fe2O3 in hydrogen

Effects of precipitation temperature and the nature of starting iron(iii) salt on the properties of iron(iii) hydroxides prepared in weakly acidic medium