Enver Murad

Mössbauer Spectroscopy of Environmental Materials and Their Industrial Utilization

-Introduction. List of Contents. Symbols and Abbreviations. Foreword. Acknowledgments. -1: Theory and Characteristics of the Mossbauer Effect. Theory of the Mossbauer Effect. Magnetism. Hyperfine Interactions. Relaxation Effects. Line Intensities. Diffusional Broadening. Mossbauer Spectroscopy in Mineralogy and Minerals Processing. Summary. -2: Mossbauer Instrumentation. General Considerations. Source and Reference Absorber. Conventional Spectrometers. Other Spectrometers. Sample Environment. Sample preparation. Summary. -3: Data Analysis and Interpretation. Fitting using Simple Lorentzians. Tests of Goodness of Fit. Convolution and Deconvolution. Thickness Effects and Recoilless Fraction. Fitting of Thickness Broadened Lines. Fitting of Lines Broadened by a Hyperfine Parameter Distribution. Magnetic Relaxation and Superparamagnetism. The Non Uniqueness Problem. Problems in Fitting and Interpretation. Presentation of Results. Summary. -4: Bulk and Clay Sized Phyllosilicates. 1:1 Minerals: Kaolinite. 2:1 Minerals: Illite. Montmorillonite. Nontronite. Summary. - 5: Iron Oxides and Oxyhydroxides. Anhydrous Oxides: Hematite. Magnetite. Maghemite. Oxyhydroxides: Goethite. Ferrihydrite. Summary. -6: Sediments. Terrestrial Sediments. Freshwater Sediments. Marine Sediments. Airborne Particles. Summary. -7: Soils and Clays. Soils. Clays. Summary. -8: Weathering. Silicate Weathering. Sulfide Weathering and Acid Mine Drainage. Summary. -9: Metastable Materials. Green Rusts. Fine Particle Magnetite. Anoxic Sediments. Fe2+ Sulfates Produced by Acid Mine Drainage. Summary. -10: Coal. Coal Characterization. Heat and Chemical Treatment. Hydroliquefaction. Summary. -11: ClayFiring. Individual Minerals: Kaolinite. Illite. Montmorillonite. Nontronite. Samples of Complex Mineralogy. Summary. -12: Mineral processing. Introduction. Iron Ores. Titanium Ores. Other Processing Operations Involving Iron. Gold. Summary. References. Index.

Properties of Goethites of Varying Crystallinity

Goethites were synthesized from ferrihydrite in 0.7 M KOH between 4° and 90°C. As temperatures increased, the goethite crystals became larger and of less domainic character, and surface areas decreased from 153 to 9 m2/g. Surface area, oxalate-soluble Fe to total Fe ratios, chemisorbed water, Mössbauer parameters, and dissolution rate in 6 M HCl at 25°C are particle-size controlled, whereas mean crystallite dimensions, a-dimension of the unit cell, differences between the two OH-bending modes, and dehydroxylation temperatures suggest the existence of a low-temperature (high-a-dimension) and a high-temperature (low-a-dimension) goethite, with a narrow transition range at a synthesis temperature of 40°–50°C. Hydrothermal treatment at 125°–180°C of a low-temperature goethite led to a healing of the multidomainic, microporous high-a-dimension goethite into a monodomainic low-a-dimension goethite of similar overall crystal size with the properties of a low-a-dimension goethite.KurzfassungGoethite wurden aus Ferrihydrit durch Lagerung in 0,7 m KOH bei Temperaturen zwischen 4° und 90°C hergestellt. Mit zunehmender Synthesetemperatur wurden die Goethitkristalle größer und deren Domänen-Charakter weniger stark ausgeprägt, während die spezifische Oberfläche von 153 auf 9 m2/g abnahm. Spezifische Oberfläche, Feo/Fet-Verhälmis, adsorptiv gebundenes Wasser, Mößauerparameter und Lösungsgeschwindigkeit in 6 m HCl bei 25°C erwiesen sich als teilchengrößenabhängig. Dagegen sprachen die mittere Teilchengröße, a-Gitterdimension, Abstand der beiden OH-Knickschwingungen und die Dehydroxylierungstemperatur für die Existenz eines Tieftemperatur- (hohe a-Gitterdimension) und eines Hochtemperatur-Goethits (niedrige a-Dimension) mit einem engen Übergangsbereich bei Synthesetemperaturen zwischen 40–50°C. Hydrothermale Behandlung eines Tieftemperatur-Goethits zwischen 125–180°C bewirkte ein Verheilen des vieldomänigen, mikroporösen Hoch-a-Goethits zu einen eindomänigen Niedrig-a-Goethit von ähnlichen Gesamtabmessungen der Kristalle mit den Eigenschaften eines Niedrig-a-Goethits.RésuméUne série de goethites a été synthétisée à partir de ferrihydrite évoluant en milieu 0,7 M KOH, à une température variable comprise entre 4° et 90°C. Pour une température croissante, la taille des particules croît, leur caractère polycristallin disparaît, la surface diminue de 153 à 9 m2/g. La surface, la quantité d’eau chimisorbée, les paramètres Môssbauer, et la vitesse de dissolution dans HCl 0,6 M sont contrôlés par le facteur taille des particules, tandis que les dimensions moyennes des domaines cohérents, le paramètre a de la maille, l’écart entre les fréquences de déformation des OH et la température de déshydroxylation reflètent l’existence de 2 types de goethites, synthétisées respectivement à basse et haute température, caractérisées respectivement par un paramètre a grand et petit. Le domaine de transition en température se situe vers 40°–50°C. Un traitement hydrothermal à 125°–180°C d’une goethite “basse température” soude les domaines à l’intérieur du volume global des particules, fait décroître le paramètre a et apporte ainsi toutes les propriétés d’une goethite “haute température,” à l’exception du volume global des cristallites qui est conservé.

Iron oxide catalysts: Fenton and Fenton-like reactions - a review

Abstract Iron is the fourth most common element by mass in the Earth’s crust and forms compounds in several oxidation states. Iron (hydr)oxides, some of which form inherently and exclusively in the nanometre-size range, are ubiquitous in nature and readily synthesized. These facts add up to render many Fe (hydr)oxides suitable as catalysts, and it is hardly surprising that numerous studies on the applications of Fe (hydr)oxides in catalysis have been published. Moreover, the abundant availability of a natural Fe source from rocks and soils at minimal cost makes the potential use of these as heterogeneous catalyst attractive. Besides those Fe (hydr)oxides that are inherently nanocrystalline (ferrihydrite, Fe5HO8·4H2O, and feroxyhyte, δ’-FeOOH), magnetite (Fe3O4) is often used as a catalyst because it has a permanent magnetization and contains Fe in both the divalent and trivalent states. Hematite, goethite and lepidocrocite have also been used as catalysts in their pure forms, doped with other cations, and as composites with carbon, alumina and zeolites among others. In this review we report on the use of synthetic and natural Fe (hydr)oxides as catalysts in environmental remediation procedures using an advanced oxidation process, more specifically the Fenton-like system, which is highly efficient in generating reactive species such as hydroxyl radicals, even at room temperature and under atmospheric pressure. The catalytic efficiency of Fe (hydr)oxides is strongly affected by factors such as the Fe oxidation state, surface area, isomorphic substitution of Fe by other cations, pH and temperature.

CLAYS AND CLAY MINERALS : WHAT CAN MOSSBAUER SPECTROSCOPY DO TO HELP UNDERSTAND THEM?

Mössbauer spectroscopy is a powerful technique for the characterization of materials formed in the weathering environment. Mössbauer studies of clay-sized phyllosilicates, however, are burdened with several problems: the samples are rarely monomineralic, they may be poor in iron, and only few iron-rich species order magnetically above 4.2 K. Site occupancies are difficult to determine, and cis and trans octahedral-OH site assignments are normally not possible. Unequivocal information that can be gained from such work thus is often restricted to the determination of the oxidation state of iron and average structural site distortions.Mössbauer data on iron oxides are generally more straightforward to interpret because these can be studied in the magnetically ordered state. A further asset of Mössbauer spectroscopy when studying iron oxides lies in its high sensitivity for magnetically ordered phases. Adverse effects ensuing from small particle size, interparticle interactions, non-stoichiometry and foreign-element substitution that often affect the Mössbauer parameters of iron oxides occurring in clays and soils can be at least partly offset by taking spectra at low temperatures.

Iron-rich precipitates in a mine drainage environment: Influence of pH on mineralogy

Abstract Ochreous precipitates deposited by waters draining a sulfide-rich lignite seam exposed in an abandoned mine in the Czech Republic show marked variations in color and mineralogy as a function of effluent pH. When initially formed under acidic (pH 3.7) conditions, the precipitates are orange in color and their mineralogy is dominated by schwertmannite. Following confluence with alkaline (pH 8.3) waters that have not been affected by mine drainage, the effluent pH rises to 7.3, the color changes abruptly to reddish-brown, and the principal precipitate mineral is a two-line ferrihydrite. This change in mineralogy and the associated variation in color can thus serve as direct indicators of the genetic environments under which the minerals were formed.

Properties and Behavior of Iron Oxides as Determined by Mössbauer Spectroscopy

During the decay of excited nuciear states, γ-rays are emitted which typically have energies that lie in the keV to MeV range. Because of these high energies, nuclei experience a significant impulse during emission and absorption of γ-quanta, i,e., they can recoil to a certain extent, and the γ-ray energy will change accordingly.

Clays and clay minerals: The firing process

Abstract57Fe Mössbauer spectroscopy reveals changes in iron valence and iron site geometry when clays and clay minerals are heated, and allows a distinction to be made between paramagnetic and magnetically ordered phases. Mössbauer spectra can thus reveal the extent of iron retention in silicate structures upon heating, the identity of iron oxides initially present or formed during the heating process and their transformations, and the character of the atmosphere under which heating was carried out. This makes Mössbauer spectroscopy the most effective tool for the characterization of changes induced by heating phyllosilicates and iron oxides.

The infrared spectrum of synthetic akaganéite, β-FeOOH

Abstract Fourier-transform infrared spectra of a synthetic akaganéite, β-FeOOH, were acquired in transmittance, attenuated total reflectance (ATR) and diffuse reflectance. The transmittance spectra showed a distinct dependence on the mode of sample preparation: measurements taken on pellets prepared by pressing the sample with alkali halides (KBr or CsI) displayed bands at 1096, 1050, and 698 cm-1 that were not observed in spectra of the neat material and must therefore be considered artifacts. Variations in the sampling environment (e.g., water and/or organic volatiles) were also observed to exert a noticeable influence on the development of the IR spectra. Infrared bands due to akaganéite were found at 3480 + 3390 (doublet), 1630, 850 + 820 (doublet), 650, 490, and ~420 cm-1. Diffuse reflectance spectra dominated by volume scattering (loose), diffuse reflectance spectra comprised of both volume and surface scattering (compacted), ATR spectra (surface only), and transmittance spectra (absorbance only, inverse of ATR) showed compatible trends for the akaganéite features both above and below ~1000 cm-1. This indicates that the multiple akaganéite measurements are consistent and confirms the band assignments.

Influence of Al substitution and crystal size on the room-temperature Moessbauer spectrum of hematite

Mössbauer spectra of 15 hematites with Al substitutions between 0 and 10 mole % were taken at room temperature. X-ray powder diffraction indicated dimensions of these hematites in the c-direction to range upwards from 27 nm to crystals large enough to show no line broadening. The Mössbauer spectra showed that magnetic hyperfine fields decreased both with increasing Al-for-Fe substitution and with decreasing crystal size. These relationships indicate that hyperfine field variations cannot, as has been done in the past, be unequivocally related to Al substitution alone. Hyperfine field reductions were paralleled by Mössbauer line broadening due to hyperfine field distributions. Only the hematites heated to 1000°C showed a significant variation of quadrupole splittings with Al substitution. No dependence of quadrupole splitting on crystal size was observed, indicating no detectable distortion of coordination polyhedra in the particle size range studied.

THE INFLUENCE OF ALUMINUM ON IRON OXIDES: XIV. AL-SUBSTITUTED MAGNETITE SYNTHESIZED AT AMBIENT TEMPERATURES

Mixtures of magnetite and goethite were formed by the slow oxidation of mixed FeCl2-AlCl3 solutions in an alkaline environment at room temperature. The compositions of the products ranged from almost exclusively magnetite in Al-free systems to goethite only at Al/(Al + Fe) ~ 0.3. The magnetic phase consisted of a partly oxidized (Fe2+/Fe3+ < 0.5), Al-substituted magnetite. The unit-cell edge length a of the magnetite decreased with increasing Al (Al = 0–0.37 per formula unit, corresponding to 0–14 mole % Al) and decreasing Fe2+ in the structure as described by the empirical relationship a (A) = 8.3455 + 0.0693 Fe2+ - 0.0789 Al. A correlation between the experimentally determined a and that calculated from the unit-cell edge lengths of end-member magnetite, maghemite, and hercynite was highly significant (r = .96) although shifted by about 0.01 Å. Mössbauer spectra showed Al to have entered preferentially the tetrahedral rather than the octahedral sites at low Al substitutions (<0.15 per formula unit), perhaps because of steric reasons. With increasing Al substitution the crystal size of magnetite decreased and structural strain increased, indicating that the structure had a limited capability to incorporate Al under these synthesis conditions. The capacity of the goethite structure to tolerate more Al may explain why goethite replaced magnetite at higher Al concentrations.