Lepidocrocite 7-FeOOH

23 MOSSBAUER SPECTROSCOPY IN THE INVESTIGATION OF THE PRECIPITATION OF IRON OXIDES 

Mossbauer spectra of sample obtained on heating of two-XRD-iine ferrihydrite at 220°C. (Reproduced from Ref. 106 with permission of Eisevier.) 

Mossbauer spectra of sample obtained on heating of two-XRD-line ferrihydrite at 245 °C [106], recorded at various temperatures in the absence of a magnetic field (top) and at 4.2 K in the applied magnetic field of 6T (bottom). (Reproduced from Ref. 106 with permission of Elsevier.) 

23 FeMOSSBAUERSPECTROSCOPYlNTHE INVESTIGATION OFTHE PRECIPITATION OF IRON OXIDES 

TABLE 23.3 The Relevant Mossbauer Parameters of Samples Prepared by Heating of two-XRD-Line Ferrihydrite at Different Temperatures 

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As shown in Figure 13.19a, phosphate and borate inhibit the dissolution of goethite by H2S. Similarly, the dissolution of lepidocrocite (7-FeOOH) by EDTA (Y" ) is inhibited by phosphate and arsenate (Figure 13.19b). Both in the reductive dissolution (by H2S) and the ligand-promoted dissolution (by... 

Lepidocrocite 7-FeOOH Orthorhombic Red, yellowish brown, blackish brown 4.0 Seasonally anaerobic soils, cool climates... 

Ladder polymer, 209-211 Lagrangian method, 441 Lag time, 376-377, 379, 443 Lanthanum silicate, 763 Laplace equation, 414, 678 Laser, 286-287 Laser fusion, 285 Latent heat, gelation, 304 Latex, 257, 258, 352 Lattice diffusion, 720, 723 Lattice mismatch, 727 Lead titanate, 848 Leatherhard point, 465 Leaving group, 24, 26, 27, 29, 30, 34, 44, 48, 131, 132, 138 LEFM, 494-496 Lens, 853 lens, silicate, 97 Lepidocrocite (FeOOH), 37 Lepidoidal silica, 505 Levitation apparatus, 350 Lewis acid, 659 Lewis acidity, of surface, 663 Ligand, 2, 3... 

Hydroxide Fe(OH)3 (Fe + plus OH ) has definite existence and there are many ill-deiined hydrates used as pigments. FeOOH has two forms goethite and lepidocrocite. Colloidal Fe(OH)3 is easily obtained as a deep red sol. Many Fe(III) hydroxy complexes are known. Fe(OH)2 may be formed from Fe and OH" in the absence of O2 but it is very readily oxidized. 

First attempts to incorporate pre-formed magnetite colloids within alginate/silica nanocomposites via a spray-drying process have been described, but formation of lepidocrocite y-FeOOH and fayalite Fe2Si04 was observed, attributed to Fe2+ release during the aerosol thermal treatment [53],... 

Sung, W. and J. J. Morgan, 1981, Oxidative removal of Mn(II) from solution catalysed by the y-FeOOH (lepidocrocite) surface. Geochimica et Cosmochimica Acta 45, 2377-2383. 

Oxide composition and lattice structure influences the coordin-ative environment of surface sites, and should have an impact on rates of ligand substitution. Hematite (Fe203), goethite (a-FeOOH), and lepidocrocite (y-FeOOH), for example, are all Fe(III) oxide/ hydroxides, but may exhibit different rates of surface chemical... 

Mn(II) oxidation is enhanced in the presence of lepidocrocite (y-FeOOH). The oxidation of Mn(II) on y-FeOOH can be understood in terms of the coupling of surface coordination processes and redox reactions on the surface. Ca2+, Mg2+, Cl, S042-, phosphate, silicate, salicylate, and phthalate affect Mn(II) oxidation in the presence of y-FeOOH. These effects can be explained in terms of the influence these ions have on the binding of Mn(II) species to the surface. Extrapolation of the laboratory results to the conditions prevailing in natural waters predicts that the factors which most influence Mn(II) oxidation rates are pH, temperature, the amount of surface, ionic strength, and Mg2+ and Cl" concentrations. 

This paper discusses the oxidation of Mn(II) in the presence of lepidocrocite, y-FeOOH. This solid was chosen because earlier work (18, 26) had shown that it significantly enhanced the rate of Mn(II) oxidation. The influence of Ca2+, Mg2+, Cl", SO,2-, phosphate, silicate, salicylate, and phthalate on the kinetics of this reaction is also considered. These ions are either important constituents in natural waters or simple models for naturally occurring organics. To try to identify the factors that influence the rate of Mn(II) oxidation in natural waters the surface equilibrium and kinetic models developed using the laboratory results have been used to predict the... 

The orange coloured lepidocrocite, y-FeOOH, is named after its platy crystal shape (lepidos scale) and its orange colour (krokus = saffron). It occurs in rocks, soils, biota and rust and is often an oxidation product of Fe ". It has the boehmite (y-AlOOH) structure which is based on cubic close packing (ccp) of anions. 

The sheets are held together solely by hydrogen bonds (Fig. 2.5 d, e). Deuteration of the bulk OH in lepidocrocite is facilitated by the layer structure ease of deuteration of FeOOH follows the order lepidocrocite > goethite > akaganeite (Ishikawa et al., 1986). 

Goethite 29-713 Lepidocrocite 44-1415 Akaganeite 13-157 Schwertmannite 6-FeOOH > Feroxyhyte Ferrihydrite 29-712 HP FeOOH > ... 

Leland and Bard (1987) found that the different iron oxides induced photooxidation of oxalate and sulphite at rates that varied by up to two orders of magnitude. For oxalate, the rate was greater for maghemite than for hematite, but this order was reversed for sulphite. Lepidocrocite (layer structure) induced faster oxidation of both compounds that did the other polymorphs of FeOOH (tunnel structures) the authors considered that the rate differences were probably associated with structural differences between the adsorbents. 

The end product of the dehydroxylation of pure phases is, in all cases, hematite, but with lepidocrocite, maghemite occurs as an intermediate phase. The amount of water in stoichiometric FeOOH is 10.4 g kg , but adsorbed water may increase the overall amount released. Thermal dehydroxylation of the different forms of FeOOH (followed by DTA or TG) takes place at widely varying temperatures (140-500 °C) depending on the nature of the compound, its crystallinity, the extent of isomorphous substitution and any chemical impurities (see Fig. 7.18). Sometimes the conversion temperature is taken from thermal analysis data (e. g. DTA), but because of the dynamic nature of the thermoanalysis methods, the temperature of the endothermic peak is usually higher than the equilibrium temperature of conversion. 

Upon adsorption of Fe " at a solid surface, the standard redox potential of the Fe /Fe pair is reduced substantially from 0.77 V to 0.35-0.45 V (Wehrli, 1990) thereby facilitating the electron transfer. Buerge and Hug (1999) have demonstrated that this higher reactivity may be responsible for the fact that solid phases (Fe oxides, Si02, and clay minerals) in natural systems accelerate Cr reduction and that goethite and lepidocrocite are by far more active in this respect than the rest of the solid phases, because these two FeOOH forms adsorb much more Fe ". The authors attribute this to better overlap and charge delocalization at the surface of the Fe oxides. 

Bechine, K. Subrt, J. Hanslik,T. ZapletafV. Tlaskal, J. Lipka, J. Sedlak, B. Rotter, M. (1982) Transformation of synthetic y-FeOOH (lepidocrocite) in aqueous solutions of ferrous sulphate. Z. anorg. allg. Chem. 489 186-196... 

Diakonov, G.G. (1998) Thermodynamic properties ofiron oxides and hydroxides. III. Surface and bulk thermodynamic properties of lepidocrocite (y-FeOOH) up to 500 K. Eur. J. Min. 10 31-41... 

A.I. Bordia, R.K. Korshin, G.V. Christensen T.H. (2000 a) Aging of iron (hydr)oxides by heat treatment and effects on heavy metal binding. Environ. Sci. Techn. 34 3991-4000 Sorensen, S. Thorling, L. (1991) Stimulation by lepidocrocite (y-FeOOH) of Fe(II)-depen-... 

Zhang, X. Zhang, F. Mao, D. (1999) Effect of iron plaque outside roots on nutrient uptake by rice Oryza sativa L.) Phosphate uptake. Plant and Soil 209 187-192 Zhang,Y Charlet, L. Schindler, P.W. (1992) Adsorption of protons, Fe(II) and Al(IIl) on lepidocrocite (y-FeOOH). Colloids Surfaces 63 259-268... 

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