Goethite structure

The goethite structure consists of an hep array of anions (0 and OH ) stacked along the [010] direction with Fe " ions occupying half the octahedral interstices within a layer (Fig. 2.4a-c). The Fe ions are arranged in double rows separated by double rows of empty sites (Fig. 2.4a, c) at the crystal surfaces the empty sites appear as grooves (Fig. 2.4d). 

The goethite structure contains two types of O atoms designated as Oi and On (Fig. 2.4e). On the 0 site, the O atom is shared between octahedra of two different double chains, whereas the On atom is shared between octahedra in the same chain and is also linked to the proton. Neutron scattering has shown that the On-On distance is... 

Although Fe "and Mn " have similar ionic radii, Mn " does not fit as readily into the goethite structure as does Fe " because owing to its four d electrons, Mn " has a tetragonally distorted coordination sphere on an octahedral site Jahn-Tdler effect). 

The carboxylate free radical rapidly decarboxylates to -hydroxy or -amine radicals which are oxidized by oxygen to formaldehyde or acetaldehyde. Substitution of Mn in the goethite structure reduces the photoredox reactivity of goethite. 

Transparent yellow iron oxide has the a-FeO(OH) (goethite) structure on heating it is converted into transparent red iron oxide with the a-Fe203 (hematite) structure. Differential thermogravimetric analysis shows a weight loss at 275 °C. Orange hues develop after brief thermal treatment of yellow iron oxide and can also be obtained by blending directly the yellow and red iron oxide powders. 

The second group includes crystalline compounds with a goethite structure. Among them there are distinguished (a-FeOOH) —hydrogoethite, formed as a result of natural aging of amorphous Fe " hydroxides due both to processes of enlargement and polymerization of amorphous particles, and... 

Properties Goethite, structure confirmed by XRD [1596], needles, 60 nm long, axial ratio of 6 [1597], particle size distribution (of length and width), XRD pattern, TEM image available (also in a series of goethites obtained under different conditions) [1596]. 

The research was focused on the acid leaching mechanism of goethite with nickel incorporated into goethite structure totally. Therefore, it was necessary to examine the synthesized nickeliferous goethite. 

The influence of the V " " ion presence during the formation of goethite from ferrihydrite in an alkaline medium was investigated using Mossbauer spectroscopy and other techniques by Kaur et al. [249]. The presence of ions reduced HMF values in the Mossbauer spectra due to a substitution of Fe by V ions (ionic radius of 0.64 A almost equal to the radius of Fe " ") in the goethite structure. Small quantities of hematite and superparamagnetic goethite were also formed. 

As in dissolution, a chemical and structural change can occur from hydrolysis as the ions replaced by or OH may be of a different size so that the crystal structure is stressed and weakened. An example of this is the weathering of feldspar or goethite by H ... 

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

Fendorf S, Eich MJ, Grossl P, Sparks DL (1997) Arsenate and chromate retention mechanisms on goethite. 1. Surface structure. Environ Sci Technol 31 315—320 Francesconi KA, Kuehnelt D (2002) Arsenic compounds in the environment. In Environmental chemistry of arsenic. In Frankenberger WT Jr (ed) Marcel Dekker, New York, Chapter 3, pp 51-94... 

Sun X, Doner, HE (1996) An investigation of arsenate and arsenite bonding structures on goethite by FTIR. Soil Sci 161 865-872 Sun X, Doner HE (1998) Adsorption and oxidation of arsenite on goethite. Soil Sci 163 278-287... 

Violante A, Krishnamurti GSR, Pigna M (2008) Mobility of trace elements in soil environments. In Violante A, Huang PM and Gadd G (eds) Wiley-JUPAC series on biophysico-chemical processes of metals and metalloids in soil environments. John Wiley Sons, Hoboken, USA Waltham AC, Eick MJ (2002) Kinetic of arsenic adsorption on goethite in the presence of sorbed silicic acid. Soil Sci Soc Am J 66 818-825 Waychunas GA, Fuller CC, Rea BA, Davis J (1996) Wide angle X-ray scattering (WAXS) study of two-line ferrihydrite structure Effect of arsenate sorption and counterion variation and comparison with EXAFS results. Geochim Cos-mochim Acta 60 1765-1781... 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

Final justification for using terms such as inner- or outer-sphere awaits direct spectroscopic confirmation. Electron Spin Resonance, Mossbauer, and Fourier Transform Infrared-Cylindrical Internal Reflection Spectroscopic techniques are being used to establish the structure of surface complexes (see, e.g., McBride, Ambe et al., and Zeltner et al., this volume). The potential for using EXAFS (extended x-ray absorption fine structure) to establish the type of surface complex for Pb + adsorbing onto goethite is currently being undertaken in our laboratory. 

Titration calorimetry and cylindrical internal reflection-Fourier transform infrared (CIR-FTIR) spectroscopy are two techniques which have seldom been applied to study reactions at the solid-liquid interface. In this paper, we describe these two techniques and their application to the investigation of salicylate ion adsorption in aqueous goethite (a-FeOOH) suspensions from pH 4 to 7. Evidence suggests that salicylate adsorbs on goethite by forming a chelate structure in which each salicylate ion replaces two hydroxyls attached to a single iron atom at the surface. 

In order to explain the similarity between the SAL-goethite spectrum and the aqueous SAL-Fe(III) spectrum, a chelate structure similar to that drawn above must be assumed to form during SAL adsorption on goethite. 

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... 

Cu and Zn enter sedimentary material in substantial proportions, both in the structure of minerals (carbonates, clays) and adsorbed on surfaces. Boyle (1981) showed that foraminiferal tests may contain Zn in excess of a few ppm. Partitioning of Cu and Zn between water and carbonates has been investigated by Rimstidt et al. (1998). The crystal chemistry of Cu and Zn in goethite has been investigated by EXAFS by Manceau et al. (2000). Typical Zn and Cu concentrations in FeMn nodules and encrustations are 500-1000 ppm and 800-6000 ppm, respectively (e.g., Albarede et al. 1997b). 

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