Lepidocrocite, analysis

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. 

Information about the corrosion of boiler piles comes from analysis of scale samples and also from laboratory experiments. Smith and McEaney (1979) used XRD and SEM to follow the initial stages in the development of scale on gray, cast iron in water at 50 °C. At first, the corrosion product was a mixture of magnetite and green rust. Whether lepidocrocite formed depended on the level of oxygen in the system. 

This method of analysis is particularly valuable when chemical methods are inadequate or inapplicable. For instance, for complex mixtures where the different elements or ions may be associated in many different ways, all compatible with the analytical figures or for mixtures of polymorphous forms of the same substance, such as the three crystalline forms of CaC03 (calcite, aragonite, and vaterite) or the three crystalline forms of FeO(OH) (goethite, lepidocrocite, and P FeO(OH)—see Bunn, 1941)—mixtures for which chemical analytical methods are irrelevant. 

Ona-Nguema, G., Morin, G., Juillot, F. et al. (2005) Jr. EXAFS analysis of arsenite adsorption onto two-line ferrihy-drite, hematite, goethite, and lepidocrocite. Environmental Science and Technology, 39(23), 9147-55. 

X-ray diffraction analysis indicates that boehmite y-AlOOH and lepidocrocite y-FeOOH crystallize in the space group Cmcm = n. 63 = D J, with Z = 4. However, such analysis does not reveal the position of the hydrogen atoms. Instead, vibrational spectroscopies can be used to obtain such information. The IR spectra are dominated by two well-split OH stretches (3305, 3090cm" for boehmite) and two OH deformations (1170 and 1074cm" ), see Figure 3.6. 

In another report. Hansel et al. (2002) studied Fe plaque associated with the rhizosphere of Typha latifolia and Phalaris arundinacea from a mine waste-contaminated site that was high in As. For both plants, the Fe and As concentrations associated with roots were approximately 10-fold concentrated on roots relative to their abundances in the bulk soil. XRF revealed that Fe plaque consisted primarily of ferrihydrite but also had appreciable levels of goethite and siderite T. latifolia also had a significant amount of lepidocrocite. Their analysis suggested that As was sequestered fairly homogeneously within the Fe plaque. A XANES analysis indicated that approximately 80% of the As was As(V) and 20% As(III). Blute et al. (2004) reported similar ratios for As(V) and As(III)... 

Of particular interest was the way in which detailed information could be derived from voltammetric studies of clay minerals treated aerobically with iron (11) solutions. This caused the precipitation of a thin, active layer of iron (HI) oxi(hydroxides) which could later be used for the sorption of arsenate(V) ions [69]. By employing the voltammetry of immobilized microparticles, it was possible to distinguish different iron species, namely (i) ion-exchangeable, labile, or sorbed iron (HI) ions (ii) ferrihydrite or lepidocrocite and (iii) crystalline hematite or goethite. Cepria et al. subsequently employed the voltammetry of immobilized microparticles in the phase analysis of iron (III) oxides and oxi(hydroxides) in binary mixtures, as well as in cosmetic formulations [70]. 

The extent of Cr(VI) reduction by the LA River magnetite is minor (Figure 9A), decreasing from 0.060 to 0.045 mM over 24 hours. No measurable Fe(III) is produced in solution suggesting that even this Cr(VI) loss is not attributable to heterogeneous reduction reactions (equation 14). As summarized by (25), chromate is adsorbed at pHs below 4 on ferric oxides such as the secondary goethite and lepidocrocite which were detected by X-ray analysis in the LA sample. 

Thermal methods of analysis are often useful in the characterization of minerals, as described in Section 7.6.5. Aluminum hydroxides such as gibbsite show a mass loss of 34.6% on dehydroxylation thus, they show an important negative peak in DTA and a marked mass loss in TGA, and so these techniques are employed for both qualitative and quantitative characterization of these minerals. The same happens with iron hydroxides and oxohydroxides, such as goethite, lepidocrocite, and so on also, the presence of OH groups in otherwise thermally inert minerals such as hematite can be detected. 

Despite these limitations, the technique has been applied in the past to characterize the forms of iron occurring in different plants [62-65]. More recently, a detailed study of iron uptake and translocation in rice [66] grown in anaerobic FeCb-enriched nutrient solutions showed primarily the presence of Fe(lll) hydrous oxide components precipitated on the root cell wall (ferrihydrite and/or lepidocrocite). No evidence of Fe(ll) was found in the leaf tissue, the spectra were characteristic of Fe(lll) present in ferritin and in other complexed forms, not further identified by the authors. Iron biomineralization was also observed In a perennial grass grown in extreme acidic environment with a high content of metals [67,68]. In this case, the Mossbauer spectral analysis indicated that iron accumulated in this plant mainly as jarosite and ferrihydrite (ferritin). ]arosite accumulated In roots and rhizomes, while ferritin was detected in all the structures. 

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