Akaganeite

In this work, the possibilities of using Fe Mossbauer spectrometry in corrosion research are reviewed in the case of three accelerated corrosion tests in chloride environments. Some suggestions for further research in this field are also proposed. Before introducing these examples, we will briefly discuss some models used to fit some of the iron phases in rust layers, such as goethite, akaganeite, and spinel phases. Additional information for other iron phases can be found elsewhere [I 1-13,19-22,26]. 

2 MOSSBAUER CHARACTERIZATION OF SOME IRON PHASES PRESENTED IN THE RUST LAYERS 

TABLE 20.1 Range of Hyperfine Parameters Derived from the Room Temperature Mossbauer Spectra of Some Iron Phases [13,19,22,26]  

S isomer shift reiative to a-Fe at room temperature, 2s quadrupole shift, A quadrupole splitting, 6 magnetic hyperfine field, f. Mdssbauer fraction relative to that of hematite [13,19], 

20 ENHANCING THE POSSIBILITIES OF MOSSBAUERSPECTROMETRY TO STUDYTHE INHERENT PROPEROES 

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Chen, M., Jiang, J., Zhou, X. and Diao, G. (2008) Preparation of akaganeite nanorods and their transformation tosphere shape hematite. Journal for Nanoscience and Nanotechnology, 8, 3942-3948. 

Akaganeite particles The first inflection point corresponds to a correlation time larger at 37°C (2.28 ns) than at 5°C (1.76 ns). This temperature dependence is typical of an electronic relaxation time (of Fe " "). The difference between the calculated correlation time and the theoretical relaxation time... 

Fig. 15. Longitudinal NMRD profile of ferritin ( ) and apoferritin ( ) aqueous solutions at 37°C. The contribution of the ferrihydrite core to the relaxation ( ) is obtained by the subtraction of the profiles. Ferritin solution has an iron concentration of 100 mM, while the protein concentration of both samples is 0.058 mM. Longitudinal NMRD profile of akaganeite particles (O) with an iron concentration of 100 mM.

Correlation Times Calculated lrom the Fittings oe the Longitudinal NMRD Proeiles oe Apoeerritin, Ferritin, and Akaganeite Particles Solutions... 

Fig. 17. Transverse NMRD profile of akaganeite particles solutions at 40°C for different pH values. The iron concentration is 100 mM.

Akaganeite particles Both Ti and T2 are strongly pH-dependent (Pigs. 17 and 19). The amplitudes of the longitudinal NMRD profiles drastically decrease when the pH increases from 3.35 to 9.45. The correlation time associated with the first dispersion is only weakly pH dependent, consistent with its former interpretation as an electron relaxation time. However, T2, the correlation time characteristic of the second dispersion, increases from 30 8 ns at pH 3.35 to 280 32 ns at pH 9.45, which eliminates its interpretation as a diffusion time T2 can be identified as a proton exchange time. 

The relaxation rates calculated from Eq. (15) are smaller than the measured ones at low field, while they are larger at high field. OST is thus obviously unable to match the experimental results. However, water protons actually diffuse around ferrihydrite and akaganeite particles and there is no reason to believe that the contribution to the rate from this diffusion would not be quadratic with the external field. This contribution is not observed, probably because the coefficient of the quadratic dependence with the field is smaller than predicted. This could be explained by an erroneous definition of the correlation length in OST, this length is the particle radius, whilst the right definition should be the mean distance between random defects of the crystal. This correlation time would then be significantly reduced, hence the contribution to the relaxation rate. 

The PEDM is able to explain the anomalous relaxation of solutions of ferritin and akaganeite particles, especially its linear dependence with Bq, the external magnetic field. The model is compatible with the observed dependence of the rate on pH. The relaxation rate predicted by the PEDM is proportional to the number of adsorption sites per particle (q) the values deduced for q from the adjustment of the model to experimental results (from NMR and magnetometry in solutions) are reasonable for hydrated iron oxide nanoparticles (63). 

Mineral name Goethite Lepidocrocite Akaganeite Schwertmannite Feroxyhyte... 

Akaganeite, P-FeOOH, is named after the Akagane mine in Japan where it was first discovered (Mackay, 1962). It occurs rarely in nature and is found mainly in Cl-rich environments such as hot brines and in rust in marine environments. Unlike the other FeOOH polymorphs, it has a structure based on body centered cubic packing of anions (bcp) (hollandite structure) and contains a low level of either chloride or fluoride ions. It has a brown to bright yellow colour. 

Schwertmannite, Fei60is(0H)y(S04)z nHyO, has the same basic structure as akaganeite, but contains sulphate instead of chloride ions. This recently recognized mineral frequently occurs in nature as an oxidation product of pyrite and can be... 

Fig. 2.3 Basic structural units and Fe-Fe distances (in nm) for hematite, goethite, akaganeite and lepidocrocite and their associated radial distribution functions as obtained from EXAFS spectra. The first peak in the radial distribution...

Lepidocrocite (Greek lepidos = scale, flake and krokoeis = saffron-coloured) is iso-structural with boehmite (Tab. 2.1). Unlike goethite and akaganeite which have a turmel structure, lepidocrocite is a layered compound. The orthorhombic unit cell contains four formula units and has the edge lengths, a = 1.2520(6) nm h = 0.3873(2) nm and c = 0.3071(6) nm (Ewing, 1935 Oles et al., 1970 Christensen Norlund-Christensen, 1978). 

Like goethite and akaganeite, lepidocrocite consists of double chains of Fe(0,0H)6 octahedra running parallel to the c-axis. The double chains share edges with adjacent double chains and each chain is displaced by half, with respect to its neighbour, thus forming corrugated sheets of octahedra (Fig. 2.5 d). These sheets are stacked perpendicular to the [100] direction and are separated by double rows of empty octahedral sites. 

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). 

Akaganeite (named after the Akagane mine in Japan) is isostructural with hollan-dite. Compounds with this structure have a tetragonal or monoclinic unit cell. Bernal et al. (1959) and Keller (1970) both concluded that the unit cell of akaganeite was tetragonal with a = 1.000 nm and c = 0.3023 nm. The structural refinement of a natural sample using XRD and Rietveld analysis indicated, however, that the unit cell is monoclinic with a = 1.060 nm, b = 0.3039 nm, c = 1.0513 nm and p = 90.24° (Post Buchwald, 1991). There are eight formula units per unit cell. 

Fig. 2.6 Structure of akaganeite. a) Arrangement of octahedral double chains in tunnels with chloride ions in the centre ofthe tunnels, b) Ball-and-stick model with unit cell outlined.

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