Hematite structural water

The structure of ferrihydrite has been the object of numerous studies in the past and several different structures have been proposed. The main difficulty affecting elucidation of the structure is the low degree of order. The original models of Towe and Bradley (1967) and Chukhrov et al. (1976) are based on XRD data and involve a defective hematite structure based on an hep array of anions with vacant Fe sites and a considerable amount of water. The Fe ions are distributed randomly over the interstices and there is more OH and H2O and less Fe in ferrihydrite than in hematite, i. e. there is a lower Fe/O ratio (< 2/3). 

Corundum is aluminum oxide, q -A1203, which has a hexagonal crystalline structure that is analogous to hematite. However, water treatment systems most often use activated alumina, which is typically produced by thermally dehydrating aluminum (oxy)(hydr)oxides to form amorphous, cubic (y), and/or other polymorphs of corundum (Clifford and Ghurye, 2002, 220 Hlavay and Poly k, 2005 Mohan and Pittman, 2007). When compared with corundum, amorphous alumina tends to have higher surface areas, greater numbers of sorption sites, and better sorption properties. 

All fine grained Fc oxides lose adsorbed water at characteristic temperatures of between 100 and 200 °C. Structural OH in gocthite and lepido-crocite is lost at 250-400°C by the dehydroxylation reaction 2OH O + H2O. Even fine grained oxides such as hematite contain some OH in the structure (Stanjek Schwertmann, 1992) and this is driven off over a wide temperature range. For Fe oxides endothermic peaks result from the release of adsorbed or structural water, whereas exothermic peaks come from phase transformations (e.g. maghemite to hematite) or from recrystallization of smaller crystals into larger ones. An example of this is observed during the transformation of ferrihydrite to hematite. 

A common feature of the dehydroxylation of all iron oxide hydroxides is the initial development of microporosity due to the expulsion of water. This is followed, at higher temperatures, by the coalescence of these micropores to mesopores (see Chap. 5). Pore formation is accompanied by a rise in sample surface area. At temperatures higher than ca. 600 °C, the product sinters and the surface area drops considerably. During dehydroxylation, hydroxo-bonds are replaced by oxo-bonds and face sharing between octahedra (absent in the FeOOH structures see Chap. 2) develops and leads to a denser structure. As only one half of the interstices are filled with cations, some movement of Fe atoms during the transformation is required to achieve the two thirds occupancy found in hematite. 

Hematite forms by a combination of aggregation-dehydration-rearrangement process for which the presence of water appears essential. Structural details about this process at 92 °C were obtained from EXAFS (Combes et al. 1989 1990) face-sharing between Fe octahedra developed before XRD showed any evidence for hematite. It is followed by internal redistribution of vacancies in the anion framework and by further dehydration. The dehydration process involves removal of a proton from an OH group and this in turn leads to elimination of a water molecule and formation of an 0X0 linkage. The local charge inbalance caused by proton loss is compensated for by migration and redistribution of Fe " within the cation sublattice. 

Kanai, H., Navarrete, R.C., Macisko, C.W. Scriven, L.E. (1992) Rheol. Acta 31 333 Kandori, K. Ishikawa,T. (1991) Selective adsorption of water on amorphous ferric oxide hydroxide. Langmuir 7 2213-2218 Kandori, K. Aoki,Y. Yasukawa, A. Ishikawa, T. (1998) Effects of metal ions on the morphology and structure of hematite particles produced from forced hydrolysis reaction. 

M. Mosselmans, J.fw. (2000) Structural chemistry of Fe, Mn, and Ni in synthetic hematites as determined by extended X-ray absorption fine structure spectroscopy. Clays Clay Min. 48 521-527 Singh, D.B. Prasad, G. Rupainwar, D.C. Singh,V.N. (1988) As(lll) removal from aqueous solution by adsorption. Water, Air, Soil Pollut. 42 373-386... 

However, most of these attempts were found unsuccessful. This is mainly due to the fact that the mechanisms and processes responsible for the low photoresponse of hematite are far from being clearly understood. A deeper understanding of the semiconducting properties of this material is needed, such as its surface structure and a better description of the hematite/electrolyte interface. Extensive knowledge is also required to understand the photoelectro- chemical behavior of hematite and the kinetics involved in the photo-oxidation of water. Finally, for effective oxidation of water an efficient catalyst must most probably be coated onto the surface of hematite. 

Although it has been suggested [35] that P-FeOOH is converted to hematite directly, possibly [1] in the presence of a low pressure of water vapour, Mackenzie and Berggren [19] recommend fiuther investigation of the system. A detailed discussion of the structure and imperfections present in the colloid system of P-FeOOH and P-FcjOj has been given. 

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