8-FeOOH formation

For steel, passivation is achieved by the surface formation of a tough, adherent mixture of oxides. The passive film is primarily gamma-magnetite (y-Fe203) but also contains gamma-hydrated ferric oxide (y-FeOOH). The film thickness is perhaps 15 A to 30 A (angstrom units). 

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],... 

The phenomena of surface precipitation and isomorphic substitutions described above and in Chapters 3.5, 6.5 and 6.6 are hampered because equilibrium is seldom established. The initial surface reaction, e.g., the surface complex formation on the surface of an oxide or carbonate fulfills many criteria of a reversible equilibrium. If we form on the outer layer of the solid phase a coprecipitate (isomorphic substitutions) we may still ideally have a metastable equilibrium. The extent of incipient adsorption, e.g., of HPOjj on FeOOH(s) or of Cd2+ on caicite is certainly dependent on the surface charge of the sorbing solid, and thus on pH of the solution etc. even the kinetics of the reaction will be influenced by the surface charge but the final solid solution, if it were in equilibrium, would not depend on the surface charge and the solution variables which influence the adsorption process i.e., the extent of isomorphic substitution for the ideal solid solution is given by the equilibrium that describes the formation of the solid solution (and not by the rates by which these compositions are formed). Many surface phenomena that are encountered in laboratory studies and in field observations are characterized by partial, or metastable equilibrium or by non-equilibrium relations. Reversibility of the apparent equilibrium or congruence in dissolution or precipitation can often not be assumed. 

He, J Ma, W Song, W Zhao, J Qian, X Zhang, S Yu JC. Photoreaction of aromatic compounds at a-FeOOH/H20 interface in the presence of H2O2 evidence for organic-goethite surface complex formation. Water Research, 2005 39, 119-128. 

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. 

Hoins, U. Charlet, L. Sticher, H. (1993) Ligand effect on the adsorption of heavy metals. The sulphate-cadmium goethite case. Water, Air, Soil Pollution 68 241-255 Holm, G. (1985) Substitution selectivity of some transition elements (Cr, Mn, Co, Ni, Cu, Zn) during formation of P-FeOOH. Geologiska Foreningsi Stockholm Forhandlingar 107 297-300... 

Inouye, K. Ichimura, H. Kaneko, K. Ishi-kawa,T. (1974) The effect of copper(II) on the formation and thermal change of synthetic P-FeOOH. Bull. Chem. Soc. Japan 47 743-744... 

Kiyama, M. Akita, T. Shimizu, S. Okuda,Y. Takada,T. (1972) Conditions favourable for the formation of y-FeOOH by aerial oxidation in an acidic suspension of iron metal powder. Bull Chem. Soc. Japan 45 3422—3426 Kiyama, M. Shamoto, S. Horushi, N. Okuda, Y. Takada,T. (1986) Growth of needle-like a-FeO(OH) particles by air oxidation of aqueous suspensions containing Fe(ll) predpi-tates. Bull. Inst..Chem. Res. Kyoto Univ. 46 150-156... 

Naono, H. Fujiwara, R. (1980) Micropore formation due to thermal decomposition of ad-cular microcrystals of a-FeOOH. J. Colloid Interface Sd. 73 406-415 Naono, H. Nakai, K. (1989) Thermal decomposition of P-FeOOH fine partides. J. Colloid Interface Sd. 128 146-156 Naono, H. Fujiwara, R. Sugioka, H. Sumiya,... 

K. Yanazawa, H. (1982) Micropore formation due to thermal decomposition of adcular microcrystals of P-FeOOH. J. Colloid Interface Sd. 87 317-332... 

Shah Singh, S. Kodama, H. (1994) Effect of the presence of aluminum ions in iron solutions on the formation of iron oxyhydroxides (FeOOH) at room temperature under acidic environment. Clays Clay Min. 42 606—613... 

Studies on the oxidation of iron(II) ions during formation of Fe304 and a-FeOOH by air oxidation of Fe(OF[)2 suspensions. J.C.S. Dalton, 1807-1811... 

Hydrolysis of FeCf Solutions. Aging of ferric chloride solutions yields, as a rule, either colloidal akageneite (p-FeOOH) or hematite (a-Fe203). However, the two forms are closely related in the formation of the precipitates (95,142). 

Fig. 1.1.13 Concentration domains of solutions containing FeClj and HCI aged at 100°C for 24 h (upper) and for 1 week (lower). N, no particle formation. Particle shapes D. double ellipsoids E, ellipsoidal 1, irregular of varying sizes R. rod-like S, spherical. Pairing of symbols indicates a mixture of corresponding particles in the suspension. Particle composition R, p-FeOOH all other particles, a-Fe2Oi. (From Ref. 65.)...

Precipitation of ferric hydroxide gel was also observed in the preparation of spindlelike hematite (a-Fe203) particles in a dilute ferric chloride solution in the presence of phosphate (9). In this case, however, the positive role of the gel was not definite since similar uniform hematite paricles were obtained as well in homogeneous systems in the presence of the same anions (9). Also, Hamada and Matijevic (10) prepared uniform particles of pseudocubic hematite by hydrolysis of ferric chloride in aqueous solutions of alcohol (10-50%) at I00°C for several days. In this reaction, it was observed that acicular crystals of (3-FeOOH precipitated first, and then they dissolved with formation of the pseudocubic particles of hematite. The intermediate P-FeOOH appears to work as a reservoir of the solute to maintain an ideal supersaturation for the nucleation and growth of the hematite. Since the (3-FeOOH as an intermediate and the pseudocubic shape tire not peculiar to the alcohol/water medium... 

When a-FeOOH particles are used for the starting material, it is very difficult to avoid pore formation completely in the dehydration process, which necessitates... 

Within this group we shall first consider iron, whose actual concentration is much higher than the calculated value. This oversaturation results in the formation of a-FeOOH or its amorphous counterpart, and the observed concentration must be attributed to the presence of these presumably colloidally dispersed solids. 

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