FeOOH, precipitation oxyhydroxides

Fe, common in surhcial oxic environments, forms hydroxides, complexes formerly called hmonite (a collection of mst-Uke precipitates, chemically designated as Fe(OH)3). The aggregate has now been shown to contain several oxyhydroxide species, FeOOH, and the oxides, hematite, Fe203, and magnetite, Fe304, and some new minerals have been identified (Schwertmann and Fitzpatrick, 1992, tables 1-3, pp. 10-11). 

Iron monosulfide, FeS, is produced in soils and sediments primarily through dissimilatory microbial reduction of sulfate to sulfide, which subsequently reacts with available iron to precipitate FeS (5, 4). The mineral mackinawite, often in poorly crystalline form (3, 23-25), is the initial FeS precipitate in the transformation of iron minerals by sulfate-reducing bacteria (3). For example, when the sulfate-reducing bacterium Desulfovibrio desulfuricans was grown at pH 8 in cultures containing a Fe(II)/Fe(III) oxyhydroxide and synthetic geothite (FeOOH), mackinawite was the predominant iron sulfide phase present after six and nine months, respectively (26). Even at lower pH values, mackinawite was the only iron sulfide phase detected after two weeks of microbial activity, and still a minor phase after that. 

We have seen above (Section 3.2) that precipitation of ferric ions in solution forms an oxyhydroxide that turns into goethite (a-FeOOH) or hematite (a-Fc203) through very different reaction mechanisms. In the presence of ferrous ions, ferric ions form a spinel oxide over a wide range of compositions. Alkalinization at pH > 9 of a Fe(ll)/Fe(lll) = 0.5 mixture forms stoichiometric magnetite Fe304 [102-104]. Small quantities of ferrous ions [Fe(lI)/Fe(III) >0.1] lead to a similar structure but containing vacancies [105,106]. 

The precipitation of ferric ions by the addition of a base or via thermohydrolysis in nitric or perchloric solutions leads to the oxyhydroxide a-FeOOH (goethile) or to the oxide a-Fe203 (hematite) (see Section 3.2. Ic). In Chapters 2 (Section 2.3.4) and 3 (Section 3.2.3), we saw that the nature of the solid phase, as well as the size and morphology of the particles, depends largely on the experimental conditions (iron concentration, pH, temperature, ageing time and ionic strength of the medium). 

Precipitation of ferric chloride in solution (at an iron concentration higher than 4 x 10" mol 1 ) by the addition of a base or thermohydrolysis leads to the oxyhydroxide /3-FeOOH (akaganeite) [58-6la]. Its structure is a parent of goethite. Double chains of octahedra are present in both structures, but they connect differently (Figure 5.16). /3-FeOOH has much wider channels containing variable amounts of chloride ions [58,62]. Unlike basic salts where polydentate ions are usually incorporated in the crystal structure, they are not part of the structure here, and can be exchanged with other ions [63-65]. 

Coprecipitation of ferric ions and divalent ions such as Mg, Cd, Zn and Pb does not lead to the spinel structure, but to M(0H)2 and the oxyhydroxide FeOOH [98]. The synthesis of the spinel requires heat treatment of the corresponding hydroxides or carbonates. Spinel feirites partially replaced with divalent elements are prepared indirectly by precipitation of Fe and as hydroxides, followed by oxidation of the suspension at 65 "C in air [99-103]. The stoichiometric phases are never obtained, however, except in the case of zinc. Precise conditions of acidity and composition (M /Fe ) must be chosen in order to prevent precipitation of hydroxides or basic salts of the divalent element and formation of the oxyhydroxide a-FeOOH. 

The oxyhydroxide /3-FeOOH is a metastable phase foniiing spontaneously during the early stages of precipitation of FeCl3. After high-temperature ageing of the suspensions, dissolution-crystallization equilibria lead to the formation of hematite [60,67,68] (see Sections 2.3.4 and 5.5.2). 

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