Iron -hydroxide Fe

Fe2+ and OH react in aqueous solution to form iron hydroxide, Fe(OH)2, which reacts with H20 and 02 to form rust, Fe203 3 H20. 

Why is the formation of iron hydroxide, Fe(OH)2, from Fe2+ and OH- not considered an oxidation-reduction reaction ... 

How many electrons are transferred from iron atoms to oxygen atoms in the formation of two molecules of iron hydroxide, Fe(OH)2 See Figure 11.18. 

Chemical reactions are those in which elements are added or removed from a chemical species. Purely chemical reactions are those in which none of the species undergoes a change in its valence, i.e., no species is either oxidized or reduced. Electrochemical rections are chemical reactions in which not only may elements be added or removed from a chemical species but also at least one species undergoes a change in the number of valence electrons. For example, the precipitation of iron hydroxide, Fe(OH)2, is a pure chemical reaction ... 

In the upstream part of the weathering profile, oxidizing conditions are prevailing, leading to the accumulation of neofor-med iron hydroxide Fe(0FI)3, while the sulfur species are completely oxidized to sulfate. 

Recent sediments of water basins. In recent basins iron sediments consist mainly of the iron hydroxides Fe(OH)3 or Fe203-nH20, but in very rare cases silicates and carbonates of Fe ", pyrite, and hydrotroilite enter into the composition of the sediment all together they constitute reactive (mobile) iron, which actively takes part in the diagenetic processes. A mixture of clastic minerals, which decompose negligibly and take practically no part in the processes of diagenesis, constitute another group. 

Ircm hydroxide (Fe(QU)g) Iron hydroxide (Fe(OH)g) Ircm hydroxide (Fe(Cffl)g)... 

Iron concentration can be reduced by precipitation of an iron mineral such as siderite (FeC03), jarosite (NaFe3(S04)2(0H) ), or amorphous iron hydroxide (Fe(OH)33). It was unclear what mineral or phase controls iron concentration, especially since precipitation was sensitive to oxidation potential (pe), and the field redox electrode measurements used in the model are not always relevant for specific redox couples. Since the mine workings flooded before the study was begun, samples of the solid phases could not be collected. 

Iron hydroxide, Fe(0H)3-nH20, decomposes with temperature to y-Fc203, which is more reactive than a-Fc203. As a result, strontium hexa-ferrite, SrFei20i9, can be obtained from coprecipitates of Fe(0H)3-nH20 and strontium laureate, Sr[CH3(CH2)ioCOO]2, at temperatures as low as 550 °C (Qian Evans, 1981). By contrast, mechanical mixtures of a-Fe203 and SrO show an appreciable reaction rate only at T > 720 °C. 

Biooxidation is decomposition of organic matter with oxidizing of its carbon. Organic matter in these reactions is donor of electrons, and the acceptors are elements or compounds outside it O, NO3. NO T Fe, iron hydroxide Fe(OH)3>, CO, some chlorinated solvents, etc. There may be aerobic and anaerobic oxidizing. In the former case acceptor of electrons is directly molecular oxygen O, in the latter oxidized forms of nitrogen (NOj", NO3 ), manganese (Mn ), iron (Fe +), sulphur (SO ), etc. 

As Figure 3.12-a shows, the dominant migration form of iron in pure water under oxidation environment and pH 4-12 is iron hydroxide Fe(OH)3. This hydroxide, because of very low solubility (about 0.024 mkg-l ), easily passes into colloidal form, precipitates and then converts to goethite - FeOOH and hematite - Fe Oj (see Figure 2.51). That is why, in the absence of humic acids, the oxide iron is mobile only in very acidic (pH < 4) or extremely alkali (pH >12) water, which is almost never encountered in nature. In neutral water this iron is present predominantly in the colloidal state. Organic acids facilitate increased water acidity, form stable complex... 

Figure 13.3 schematizes reactions (13.17 and 13.18) for iron in water. Iron hydrates into Fe " ions and free electrons e while oxygen O2 is reduced by electrons and reacts with water forming OH ions, according to Eq. (13.18). OH ions, in turn, allow iron reduction that precipitates as Fe(OH)2 or Fe(OH)3 (rust). The iron hydroxide Fe(OH)2 that forms from reaction (4) in Eq. (13.17), in fact, precipitates because it is insoluble in water solution or is further oxidized into ferric hydroxide Fe(OH)3 that gives rust that characteristic reddish color, through the reaction... 

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