Magnetite partly oxidized

The method of Baudiseh and Welo (1925) in whieh a mixed solution of Fe " and Fe with a Fe VFe ratio of 2 is reaeted at pH 9-10 at RT leads to a poorly erystalline magnetite whieh is partly oxidized unless air is rigorously exeluded. The proeess probably involves reaetion of the Fe ions with the 2-line ferrihydrite whieh precipitates as the pH is brought to pH 9 (Taylor ct al. 1987). 

The principal cathodic reaction on the upper surface of the membrane is the reduction of Cu " that is formed by the reaction of Cu with dissolved oxygen in the water these Cu ions are provided partly from the diffusion through the pores in the oxide membrane from within the pit and partly from those produced by cathodic reduction (equation 1.154). Lucey s theory thus rejects the conventional large cathode small anode relationship that is invoked to explain localised attack, and this concept of an electronically conducting membrane has also been used by Evans to explain localised attack on steel due to a discontinuous film of magnetite. 

Biological reductive dissolution by Shewanella putrifaciens of Fe oxides in material from four Atlantic pleistocene sediments (ca. 1.5-41 g/kg Fe oxides) was compared with that of the synthetic analogues (ferrihydrite, goethite, hematite) (Zachara et al. 1998). In the presence of AQDS as an electron shuttle, the percentage of bio-reduc-tion of the three oxides was increased from 13.3 %(fh) 9.2%(gt) and 0.6%(hm) to 94.6% 32.8% and 9.9% with part of the Fe formed being precipitated as vivianite and siderite, but not as magnetite. The quinone was reduced to hydroquinone which in turn, and in agreement with thermodynamics, reduced the Fe as it had much better access to the oxide surface than did the bacteria themselves. 

The details of enzymatic magnetite formation in bacteria, especially the valence and chemical form in which the Fe enters the cell, are still not fully understood. At low oxygen concentrations in the bacterial habitats dissolved Fe may exist in bivalent form, but Fe added as a soluble Fe " complex, such as Fe " citrate (Schuler Bauerlein, 1996) can also function as an Fe source. Within the cell, part of the Fe will then form a highly reactive Fe "oxide, probably ferrihydrite, which in turn, reacts with the dissolved Fe to form magnetite (Mann et al. 1989) by a via-solution process (Fig. 17.6) ... 

Iron oxides have played their part in navigation. Around the first millenium AD, magnetite, in the form of lodestone, was used in the earliest, crude mariners compasses. These enabled more difficult voyages to be undertaken and thus contributed to a furthering of trade and exploration. 

After performing FT synthesis on an unreduced iron oxide catalyst, Kuivila et al.12 observed 22% carbide in the bulk by Mossbauer spectroscopy, but only —3% carbide on the surface by XPS, and therefore concluded that a sub-surface carbide phase had formed beneath a magnetite surface layer. Based in part on this result, they conclude that magnetite is the active phase for FT synthesis. Reymond et a/.10 also observed substantial amounts of carbide by XRD, but little or no carbide by XPS. The observation of a 2-4 nm thick carbon layer on the carbide phase, but not on the magnetite, allows a reinterpretation of the data in these two papers. Sputtering of the surface carbon layer permits the XPS to see the underlying carbide, and therefore it is not necessary that the carbide be present beneath an oxide layer. Thus, measurement of low carbide signals by XPS cannot be interpreted to mean that carbide is absent from the catalyst surface, and therefore not an important phase in FT... 

Elemental iron, the major element in Earth s core, is the fourth most abundant element in Earth s crust (about 5.0% by mass overall, 0.5%-5% in soils, and approximately 2.5 parts per billion in seawater.) In the crust, iron is found mainly as the oxide minerals hematite, Fe203, and magnetite, Fe304. Other common mineral forms are siderite, FeC03, and various forms of FeO(OH). Iron is an essential element in almost all living organisms. In the human body, its concentration ranges between 3 and 380 parts per million (ppm) in bone, 380-450 ppm in blood, and 20-1,400 ppm in tissue. 

Thus, ammonia does not reduce magnetite at an appreciable rate at temperatures below 450°C., and it appeal s that at 450°C. and above, the reduction may be accomplished by decomposition products of ammonia rather than by ammonia itself. This contention is based on the fact that the reduction of fused catalysts with ammonia at 450°C. and 550°C. appeared to be an autocatalytic process that is, the rate of reduction increased with time in the initial part of the experiment. Reduction with hydrogen does not appear to be autocatalytic. It may be postulated that a-iron and nitride formed in the reduction are better catalysts for the ammonia decomposition than iron oxide. 

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