Solubility of iron oxides

Solubility diagrams have nearly always been calculated using solubility and stability constants. Experimental determination of the solubility of iron oxides as a function of pH has been concerned predominately with ferrihydrite. Lengweiler et al. 

The solubility of iron oxides in water and in alkaline drilling fluids is extremely low. Thus, scavenging reactions between soluble sulfides and iron oxide occur at the interface, where the iron in the solid surface and the liquid meet. Such heterogeneous reactions depend on exposure of a large, unreacted surface area and efficient contact with the sulfide species. In other words, both physical and chemical factors are important for heterogeneous reactions to proceed rapidly (Garrett et al. 1979). 

Sulfide collectors ia geaeral show Htfle affinity for nonsulfide minerals, thus separation of one sulfide from another becomes the main issue. The nonsulfide collectors are in general less selective and this is accentuated by the large similarities in surface properties between the various nonsulfide minerals (42). Some examples of sulfide flotation are copper sulfides flotation from siUceous gangue sequential flotation of sulfides of copper, lead, and zinc from complex and massive sulfide ores and flotation recovery of extremely small (a few ppm) amounts of precious metals. Examples of nonsulfide flotation include separation of sylvite, KCl, from haUte, NaCl, which are two soluble minerals having similar properties selective flocculation—flotation separation of iron oxides from siUca separation of feldspar from siUca, siUcates, and oxides phosphate rock separation from siUca and carbonates and coal flotation. 

Since Mn is both soluble in iron oxides and mobile to the same extent as Fe, the addition of Mn to steels has little effect on the overall scaling rate in air or oxygen. Jackson and Wallwork have shown that between 20% and 40% manganese must be added to steel before the iron oxides are replaced by manganese oxides. However, Mn supresses breakaway oxidation in CO/CO2 possibly by reducing the coalescence of pores in the oxide scale. 

U. Schwertmann, Solubility and dissolution of iron oxides. Iron Nutrition and Interactions in Plants (Y. Chen and Y. Hadar, eds.), Kluwer Academic Publishers, Boston, 1990, pp,. 7-28. 

The oxide surface has structural and functional groups (sites) which interact with gaseous and soluble species and also with the surfaces of other oxides and bacterial cells. The number of available sites per unit mass of oxide depends upon the nature of the oxide and its specific surface area. The specific surface area influences the reactivity of the oxide particularly its dissolution and dehydroxylation behaviour, interaction with sorbents, phase transformations and also, thermodynamic stability. In addition, specific surface area and also porosity are crucial factors for determining the activity of iron oxide catalysts. 

The solubility and the hydrolysis constants enable the concentration of iron that will be in equilibrium with an iron oxide to be calculated. This value may be underestimated if solubility is enhanced by other processes such as complexation and reduction. Solubility is also influenced by ionic strength, temperature, particle size and by crystal defects in the oxide. In alkaline media, the solubility of Fe oxides increases with rising temperature, whereas in acidic media, the reverse occurs. Blesa et al., (1994) calculated log Kso values for Fe oxides over the temperature range 25-300 °C from the free energies of formation for hematite, log iCso fell from 0.44 at 25 °C to -10.62 at300°C. 

The most important physical property of the solid that will affect solubility is particle size. For crystals < 1 pm, the high surface area may increase solubility. This occurs because it is the surface properties, especially the surface free energy, rather than the properties of the bulk solid, that govern the dissolution behaviour. Because the surface free energies of iron oxides are relatively high, particle size will have a marked... 

Schwertmann, U. (1988b) Occurrence and formation of iron in various pedoenvironments. In Stucki, J.W. Goodman, B.A. Schwertmann, U. (eds.) Iron in soils and clay minerals. Reidel Publ. Co., Dordrecht, Holland, NATO ASI Ser. 217 267-302 Schwertmann, U. (1991) Solubility and dissolution of iron oxides. Plant Soil 130 1-25 Schwertmann, U. (1993) Relations between iron... 

Since realization of the expected performance depends on rate of combustion and combustion efficiency, many studies have been made on catalysis. The studies of iron oxide have been particularly fruitful. The activity, amount and particle size of the iron additive can be an important tool in adjusting the performance of a propellant formulation. In many propellant formulations, performance can be improved by using soluble catalysis. 

With reference to the discussion of the bimetallic corrosion of iron given in Section 16.1, confirm (a) that the solubility of iron(II) hydroxide is 5.8 /zmol L-1 if OH- is produced along with iron(II) according to reaction 16.5 and the reverse of reaction 16.6 and (b) that the timescale of the oxidation of iron(II) in solution at pH 7 is as given following Eq. 16.8. Hint Incorporate [O2] and pH into a rate constant ki for a first-order process, then calculate the half-period h/2 = (ln2)/fci. 

Schwertmann, U. (1991). Solubility and dissolution of iron oxides, Plant Soil 130,1-25. 

A decisively higher water content of the solid material and the considerably better absorption and solubility properties of hydrogen cyanide in water are the reasons for the tendency of solid materials to accumulate more cyanides with lower temperatures. An increase in the reactivity of iron oxide (rust) in the solid body with relation to hydrogen cyanide with a higher water content of the solid material at lower temperatures must be anticipated, as well as with a general increase in the reactivity of all agents. A cooler, and thus moister, solid material is therefore better suited to the formation of Iron Blue than a warm, dry body.351... 

If the original rock contains up to 2 per cent, of iron oxide the resulting phosphate of iron is soluble, but with more than 4 per cent, of iron oxide the phosphate is insoluble—hence such a rock is considered unsuitable for the manufacture of superphosphate. The regression ... 

The slight solubility of the perchlorate in dilute alcohol affords a means of separating it from the chloride. According to Blau and Weingland, the decomposition of the chlorate is best carried out in quartz flasks at 480° C. without a catalyst, but when between 96 and 97 per cent, of the chlorate has been transformed the perchlorate begins to decompose, so that the change is never complete. The decomposition of the perchlorate is much accelerated by the presence of traces of iron oxide,... 

At high potentials (about +1 volt) the corrosion rate increases with the potential in agreement with the increasing solubility of the oxide, fee composition of which lies beyond its normal stoichiometric composition. As a matter of fact, iron, which does not oxidize easily beyond the trivalent state in acid solutions, does not undergo any important increase of its corrosion rate at high potentials. On the other hand, alloys with chromium, like 18/3 steels, are etched because chromium can be oxidized to a valence of six. In fact, the corrosion products contain mainly hexavaieat chromium and only traces of fcriva-lent iron. 

The largest proportion of the total Fe was removed by ammonium oxalate, which attacks the amorphic fraction of iron oxide in the sediments (23). Among the low pH extractants, hydroxylamine was the least efficient in extracting Fe. The difference between the extraction of Fe by acid and extraction by hydroxylamine was related to the crystallinity of the hydrous iron oxide. As pure iron oxides aged (and crystallized) in the laboratory, Fe solubility in hydroxylamine declined relative to solubility in acetic acid (Table III). In San Francisco Bay sediments, the ratio of hydroxylamine-soluble Fe to acetic acid-soluble Fe increased during the period of maximum runoff to the estuary (27) suggesting the proportion of the Fe in the sediments that was freshly precipitated varied seasonally. This was expected, since periods of heavy runoff are also times of maximum Fe movement from the watershed to the tributaries of the estuary ( ). 

Other molar weathering ratios can be devised to reflect leaching (Ba/Sr), oxidation (Fe0/Fe203), calcification (CaO + MgO/AlaOs), and salinization (Na20/K20). Two of these ratios reflect differential solubility of chemically comparable elements, but calcification ratio quantifies the accumulation of pedogenic calcite and dolomite, and the ratio of iron of different valence gives reactant and product of iron oxidation reactions. In the Precambrian paleosol illustrated (Figure 4), these molar ratios indicate that the profile was oxidized and well drained, but little leached, calcified or salinized. 

The following descriptive material concerns the solubility of iron in the reduced form (ferrous iron, Fe ) and the oxidized form ferric iron, Fc ). In a solution of strong acid, ferrous iron exists as a complex with water, Fe(M2D)h The complex contains six molecules of water. At higher pH, some of the protons of the complex are released, generating Fe(OH)2. Fe(OH)2 is pale green and, if present at sufficiently high concentrations, can form a gelatinous precipitate. Its maximal solubility at pH 7 is about 0.1 M. Hence, this form of iron is fairly soluble at neutral pH. 

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