Ferric oxide solubility

Iron hydrolysis and solubility revisited Observations and comments on iron hydrolyses characterizations. Marine Chem. 70 23—38 Byrne, R.H. Kester, D.R. (1976) Solubility of hydrous ferric oxide and iron speciation in seawater. Marine Chem. 4 255—274 Byrne, R.H. Luo,Y.-R. (2000) Direct observations of nonintegral hydreno ferric oxide solubility products K Sq = [Fe ][H ] Geo-chim. Cosmochim. Acta 64 1873-1877 Cabrera, F. de Arambarri, P. Madrid, L. ... 

Byrne R. H. and Luo Yu-Ran (2000) Direct observations of nonintegral hydrous ferric oxide solubility products K so = [Fe ][H ] —2.86. Geochim. Cosmochim. Acta 64, 1873-1877. 

In the double-neutralization process, Na2SiFg is precipitated and removed by filtration at a pH of 3—4 (9). Upon raising the pH to 7—9, insoluble phosphates of Fe, Al, Ca, and Mg form and separate. Iron can be precipitated as hydrous ferric oxide, reducing the phosphate loss at the second filter cake. Both the fluorosihcate and metal phosphate filter residues tend to be voluminous cakes that shrink when dewatered recovery of soluble phosphates trapped within the cakes is difficult. 

Iron fouling is caused by both forms of iron ions the insoluble form will coat the resin bead surface and the soluble form can exchange and attach to exchange sites on the resin bead. These exchanged ions can be oxidized by subsequent cycles and precipitate ferric oxide within the bead interior. 

As pH rises, the metal content of drainage water tends to decrease. Some metals precipitate directly from solution to form oxide, hydroxide, and oxy-hydroxide phases. Iron and aluminum are notable is this regard. They initially form colloidal and suspended phases known as hydrous ferric oxide (hfo, FeOOH n O) and hydrous aluminum oxide (HAO, AlOOH nH.2O), both of which are highly soluble under acidic conditions but nearly insoluble at near-neutral pH. 

Although there are no universal rules for metal selectivity as it depends on numerous factors, the obtained sequence is in agreement with the results reported in literature for the Kd sorption values on fresh precipitates of hydrous ferric oxide (Dzombak Morel 1990 Munk et al. 2002). The lowest factor observed for Mn comes in agreement with the well-known difficulty to remove this metal from mine waters due to its high solubility over a wide pH range (e.g., Hedin etal. 1994). 

Typically, mammalian ferritins can store up to 4500 atoms of iron in a water-soluble, nontoxic, bioavailable form as a hydrated ferric oxide mineral core with variable amounts of phosphate. The iron cores of mammalian ferritins are ferrihydrite-like (5Fe203 -9H20) with varying degrees of crystallinity, whereas those from bacterioferritins are amorphous due to their high phosphate content. The Fe/phosphate ratio in bacterioferritins can range from 1 1 to 1 2, while the corresponding ratio in mammalian ferritins is approximately 1 0.1. 

Stability constants (ethylendiamine, glycinate, oxalate), surface complex formation constants and solubility products (sulfides) of transition ions. The surface complex formation constant is for the binding of metal ions to hydrous ferric oxide =Fe-OH + Me2+ =FeOMe++ H+ K. ... 

Compare the solubility of amorphous Fe(OH)3(s), as given in the figure below with the acid base properties of a solid hydrous ferric oxide (cf. Fig. 2.3). Is there a connection between the solubility minimum and the pHpznpc ... 

In the sorption experiments of Icopini et al. (2004), the measured isotopic contrast between Fe(II)aq and the goethite starting material was -0.8%o after Fe(II) had sorbed to the surface over 24 hours in this case, the isotopic fractionation between sorbed Fe(II) and Fe(II)aq is not the 0.8%o measured difference, but is approximately +2.1%o based on an inferred 8 Fe value for the sorbed component as calculated from Fe mass balance (Fig. 4), as was noted in that study. Measured differences in Fe isotope compositions between ferric oxide/hydroxide and Fe(II)aq during dissimilatory Fe(III) reduction and photosynthetic Fe(II) oxidation have been proposed to reflect fractionation between soluble Fe(II) and Fe(III) species, where the soluble Fe(III) component is postulated to be bound to the cell and is not directly measured (Beard et al. 2003a Croal et al. 2004). In the case of dissimilatory Fe(III) reduction, assuming a static model simply for purposes of illustration, if 50% of the Fe in a pool that is open to... 

Figure 5. Possible pathways by which Fe isotopes may be fractionated during dissimilatory Fe(III) reduction (DIR). Dissolution, if it occurs congruently, is unlikely to produce isotopic fractionation (Afi. If Fe(II) is well complexed in solution and conditions are anaerobic, precipitation of new ferric oxides (A3) is unlikely to occur. Significant isotopic fractionation is expected during the reduction step (A2), possibly reflecting isotopic fractionation between soluble pools of Fe(III) and Fe(II). The soluble Fe(III) component is expected to interact with the cell through an electron shuttle compound and/or an outer membrane protein, and is not part of the ambient pool of aqueous Fe. Sorption of aqueous or soluble Fe(II) to the ferric oxide/hydroxide substrate (A4) is another step in which isotopic fractionation may occur. Modified from Beard et al. (2003a).

Figure 12. Possible isotope fractionation steps during anaerobic photosynthetic Fe(II) oxidation (APIO). It is assumed that the process of oxidation proceeds through an oxidation step, where Fe(II),q is converted to soluble Fe(III) in close proximity to the cell, followed by precipitation as ferric oxides/hydroxides. As in DIR (Fig. 5), the most likely step in which the measured Fe isotope fractionations are envisioned to occur is during oxidation, where isotopic exchange is postulated to occur between pools of Fe(II) and Fe(III) (Aj). As discussed in the text and in Croal et al. (2004), however, it is also possible that significant Fe isotope fractionation occurs between Fe(III), and the ferrihydrite precipitate (Aj) in this case the overall isotopic fractionation measured between Fe(II), and the ferrihydrite precipitate would reflect the sum of A and Aj, assuming the proportion of Fe(III) is small (see text for discussion). Isotopic exchange may also occur between Fe(II),q and the ferric hydroxide precipitate (Aj), although this is considered unlikely.

Since the hydrous ferric oxide has the largest p, it should be the most soluble. The equilibrium concentration of Fe can be predicted for this mineral at a given pH, as follows ... 

Fowkes and co-workers also clearly demonstrated that the physical Interaction of polymers with neighboring molecules was determined by only two kinds of interactions London dispersion forces and Lewis acid-base interactions (21) Calculations based on this concept were shown to correct many of the problems inherent in the solubility approach. They were also able to use the concept to study the distribution of molar heats of absorption of various polymers onto ferric oxides, and thereby more accurately described the requirements for adequate adhesion to steel substrates (21) In the symposium on which this book is based, Fowkes summarized work showing that the polar Interactions between polymers and metal surfaces that are... 

Soluble complexes are formed with metallic oxides, especially in the presence of alkali hydroxides. The strong tendency of hexitols to dissolve metallic oxides presents considerable technical difficulty in their manufacture and for this reason glass, rubber or stainless-steel equipment is used. In some instances well defined complexes can be isolated, particularly with alkaline earth oxides or mixtures with ferric oxide. These complexes absorb carbon dioxide and water and are unstable in dilute aqueous solution. Their structures are not established, but are inferred from analytical and physical measurements. Diehl has reviewed the subject. 

Metastability of Hydrolyzed Iron (III) Solutions The low solubility of ferric hydroxide has been alluded to in the Introduction. Feitknecht and Michaelis (29) have observed that aU ferric perchlorate solutions to which base has been added are unstable with respect to eventual precipitation of various forms of hydrated ferric oxides. In 3 M NaC104 at 25° C the two phase system reaches an apparent equilibrium after 200 hours, according to Biedermann and Schindler (6), who obtained a reproducible solubility product constant for ferric hydroxide at varying degrees of hydrolysis. It appears that many of the solutions used in the equilibrium studies of Hedstrom (9) and Biedermann (22) were metastable, and should eventually have produced precipitates. Nevertheless, since the measured potentials were reversible, the conclusions reached about the species present in solution remain valid. 

Within tissues of animals, plants, and fungi much of the iron is packaged into the red-brown water-soluble protein ferritin, which stores Fe(III) in a soluble, nontoxic, and readily available form.61 64 Although bacteria store very little iron,65 some of them also contain a type of ferritin.66-67 On the other hand, the yeast S. cerevisieae stores iron in polyphosphate-rich granules, even though a ferritin is also present.65 Ferritin contains 17-23% iron as a dense core of hydrated ferric oxide 7 nm in diameter surrounded by a protein coat made up of twenty-four subunits of mo-... 

The precipitated hexahydrate gradually undergoes oxidation on exposure to moist air, yielding ferric arsenate and ferric oxide.3 It is sparingly soluble in aqueous ammonia,4 but is insoluble in the presence of ammonium salts. 

If a soap is added to a lead hydrogen arsenate-hydrated ferric oxide mixture considerable damage and leaf drop is caused, especially if the soap contains a strong base, as in the case of commercial potash fish-oil soap or potassium oleate. Laboratory tests show that more soluble arsenic is formed than with a soap of a weak base such as triethanolamine oleate.7 The addition of cryolite to an arsenate inhibits the formation of soluble arsenic 8 fluosilicates cause decomposition. These fluorine... 

Unfortunately, any attempt to reduce ferric oxide results in the formation of magnetite as a distinct separate phase, and there is no solubility of this spinel in the corundum structure ( 5, 6). Thus, all the properties reported above for were... 

Good lithopone should contain not more than traces of soluble salts and impurities, especially ferric oxide, and should not contain added extraneous substances. The proportion of zinc oxide should not be more than 2—3% and the moisture usually amounts to 0-2-0-3%. In some cases,however, zinc oxide is present in considerable amount and moisture to the extent of x-2%. 

---