Solubility, ferric oxyhydroxides

As we pointed out earlier, the H subunit catalyses the ferroxidase reaction, which occurs at all levels of iron loading, but decreases with increasing amounts of iron added (48-800 Fe/ protein). Reaction (19.8) catalysed by both FI- and L-chain ferritins, occurs largely at intermediate iron loadings of 100-500 Fe/protein. Once nucleation has taken place, the role of the protein is to maintain the growing ferrihydrite core within the confines of the protein shell, thus maintaining the insoluble ferric oxyhydroxide in a water-soluble form. 

Macalady, D.L., D. Langmuir, T. Grundl, and A. Elzerman. 1990. Use of model-generated Fe3+ ion activities to compute Eh and ferric oxyhydroxide solubilities in anaerobic systems. In D.C. Melchior and R.L. Bassett, ed., Chemical Modeling of Aqueous Systems 11, Vol. 416 pp. 350-367. American Chemical Society, Washington, DC. 

The aluminum oxyhydroxides, kaolinite, and halloysite dissolve to form cationic aluminum species at low pH and the anionic species (Al(OH)4) at high pH. The same amphoteric behavior is also true of the Fe(III) oxyhydroxides, although the latter are much less soluble, in general, under oxidizing conditions. We next compute the solubilities of the ferric oxyhydroxides as a function of pH. The approach is identical to that described above for the Al-oxyhydroxides. 

Starting with a mass-balance equation for total iron, and using an approach identical to that used to derive Eq. (7.32) for the Al-oxyhydroxides, we obtain Eq, (7.49), which describes the solubility of the ferric oxyhydroxides as a function of pH,... 

In oxidized surface waters and sediments, dissolved iron is mobile below about pH 3 to 4 as Fe and Fe(lII) inorganic complexes. Fe(III) is also mobile in many soils, and in surface and ground-waters as ferric-organic (humic-fulvic) complexes up to about pH 5 to 6 and as colloidal ferric oxyhydroxides between about pH 3 to 8. Under reducing conditions iron is soluble and mobile as Fe(II) below about pH 7 to 8, when it occurs, usually as uncomplexed Fe ion. However, where sulfur is present and conditions are sufficiently anaerobic to cause sulfate reduction, Fe(H) precipitates almost quantitatively as sulfides. Discussion and explanation of these observations is given below. Thermodynamic data for iron aqueous species and solids at 25°C considered in this chapter are given in Table A12.1. Stability constants and A//° values computed from these data are considered more reliable than their values in the MINTEQA2 data base for the same species and solids. 

The pH at which the surface of the oxyhydroxides is uncharged in pure water is called the point of zero net proton chaige (PZNPC) (see Chap. 10). For the ferric oxyhydroxides in pure water the PZNPC corresponds to the pH of minimum solubility, which is near pH 8 (Fig. 12.4). Below this pH, consistent with the charge of the predominant Fe(III)-OH aqueous complex, solid surfaces are OH deficient (relative to Fe(III)/OH = 1/3) and so have a net positive charge. Above pH 8 the surface has... 

D. O. Whittemore and D. Langmuir, The solubility of ferric oxyhydroxides in natural waters. Copyright 1975 by Ground Water Publishing Company. Used by permission. 

The insolubility of such phases and of their solid solutions makes them potential sinks for heavy metals in acid sulfate systems. This is important, in part, because such metals are poorly adsorbed by phases such as hydrous ferric oxyhydroxide (HFO) under acid conditions (see Chap. 10) and in fact the HFO itself becomes soluble below pH 2 to 3 (Fig. 12.4). 

Results of batch leach tests performed on tailings from the Grants Mineral Belt, New Mexico are summarized in Tables IV and V. These results represent the average of two analyses on the acid leach tailings samples. Table IV contains the elemental concentrations of the sand and silt and clay-sized fractions, as well as the soluble concentration of each element found when 100 g of material is leached with 250 mL of sodium hexametaphosphate solution. The right-most column presents the ratio of the elemental concentration associated with the silt and clay-sized fraction to that of the sand-sized fraction, and it is presented as an enrichment factor. The high enrichment factors for As, Mo, Cr, Se, and V may be due to the fact that these elements are adsorbed onto jarosite, montmorillonite, and ferric oxyhydroxides (9-141. Speciation... 

Use of Model-Generated Fe Ion Activities To Compute Eh and Ferric Oxyhydroxide Solubilities in Anaerobic Systems... 

The measurement of dissolved Fe(III) and calculation of the activities of aqueous ferric species have challenged geochemists and others in their efforts to understand the chemistry of iron in natural waters (i-5). A new method for determining the activities of dissolved Fe(IlI) species in anaerobic systems containing Fe(II) is described in this paper (see also Q, along with conclusions about the behavior of redox electrodes in such systems and solubilities of associated ferric-oxyhydroxide solids. Redox conditions in dramatically different anaerobic aquifers, at Otis Air Force Base, MA and near Leadville, CO are also discussed. 

The solubility products of stoichiometric ferric oxyhydroxides can be expressed in terms of ion activities as ... 

Water Compositions and Calculated Ferric Oxyhydroxide Solubilities at Different Depths in Two Cape Cod Wells... 

Well 16-17 has Fe(II) values near the detection limit of 0.05 mg/L and contains O2 at low levels, which may be the cause of the higher Pt electrode reading. The site is in a transition zone, out of redox equilibrium, and contains nitrite at ca. 0.5 mg N/L. However, the laboratory results discussed above indicate that Ehj at the WIG electrode may still provide an accurate representation of the distribution of ferric/ferrous species in the system. Thus, the pQ value (39.4) calculated from the Ehj at the WIG electrode may be reliable. This pQ indicates that a ferric oxyhydroxide with a solubility similar to that of colloid-sized goethite is present. 

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