Hydrous ferric oxides

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. 

D. A. D2ombak and P. M. M. Morel, Suface Complexation Modeling Hydrous Ferric Oxide, Wiley-Interscience, New York, 1990. 

In oxygenated water of near neutral pH and at or slightly above room temperature, hydrous ferric oxide [FelOHla] forms on steel and cast irons. Corrosion products are orange, red, or brown and are the major constituent of rust. This layer shields the underl3dng metal surface from oxygenated water, so oxygen concentration decreases beneath the rust layer. 

Rusting (rust) corrosion of iron or ferrous alloys resulting in a corrosion product which consists largely of hydrous ferric oxide. 

Pigna M, Colombo C, Violante A (2003) Competitive sorption of arsenate and phosphate on synthetic hematites (in Italian). Proceedings XXI Congress of Societa Italiana Chimica Agraria SICA (Ancona), pp 70-76 Quirk JP (1955) Significance of surface area calculated from water vapour sorption isotherms by use of the B. E. T. equation. Soil Sci 80 423-430 Rancourt DG, Fortin D, Pichler T, Lamarche G (2001) Mineralogical characterization of a natural As-rich hydrous ferric oxide coprecipitate formed by mining hydrothermal fluids and seawater. Am Mineral 86 834-851 Raven K, Jain A, Loeppert, RH (1998) Arsenite and arsenate adsorption on ferrihydrite kinetics, equilibrium, and adsorption envelopes. Environ Sci Technol 32 344-349... 

In this chapter, we discuss double layer theory and how it can be incorporated into a geochemical model. We will consider hydrous ferric oxide (FeOOH //IFO), which is one of the most important sorbing minerals at low temperature under oxidizing conditions. Sorption by hydrous ferric oxide has been widely studied and Dzombak and Morel (1990) have compiled an internally consistent database of its complexation reactions. The model we develop, however, is general and can be applied equally well to surface complexation with other metal oxides for which a reaction database is available. 

In Dzombak and Morel s (1990) development, hydrous ferric oxide holds two site types, one weakly and the other strongly binding. In their uncomplexed forms, the sites are labeled >(w)FeOH and >(s)FeOH the notation > represents bonding to the mineral structure, and (w) and (s) signify the weak and strong sites. 

To cast the equations in general terms, we use the label Ap to represent each type of surface site. In the case of hydrous ferric oxide, there are two such entries, >(w)FeOH and >(s)FeOH. There are Mp total moles of each site type in the system, divided between uncomplexed and complexed sites. This value is the product of the mass (in moles) of the sorbing mineral and the site density (moles of sites per mole of mineral) for each site type. 

As an example of an equilibrium calculation accounting for surface complexation, we consider the sorption of mercury, lead, and sulfate onto hydrous ferric oxide at pH 4 and 8. We use ferric hydroxide [Fe(OH)3] precipitate from the LLNL database to represent in the calculation hydrous ferric oxide (FeOOH /1H2O). Following Dzombak and Morel (1990), we assume a sorbing surface area of 600 m2 g-1 and site densities for the weakly and strongly binding sites, respectively, of 0.2 and 0.005 mol (mol FeOOH)-1. We choose a system containing 1 kg of solvent water (the default) in contact with 1 g of ferric hydroxide. 

In a second example, we calculate how pH affects sorption onto hydrous ferric oxide, expanding on our discussion (Section 10.4) of Dzombak and Morel s (1990) surface complexation model. We start as before, setting the dataset of surface reactions, suppressing the ferric minerals hematite (Fe203) and goethite (FeOOH), and specifying the amount of ferric oxide [represented in the calculation by Fe(OH)3 precipitate] in the system... 

Fig. 14.8. Concentrations (mmolal) of sites on a hydrous ferric oxide surface exposed at 25 °C to a 0.1 molal NaCl solution, calculated using a sliding pH path.

Fig. 14.9. Variation of surface potential (mV) with pH for a hydrous ferric oxide surface in contact at 25 °C with a 0.1 molal NaCl solution (bold line) and a more complex solution (fine line) that also contains Ca, SO4, Hg, Cr, As, and Zn.

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. 

The concentrations of other metals attenuate when the metals sorb onto the surfaces of precipitating minerals (see Chapter 10). Hydrous ferric oxide, the behavior of which is well studied (Dzombak and Morel, 1990), has a large specific surface area and is capable of sorbing metals from solution in considerable amounts, especially at moderate to high pH HAO may behave similarly. The process by which hfo or HAO form and then adsorb metals from solution, known as coprecipitation, represents an important control on the mobility of heavy metals in acid drainages (e.g., Chapman etal., 1983 Johnson, 1986 Davis etal., 1991 Smith et ai, 1992). 

We construct in this section a model of how inorganic lead reacts as it infiltrates and contaminates an aquifer, and then as the aquifer is flushed with fresh water during pump-and-treat remediation (Bethke, 1997 Bethke and Brady, 2000). We assume groundwater in the aquifer contacts hydrous ferric oxide [Fe(OH)3, for simplicity] which sorbs Pb++ ions according to the surface complexation model of Dzombak and Morel (1990), as discussed in Chapter 10. 

Iron-reducing bacteria from a copper-contaminated sediment were more tolerant of copper adsorbed to hydrous ferric oxide (HFO) than were pristine-sediment bacteria (Markwiese et al. 1998). Copper-tolerant bacteria were more efficient in reducing contaminated HFO, with greater potential for copper mobilization in aquatic sediments (Markwiese et al. 1998). 

Reactions between Fe(ll) in contaminated groundwater (5.8 mg/L) and oxic sediment also affected As mobility. Ferrous iron was oxidized by manganese oxides to ferric iron which precipitated as hydrous ferric oxide, creating additional sorption sites. Evidence for this reaction included an increase in ferric oxide concentrations in reacted column sediments and manganese concentrations in leachate that were greater than in the initial eluent. 

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). 

Dzombak, A. Morel, M. 1990. Surface complexation modeling hydrous ferric oxide. Wiley-lnterscience, New York. 

Dzombak, D. A., and F. M. M. Morel (1990), Surface Complexation Modeling Hydrous Feme Oxide, Wiley-lnterscience, New York. (This book addresses general issues related to surface complexation and its modeling, using the results obtained for hydrous ferric oxide as a basis for discussion. 

Surface charge as a function of pH and ionic strength (1 1 electrolyte) for a 90-mg/e (TOTFe = 10 3 M) suspension of hydrous ferric oxide. 

As evidenced by the mass laws of Eqs. (2.8), (2.9), the binding of a metal ion by. surfaceJigands - similar to the binding of a metal ion by a solute ligand - js strongly pH dependent (Fig. 2.5a). Complex formation is competitive (e.g., metal ion vs H+ ion or vs.another metal ion). Fig. 2.5b illustrates the sorption of various metal ions on hydrous ferric oxide. For each metal ion there is a narrow interval of 1 - 2 pH un.ita where the extent of sorption rises from zero to almost 100 %. 

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 ... 

Dzombak and Morel, 1990, have illustratively and compactly summarized (Fig. 3.3) the interdependence of the Coulombic interaction energy with pH and surface charge density at various ionic strengths for hydrous ferric oxide suspensions in... 

Relationship between pH, surface potential, xp or Coulombic term, log P, or Coulombic free energy, AGcoui), and surface charge density, a (or surface protonation) for various ionic strengths of a 1 1 electrolyte for a hydrous ferric oxide surface (P = exp(-Fi //RT). 

From alkalimetric-acidimetric titration curves on hydrous ferric oxide the following intrinsic acidity constants have been obtained (I = 0.1 M, 25° C)... 

Furthermore the surface complex formation with Zn(II) has been determined from adsorption studies in 10 3 M suspensions of hydrous ferric oxide with dilute (10 7 M) Zn(II) solution. It can be described by the reaction... 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

Schematic representation of surface precipitation on hydrous ferric oxide (Fe(OH)3(s))...

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