Iron oxyhydroxide, amorphous

Fig. 15-5 Comparative adsorption of several metals onto amorphous iron oxyhydroxide systems containing 10 M Fej and 0.1 m NaNOs. (a) Effect of solution pH on sorption of uncomplexed metals, (b) Comparison of binding constants for formation of soluble Me-OH complexes and formation of surface Me-O-Si complexes i.e. sorption onto Si02 particles, (c) Effect of solution pH on sorption of oxyanionic metals. (Figures (a), (c) reprinted with permission from Manzione, M. A. and Merrill, D. T. (1989). "Trace Metal Removal by Iron Coprecipitation Field Evaluation," EPRI report GS-6438, Electric Power Research Institute, California. Figure (b) reprinted with permission from Balistrieri, L. et al. (1981). Scavenging residence times of trace metals and surface chemistry of sinking particles in the deep ocean, Deep-Sea Res. 28A 101-121, Pergamon Press.)...

Davis, J.A. and Leckie, J.O., Surface ionization and complexation at the oxide/water interface, II surface properties of amorphous iron oxyhydroxide and adsorption of metal ions, J. Colloid Interface Sci. 67, 90-107, 1978. 

Addition of sufficient base to give a > 3 to a ferric solution immediately leads to precipitation of a poorly ordered, amorphous, red-brown ferric hydroxide precipitate. This synthetic precipitate resembles the mineral ferrihydrite, and also shows some similarity to the iron oxyhydroxide core of ferritin (see Chapter 6). Ferrihydrite can be considered as the least stable but most reactive form of iron(III), the group name for amorphous phases with large specific surface areas (>340 m2 /g). We will discuss the transformation of ferrihydrite into other more-crystalline products such as goethite and haematite shortly, but we begin with some remarks concerning the biological distribution and structure of ferrihydrite (Jambor and Dutrizac, 1998). 

Benjamin, M. M. and Leckie, J. O. (1981). Multiple-site adsorption of cadmium, copper, zinc, and lead on amorphous iron oxyhydroxide, J. Coll. Inter/. Sci., 79, 209-221. 

Davis, J. A., and J. O. Leckie (1978a), "Surface Ionization and Complexation at the Oxide/Water Interface. II. Surface Properties of Amorphous Iron Oxyhydroxide and Adsorption of Metal Ions," J. Colloid Interface Sci. 67, 90-107. 

Effects of Pentavalent Sb Ions on the Adsorption of Divalent Co-57 on Hematite. Benjamin and Bloom reported that arsenate ions enhance the adsorption of cobaltous ions on amorphous iron oxyhydroxide (J 6). Similarly, when divalent Co-57 ions were adsorbed on hematite together with pentavalent Sb ions, an increase of adsorption in the weakly acidic region was observed. For example, when 30 mg of hematite was shaken with 10 cm3 of 0.1 mol/dm3 KC1 solution at pH 5.5 containing carrier-free Co-57 and about 1 mg of pentavalent Sb ions, 95 % of Co-57 and about 30 % of Sb ions were adsorbed. The emission spectra of the divalent Co-57.ions adsorbed under these conditions are shown in Figure 8 together with the results obtained under different conditions. As seen in Figure 8, the spectra of divalent Co-57 co-adsorbed with pentavalent Sb ions are much different from those of Co-57 adsorbed alone (Figure 3). These observations show a marked effect of the.co-adsorbed pentavalent Sb ions on the chemical structure of adsorbed Co-57. 

Some metals are irreversibly adsorbed, probably via incorporation into the mineral phases, such as amorphous iron oxyhydroxides, as shown in Figure 11.6d. Some of these amorphous phases form by direct precipitation from seawater. As noted earlier, hydrothermal fluids are an important source of iron and manganese, both of which subsequently precipitate from seawater to form colloidal and particulate oxyhydroxides. Other metals tend to coprecipitate with the iron and manganese, creating a polymetallic oxyhydroxide. It is not clear the degree to which biological processes mediate the formation of such precipitates. Since the metals are incorporated into a mineral phase, this type of scavenging is better referred to as an absorption process. 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

Benjamin, M.M. Leckie, J.O. (1981) Multiple-site adsorption of Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide. J. Colloid Interface Sci. 79 209-221 Benjamin, M.M. Leckie, J.O. (1981a) Competitive adsorption of Cd, Zn, Cu and Pb on amorphous iron oxyhydroxide. J. Colloid Interface Sci. 83 410-419 Benjamin, M.M. Leckie, J.O. (1982) Effects of complexation by Cl, SO4, and S2O3 on the adsorption behavior of cadmium on oxide surfaces. Environ. Sci. Tech. 16 162-170 Benjamin, M.M. (1978) Effects of competing metals and complexing ligands on trace metal adsorption. Ph.D. Thesis Benjamin, M.M. Hayes, K.E. Leckie, K.O. 

McGarrah, J.E. (1991) Neptunium adsorption on synthetic amorphous iron oxyhydroxide. 

Zachara, J.M. Girvin, D.C. Schmidt, R.L. Resch, C.J. (1987) Chromate adsorption on amorphous iron oxyhydroxide in the presence of major ground water ions. Envir. Sci. Technol. 21 589-594... 

Girvin, D. C., Ames, L. L., Schwab, A. P. McGaRRAH, J. E. 1991. Neptunium adsorption on synthetic amorphous iron oxyhydroxide. Journal of Colloid and Interface Science, 141, 67-78. 

Johnson, C.A. (1986) The regulation of trace element concentrations in river and estuarine waters contaminated with acid mine drainage the adsorption of copper and zinc on amorphous iron oxyhydroxides. Geochim. Cosmochim. Acta, 50, 2433-2438. 

Figure 4. Predictive model calculation of Cd(II) adsorption on amorphous iron oxyhydroxide as a function of pH and amount of solid substrate present. Cdx 5 lO M, 0.1M NaNOs. (------) Model calculations. —4.6 pKcdoH —H-1.

This approach successfully described the experimental results of several adsorption studies with various metal ions and oxide substrates ( 2). In addition, one can make predictive calculations of metal ion uptake, if the surface parameters of an oxide/elec-trolyte can be estimated. For example. Figure 4 shows predicted and experimental adsorption behavior of Cd(II) on amorphous iron oxyhydroxide. Surface stability constants for Cd(II) were estimated ( ) from an experimental study of Cd(II) uptake by a-FeOOH (21, %). 

For example. Figure 6 shows experimental data and model calculations for sulfate adsorption on amorphous iron oxyhydroxide. The dashed gurve represents a model calculation using Equation 8 only, with p 10.1. At low pH the calculated adsorption is less... 

Sorption to iron-oxyhydroxide can be computed with the surface complexation model of Dzombak and Morel (1990). This model assembles the results of numerous laboratory experiments on sorption of trace elements to ferrihydrite (Hfo, hydrous ferric oxide, EeOOH). Eerrihydrite is a more or less amorphous substance which is found in nature in seepage zones of reduced, iron containing groundwater. Probably, it will be representative for the iron-oxyhydroxide which forms during in situ iron removal in aquifers, but this has not been verified yet. 

At the end of the seventies Garcia-Miragaya and Page [8,9] and Street et al. [10] reported a successful correlation by Freundlich equation of the data of trace Cd + adsorption by both clay minerals and soils. Benjamin and Leckie [11] found the same for the trace adsorption of Cu ", Zn +, Cd + and Pb + onto amorphous iron oxyhydroxide. The use of Freundlich equation was also suggested in the works by Sposito [12,13]. 

Values of the parameters obtained by Davies and Leckie [104], who analyzed their titration data for the amorphous iron oxyhydroxide in terms of the homogeneous surface model... 

Figure 27. The effects of pH on the isotherms of cadmium ions, adsorbed on the amorphous iron oxyhydroxide investigated by Benjamin and Leckie [11]. The solid lines were drawn by Benjamin and Leckie [11] as the best linear regression for their experimental points. Our theoretical lines match exactly the Benjamin and Leckie solid lines if the best fit parameter pK = 3.9. Theoretical lines were calculated from Eq. (79) by using the parameters pK , p K , PZC, Ns, Cl, I and T collected in Table 3, kT/c = 0.7 (i=0,+,A,C) and the parameters pK = 10.7 and p K]C = 6.8 calculated by us from Eqs. (81).

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