Iron anion adsorption

Liu F, De Cristofaro A, Violante A (2001) Effect of pH phosphate and oxalate on the adsorption/desorption of arsenate on/from goethite. Soil Sci 166 197-208 Livesey NT, Huang PM (1981) Adsorption of arsenate by soils and its relation to selected properties and anions. Soil Sci 131 88-94 Manceau A (1995) The mechanism of anion adsorption on iron oxides Evidence for the bonding of arsenate tetrahedra on free Fe(0, OH)6 edges. Geochim Cosmochim Acta 59 3647-3653. 

Anion adsorption on iron oxides is frequently described by this equation whereas cation adsorption data is often fitted to the Freundlich equation, i.e. 

Tab. 11.1 Inorganic anion adsorption studies on iron oxides.

The desorption of anions from iron oxides as a result of changing the anion concentration in solution is often very slow. It can be accelerated by increasing the pH. The partial irreversibility of anion adsorption has been attributed by some authors to a high activation energy of adsorption resulting from the formation of multiden-tate surface complexes, whereas others attribute it to a slow diffusion out of micropores. 

Borggaard, O.K. (1983) Effect of surface area and mineralogy of iron oxides on their surface charge and anion-adsorption properties. Clays Clay Min. 31 230-232 Borggaard, O.K. (1983 a) The influence of iron oxides on phosphate adsorption by soil. J. 

Anion adsorption by goethite and gibbsite. II. Desorption of anions from hydrous oxide surfaces. J. Soil Sci. 25 16-26 Hiradate, S. Inoue, K. (1998) Interaction of mugineic acid with iron (hydr)oxides Sulfate and phosphate influences. Soil Sci. Soc. 

Manceau, A. (1995) The mechanism of anion adsorption on iron oxides. Evidence for the bonding of arsenate tetrahedra at free Fe(0, OH) edges. Geochim. Cosmochim. Acta 59 3647-3653... 

Anion adsorption and then the exchange of anions mainly takes place on the protonated surface sites of silicates and other oxides (e.g., iron, manganese oxides) and hydroxides, as well as on the positive functional group (e.g., protonated amino groups) of humic substances. It is directed by electrostatic forces. The degree of anion exchange of rocks and soils is usually much less than that of cation exchange. 

Finally, in the group of acids whose 0 < pKai < 4, sigmoidal uptake curves and uptake curves with a maximum are reported in different sources, and there is no apparent correlation between the type of uptake curve, and the nature of the adsorbent (actually most available data on anion adsorption are for aluminum and iron III oxides and hydroxides as adsorbents) or the experimental conditions (e.g. the initial concentration of the adsorbate). Arsenate V, chromate VI, phosphate, and molybdate are typical examples of such behavior. For three former anions the number of publications reporting sigmoidal uptake curves on the one hand and uptake curves with a maximum on the other are approximately equal, but for molybdate the sigmoidal curves are more abundant. Comparison of molybdate with other anions in terms of pKa, is difficult in view of tendency to form polyacids (condensation). 

Hydroxy aluminium and hydroxy iron polymers also can adsorb anions with concurrent release of hydroxyl ions. The pH increase due to this anion exchange can be masked, however, by the simultaneous hydrolysis of desorbed aluminium ions (Bq. 10.2). Adsorption of multicharged anions can also decrease the net positive charge on hydroxy aluminium or hydroxy iron polymers, and thus increase the net negative charge of the soil-polymer mixture. The anion adsorption capacity of soils decreases with increasing pH and becomes virtually zero for all anions except phosphate and arsenate at pH values greater Ilian 5.5 or 6. 

According to Dzombak and Morel (1990) only one type of surface site is necessary to model the anion adsorption onto the surface of amorphous iron hydroxide. The input parameters specifying the availability of these sites are total amount of sites (moles/L), specific area (m /g) and mass (g/L). 

The site density for anion adsorption onto the amorphous iron hydroxide surface ranges mainly between 0.1 and 0.3 mol sites/mol Fe(goii(i) (see data collection of Dzombak and Morel, 1990). Dzombak and Morel used 0.2 mol sites/mol Fe(goiid) as an approximate value and assumed a specific area of 600 m /g when deriving the complexation constants from the collected experimental batch test data. 

V. Marecek, Z. Samec, and J. Weber, The Dependence of the Electrochemical Charge-Transfer Coefficient on the Electrode Potential. Study of the Hexacyanoferrate (III) Hexacyanferrate (IV) Redox Reaction on Polycrystalline Gold Electrode in Potassium Fluoride Solutions, J. Electroanal. Chem. Interfacial Electrochem. 94, 169-185 (1978) cf. also J. Weber, Z. Samec, and V. Marecek, The Effect of Anion Adsorption on the Kinetics of the Iron (3 + )/Iron (2. ) Reaction on Platinum and Gold Electrodes in Perchloric Acid, J. Electroanal. Chem. Interfacial Electrochem. 89, 271-288 (1978). 

Fe(0H)2)ajg, (Fe(OHA))j,j, and (FeA2)ads- The kinetics of iron dissolution in the active range in the presence of halide ions X is largely dominated by the competitive adsorption of X [88] with the dissolution activating OH. A critical survey of the possible reaction paths in which Cl competes with other anion adsorption is given in Ref 73. The mechanism is claimed to depend on the pH range. The catalytic step of dissolution in Cl-flee media [63] is considered to become at medium acidities (pH > 0.6) ... 

Z. A. lofa, V. V. Batrakov, and Cho-Ngok-Ba, Influence of anion adsorption on the action of inhibitors on the acid corrosion of iron and cobalt, Electrochim. Acta P 1645 (1964). 

Much work has been published on the dissolution of iron oxides in connection with the iron cycle in geochemistry, decontamination processes or the clean-up of industrial facilities. We have already seen that strong chelating agents such as EDTA or amino acids can adsorb on the surface of oxides and promote their dissolution because they can form anion complexes that are more stable than the oxide [52,63,64], Citrates and oxalates, among others, act in a similar way [65], Dissolution of oxides is markedly accelerated if oxidation-reduction processes occur in conjunction with anion adsorption [66]. The adsorption of ascorbate on hematite is a good example [67] (Figure 9.16). The reduction of ferric ions is shown... 

Early studies on oxide films stripped from iron showed the presence of chromium after inhibition in chromate solutionand of crystals of ferric phosphate after inhibition in phosphate solutions. More recently, radio-tracer studies using labelled anions have provided more detailed information on the uptake of anions. These measurements of irreversible uptake have shown that some inhibitive anions, e.g. chromateand phosphate are taken up to a considerable extent on the oxide film. However, other equally effective inhibitive anions, e.g. benzoate" pertechnetate and azelate , are taken up to a comparatively small extent. Anions may be adsorbed on the oxide surface by interactions similar to those described above in connection with adsorption on oxide-free metal surfaces. On the oxide surface there is the additional possibility that the adsorbed anions may undergo a process of ion exchange whereby... 

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

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