Iron oxyhydroxides, dissolution

Bertrand, I., and Hinsinger, P. (2000). Dissolution of an iron oxyhydroxide in the rhizosphere of various crop species. J. Plant Nutr. 23,1559-1577. 

Figure 3. Diagram of a section through the cell wall of Acidithiobacillus ferrooxidans modified from Blake et al. (1992) showing the relationship between iron oxidation and pyrite dissolution. OM =outer membrane, P = periplasm, IM = inner or (cytoplasmic) membrane, cty = cytochrome, pmf = proton motive force. Passage of a proton (driven by proton motive force) into the cell catalyzes the conversion of ADP to ATP. Ferrous iron binds to a component of the electron transport chain, probably a cytochrome c, and is oxidized. The electrons are passed to a terminal reductase where they are combined with O2 and to form water, preventing acidification of the cytoplasm. Ferric iron can either oxidize pyrite (e.g. within the ore body) or form nanocrystalline iron oxyhydroxide minerals (often in surrounding groundwater or streams).

These observations indicate that reduction of As(V) to As(III) does not, in itself, result in the mobilization of arsenic. This conclusion is supported by laboratory adsorption studies showing similar affinities of As(III) and As(V) for hydrous ferric oxide, goethite, and magnetite.16 However, outstanding questions remain regarding the factors that control the rate and extent of the reductive dissolution of iron in these sediments and whether the arsenic (and iron) that is released into the porewater is (re)sorbed onto the residual iron oxyhydroxides in... 

Fig. 7.16 Pore water and extraction results from hemipelagic sediments off Uruguay (redrawn from Haese et al. 2000). Dissolution and precipitation of Fe is reflected by the easy reducible iron oxyhydroxide fraction whereas less reducible iron oxides soluble by subsequent citrate/dithionite/bicarbonate (CDB) extraction remain constant. A concurrent liberation of Mn and Fe indicates dissimilatory iron reduction and subsequent iron reoxidation by manganese oxides, which results in the build-up of Mn Under these conditions the actual dissimilatory iron reduction rate is higher than deduced from iron pore water gradients.

It is well established that oxides of Fe and perhaps Mn and Al effectively adsorb or occlude most toxic metals. These oxides exist in mineral soils in large quantities. When soils become reducing, the metals bound to Fe and Mn oxides are transformed into readily available forms due to dissolution of Fe and Mn oxides. During flooding and drainage cycles of wetlands, the formation of iron oxyhydroxides is important in retaining metals in surface soils (Gambrell, 1994). 

The highest residual traces of Cr(VI) occur in the anodic sections of the experimental cells. Cr(VI) removal from aqueous solutions is enhanced by the presence of ferric iron oxyhydroxide phases, as Cr(VI) adsorbs onto FeOOH (e.g. Aoki and Munemori, 1982 Mesuere and Fish, 1992a,b Mukhopadhyay, Sundquist, and Schmitz, 2007). The amount of released by anodic electrode dissolution primarily depends on the applied current and the duration of the passage of the current through the electrodes (e.g. Mukhopadhyay, Sundquist, and Schmitz, 2007). Differences in the lateral extent of iron mineralization in the three experiments illustrate that the buffering capacity of the soils influenced the spatial extents of the zone of Cr(VI) reduction and complementary alkaline zone. The Warwick soil (experiment A) operated at half the applied voltage to experiments B and C, experienced the furthest advance of iron mineralization from the anode array, quickly developed a sharp pH jump, and attained the most acidic conditions. Collectively, these attributes indicate that the Warwick soil had a comparatively low buffering capacity relative to the other two soils examined. 

Although mostly it is accepted that the corrosion reaction requires the existence of the oxygen reduction reaction, Hoffmann has concluded that, in the absence of oxygen, dissolution can be compensated by reduction of iron oxyhydroxides [87]. 

An interesting observation of Hickiing and Ives is that the redox potential of dissolving magnetite and the Flade potential (the potential at which the passive film dissolves) in a similar solution are almost identical, implying that passivation and dissolution of oxides are simply reversible reactions of the same type. A similar observation by Allen et al. supports this idea. In contrast, there is evidence that the passive film on iron is composed primarily of a highly protonated, trivalent iron oxyhydroxide. ... 

Fig. 6. Scanning Electron photomicrographs illustrating key mineral textures in core samples. A, Plagioclase surface showing dissolution textures (DH-3/-484m) B, plagioclase surface coated by kaolinite (DH-3/-484m) C and D, biotite and iron oxyhydroxide mineral (DH-3/-484m) E, pyrite dissolution texture (DH-4/80m) ...

Inorganic reactions in the soil interstitial waters also influence dissolved P concentrations. These reactions include the dissolution or precipitation of P-containing minerals or the adsorption and desorption of P onto and from mineral surfaces. As discussed above, the inorganic reactivity of phosphate is strongly dependent on pH. In alkaline systems, apatite solubility should limit groundwater phosphate whereas in acidic soils, aluminum phosphates should dominate. Adsorption of phosphate onto mineral surfaces, such as iron or aluminum oxyhydroxides and clays, is favored by low solution pH and may influence soil interstitial water concentrations. Phosphorus will be exchanged between organic materials, soil inter-... 

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). 

Corrosion is a mixed-electrode process in which parts of the surface act as cathodes, reducing oxygen to water, and other parts act as anodes, with metal dissolution the main reaction. As is well known, iron and ferrous alloys do not dissolve readily even though thermodynamically they would be expected to, The reason is that in the range of mixed potentials normally encountered, iron in neutral or slightly acidic or basic solutions passivates, that is it forms a layer of oxide or oxyhydroxide that inhibits further corrosion. 

Blesa, M.A. Maroto, A.J.G. (1986) Dissolution of metal oxides. J. chim. phys. 83 757—764 Blesa, M.A. Matijevic, E. (1989) Phase transformation of iron oxides, oxyhydroxides, and hydrous oxides in aqueous media. Adv. Colloid Interface Sci. 29 173-221 Blesa, M.A. Borghi, E.B. Maroto, A.J.G. Re-gazzoni, A.E. (1984) Adsorption of EDTA and iron-EDTA complexes on magnetite and the mechanism of dissolution of magnetite by EDTA. J. Colloid Interface Sci. 98 295-305 Blesa, M.A. Larotonda, R.M. Maroto, A.J.G. Regazzoni, A.E. (1982) Behaviour of cobalt(l 1) in aqueous suspensions of magnetite. Colloid Surf. 5 197-208... 

Chem. Soc. Faraday Trans. I. 71 1623-1630 Rustad, J.R. Felmy A.R. Hay, B.P. (1996) Molecular statics calculations for iron oxide and oxyhydroxide minerals Toward a flexible model of the reactive mineral-water interface. Geochim. Cosmochim. Acta 60 1553—1562 Ryan, J.N. Gschwend, P.M. (1991) Extraction of iron oxides from sediments using reductive dissolution by titanium(III). Clays Clay Min. 39 509-518... 

Sidhu, P.S. Gilkes, R.J. Posner, A.M. (1981a) Oxidation and ejection of nickel and zinc from natural and synthetic magnetites. Soil Sci. Soc. Am. J. 45 641-644 Sidhu, P.S. Gilkes, R.J. Cornell, R.M. Posner, A.M. Quirk, J.P. (1981) Dissolution of iron oxides and oxyhydroxides in hydrochloric and perchloric acids. Clays Clay Min. 29 269-276... 

In very acidic solutions (pH < 2.4-3) with ionic strengths below 0.1 M and at 25 °C and 1 bar pressure, scorodite has a pK of about 25.83 0.07. The pK of amorphous Fe(III) arsenate is approximately 23.0 0.3 under the same conditions (Langmuir, Mahoney and Rowson, 2006). At higher pH values, scorodite dissolves incongruently, which means that at least one of its dissolution products precipitates as a solid. The incongruent dissolution of scorodite in water leads to the formation of Fe(III) (oxy)(hydr)oxide precipitates that is, Le(III) (hydrous) oxides, (hydrous) hydroxides and (hydrous) oxyhydroxides (Chapter 3). During the formation and precipitation of the iron(III) (oxy)(hydr)oxides, As(V) probably coprecipitates with them (Chapter 3 also see Section 2.7.6.3). The dissolution rate of scorodite at 22 °C in pH 2-6 water is slow, around 10—9 —10—10 mol m-2 s-1, which explains its presence in many mining wastes (Harvey et al., 2006). 

Foley and Ayuso (2008) suggest that typical processes that could explain the release of arsenic from minerals in bedrock include oxidation of arsenian pyrite or arsenopyrite, or carbonation of As-sulfides, and these in general rely on discrete minerals or on a fairly limited series of minerals. In contrast, in the Penobscot Formation and other metasedimentary rocks of coastal Maine, oxidation of arsenic-bearing iron—cobalt— nickel-sulfide minerals, dissolution (by reduction) of arsenic-bearing secondary arsenic and iron hydroxide and sulfate minerals, carbonation and/or oxidation of As-sulfide minerals, and desorption of arsenic from Fe-hydroxide mineral surfaces are all thought to be implicated. All of these processes contribute to the occurrence of arsenic in groundwaters in coastal Maine, as a result of the variability in composition and overlap in stability of the arsenic source minerals. Also, Lipfert et al. (2007) concluded that as sea level rose, environmental conditions favored reduction of bedrock minerals, and that under the current anaerobic conditions in the bedrock, bacteria reduction of the Fe-and Mn-oxyhydroxides are implicated with arsenic releases. 

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