Ferric oxides, reductive dissolution

On the basis of this model, Lovley et al. (17) argued that reductive dissolution of ferric oxides must be a microbiological process because the zone of sulfide generation is distinct from the zone of maximum ferric oxide reduction. Highly eutrophic environments would be an exception. In these systems the zone of decomposition with oxygen as terminal electron acceptor directly overlies the zone of sulfate reduction. 

We exemplify the reductive dissolution of minerals by illustrating the reductive dissolution of hydrous ferric oxide. With this oxide (Sulzberger et al. 1989, Suter et al.,... 

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

Figure 5. Possible pathways by which Fe isotopes may be fractionated during dissimilatory Fe(III) reduction (DIR). Dissolution, if it occurs congruently, is unlikely to produce isotopic fractionation (Afi. If Fe(II) is well complexed in solution and conditions are anaerobic, precipitation of new ferric oxides (A3) is unlikely to occur. Significant isotopic fractionation is expected during the reduction step (A2), possibly reflecting isotopic fractionation between soluble pools of Fe(III) and Fe(II). The soluble Fe(III) component is expected to interact with the cell through an electron shuttle compound and/or an outer membrane protein, and is not part of the ambient pool of aqueous Fe. Sorption of aqueous or soluble Fe(II) to the ferric oxide/hydroxide substrate (A4) is another step in which isotopic fractionation may occur. Modified from Beard et al. (2003a).

Bridge TAM, Johnson DB. 1998. Reduction of soluble iron and reductive dissolution of ferric iron containing minerals by moderately thermophilic iron-oxidizing bacteria. Appl Environ Microbiol 64 2181-6. 

Pryor, M.J. Evans, U.R. (1950) The reductive dissolution of ferric oxide in acid. I. The reductive dissolution of oxide films present on iron. J. Chem. Soc., 1259-1266 Puchelt, H. (1973) Recent iron sediment formation at the Kameni Islands, Santorini (Greece). In Amstutz, G.C. Bernard, A.J. (eds.) Ores in sediments. Springer Berlin, 227-245... 

The ferrous iron supply stems from reductive dissolution of ferric oxides, another alkalinity-generating process ... 

By following the reaction scheme proposed by dos Santos Afonso and Stumm (22) for the reductive dissolution of hematite surface sites (Scheme 1), we were able to explain perfectly the observed pH pattern of the oxidation rate of H2S. The rate is proportional to the concentration of inner-sphere surface complexes of HS" formed with either the neutral (>FeOH) or the protonated (>FeOH2+) ferric oxide surface sites. 

We were not able to measure an increase of dissolved iron at a pH higher than 5.7. Even at this pH, the recovery of dissolved iron accounted for only 28% of the dissolved sulfide consumed. This finding agrees with other studies of the reductive dissolution of ferric oxides in which dissolution rates frequently are not detectable at pH 7 (26, 27). 

Pore-water profiles are frequently interpreted according to this concept. For example, White et ah (35) described a conceptual model of biogeo-chemical processes of sediments in an acidic lake (cf. Figure 4). They discussed the numbered points in Figure 4 as follows Diffusion of dissolved oxygen across the sediment-water interface leads to oxidation of ferrous iron and to an enrichment of ferric oxide (point 1). Bacterial reductive dissolution of the ferric oxides in the deeper zones releases ferrous iron (point 2). The decrease in sulfate concentration stems from sulfate reduction, which produces H2S to react with ferrous iron to form mostly pyrite in the zone below the ferric oxide accumulation (point 3). 

Substances in the bulk solution diffuse into the biofilm, where they are consumed (such as oxygen, point 1 in Figure 6) or recycled (such as sulfate through stepwise reoxidation of H2S from sulfate reduction, point 5). Within the biofilm, very steep gradients exist for oxygen or hydrogen sulfide and also for ferrous iron from reductive dissolution of ferric oxides. These gradients result from the coexistence of anaerobic and aerobic metabolisms such as aerobic respiration (point 1), reduction of ferric oxides (point 3), and sulfate... 

Reductive Dissolution. Many substances in nature contain the same metal or metalloid, but under different oxidation states. For example, the metalloid arsenic may exist as arsenite (AsIII, As03) or arsenate (AsIV, As04) in the forms of ferrous-arsenite or ferric-arsenate, respectively. Ferrous-arsenite is more soluble than ferric-arsenate for this reason, one may be interested in studying the kinetics of arsenate reduction to arsenite. Similar chemistry applies to all elements present in soil-water systems with more than one oxidation state (e.g., iron, manganese, selenium, and chromium). 

One of the cathodic dissolution of metal oxides frequently encountered in the corrosion of metallic iron is the reductive dissolution of ferric oxide ... 

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... 

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