Iron oxyhydroxides, dissolution ferric oxides

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... 

As indicated in Fig. 1, light may induce the reduction of Fe(III) present as particulate iron oxides and oxyhydroxides (Processes 13 and 14). The resulting Fe(II) species could induce the formation of a mixed valence oxide at the particle surface or, more hkely, be released to solution resulting in dissolution of the solid phase. Whether the ferrous iron remains in solution or oxidises and reprecipitates as a ferric oxyhydroxide will be dependent particularly upon solution pH. If other species are adsorbed to the oxide surface, they may also undergo redox transformation as a result of reduction of the metal centre. Such photo-transformations are described in this section. 

XPS study by Buckley and Woods (1984b) showed that freshly fractured chalcopyrite surfaces exposed to air formed a ferric oxyhydroxide overlayer with an iron-deficient region composed of CuSi. Acid-treated surfaces of fractured chalcopyrite showed an increase in the thickness of the CuS2 layer and the presence of elemental sulfur. Hackl et al. (1995) suggested that dissolution of chalcopyrite is passivated by a thin (< 1 pm) copper-rich surface layer that forms as a result of solid-state changes. The passivating surface layer consists of copper polysulhde, CuS , where n > 2. Hackl et al. (1995) described the dissolution kinetics as a mixed diffusion and chemical reaction whose rate is controlled by the rate at which the copper polysulhde is leached. The oxidation of chalcopyrite in the presence of ferric ions under acidic conditions can be expressed as... 

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