Iron oxides oxic-anoxic interface

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). 

The occurrence of anoxic conditions causes cycling of iron and manganese at the oxic-anoxic interface (6-10). In lakes with a significant seasonal cycle, iron and manganese oxides are reduced during anoxia, and Fe(II) and Mn(II) are released into solution. The Fe(II) and Mn(II) species are reoxidized, and Fe(III) and Mn(III,IV) precipitate as oxides during lake overturn, when the reduced species come into contact with oxygen. 

The sulfur budget for the Black Sea has been considered in several papers [23, 24,74-77]. Sulfide sources are sulfide production in sediments, sulfide flux at the sediment/water interface, and sulfide production in the water column. Sulfide sinks are sulfide oxidation at the oxic/anoxic interface and in the basin interior by dissolved oxygen of the modified Mediterranean water and iron sulfide formation in the water column. 

When oxides in soils or sediments dissolve, substances adsorbed to the oxide surfaces will also be released into solution. Thus, for example, phosphate release into sediment pore waters accompanies the reductive dissolution of iron oxides in anoxic sediments. Release of phosphate into the overlying (oxic) water column is limited by phosphate adsorption on freshly precipitated amorphous iron oxides at the oxic-anoxic interface (9, 10). A similarly coupled cycle of phosphate and iron is observed in surface waters where photochemical reductive dissolution of iron oxides results in increased dissolved concentrations of ferrous iron and phosphate during the day (11). 

This bacterial production occurs in the pore fluids of sediments and in stagnant basins (seas, lakes, rivers and fiords). At the interface between anoxic and oxic waters the H2S can be oxidized. This oxidation is frequently coupled to changes in the redox state of metals (1.2) and non-metals (2). Another major interest in the H-jS system comes from an attempt to understand the authigenic production of sulfide minerals as a result of biological or submarine hydrothermal activity and the transformation and disappearance of these minerals due to oxidation (4). For example, hydrothermally produced H2S can react with iron to form pyrite, the overall reaction given by... 

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