Manganese 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. 

Few examples of studies on cycling of trace elements (other than iron and manganese) at oxic-anoxic interfaces are found in the literature (11-17). Trace element cycling in the water column of a eutrophic lake (Figure 1) is affected by a number of processes related to the redox conditions. 

Pimenov NG, Neretin LN (2006) Composition and activities of microbial communities involved in carbon, sulfur, nitrogen and manganese cycling in the oxic/anoxic interface of the Black Sea. In Neretin LN (ed) Past and present water column anoxia. NATO Sciences Series. Springer, Dordrecht, p 501... 

Example 3.2 In oxic waters, Mn2+ is normally oxidized to MnCfys)- Under certain circumstances, Mn3+ is also produced. In order to better understand manganese cycling at the oxic/anoxic interface, thermodynamics offers its predictive power. From this point of view, and assuming standard conditions and concentrations,... 

Fig. 3.5 The cycling of sulfur, manganese and uranium at the oxic-anoxic interface.

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