Organic matter manganese transformations

Figure 6.5 Electron transfer reactions for sediments (Ruddy, 1993). CHjO.N.P (organic matter) is transformed by bacterial decomposition reactions to bicarbonate in the pore-waters. This is the primary source of electrons (and therefore energy) for the remainder of the sediment redox chemistry. Most of the primary flux of electrons may pass through the sulphur, iron and manganese cycles, but will eventually react with oxygen. Only a small part of the total electron flux will ultimately be buried as reduced minerals.

Soil pH affects the transformation of Cr between Cr(III) and Cr(VI) in soils. Since Cr(VI) has greater bioavailability and mobility in soils than Cr(III), which is strongly bound by soil solid matrix (Han and Banin, 1997). Cr(III) can be oxidized by soil manganese oxides into Cr(VI), while Cr(VI) can be reduced by organic matter, Fe(II) and microorganisms in soils. Reduction of Cr(VI) has been found to occur much slower in alkaline soils compared to acid soils (Cary et al., 1997). 

Mandal L.N., Mitra R.R. Transformation of iron and manganese in rice soils under different moisture regimes and organic matter applications. Pland Soil 1982 69 45-56. 

By bacterial disproportionation H S and are produced concurrently without participation of an external electron acceptor or donor (Bak and Pfennig 1987 Thamdrup et al. 1993). The biogeo-chemical transformations of sulfur in marine sediments are closely coupled to the cycles of iron and manganese. Sulfate, iron oxides, and manganese oxides all serve as electron acceptors in the respiratory degradation of organic matter. As there are also non-enzymatic reactions between iron, manganese and H S within the sediment, the quantification of dissimilatoiy, heterotrophic Fe and Mn reduction is particularly difficult. 

The remediation of chromium-contaminated sites requires knowledge of the processes that control migration and transformation of chromium. Chromium(VI) can be reduced to chromium(III) in the presence of ferrous iron, reduced sulfur compounds, or organic matter in soil. However, chromiu-m(III) also can be oxidized by manganese dioxide, a common mineral found in many soils (Bartlett 1991 Palmer and Wittbrodt 1991 Pandey etal. 2003). Usually, Part of any chromium(VI) added to a soil or sediment will be reduced very rapidly, especially under acid conditions. On the other hand, excess chromium(VI) may persist for years in soils or lagoons without reduction (Bolan et al. 2003). The addition of organic amendments such as manure enhanced the rate of reduction of chromium(VI) to chromium(III) in soils low in organic matter (Bolan et al. 2003). 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

The second pathway for the transformation products, cis-DCE and vinyl chloride (VC) is an anaerobic oxidation to carbon dioxide. This may occur with a variety of electron acceptors including oxygen, iron (III), and manganese (IV), or native organic matter. 

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