Sediments, oxidation-reduction reactions

Soil and sediment oxidation-reduction reactions play an important role in determining arsenic solubility, mobility, bioavailability, and toxicity. Under natural environmental conditions, arsenate... 

Because chlorite is an anion, sorption of chlorite ions onto suspend particles, sediment, or clay surfaces is expected to be limited under enviromnental conditions. Thus, chlorite ions may be mobile in soils and leach into groundwater. However, chlorite (ions or salts) will undergo oxidation-reduction reactions with components in soils, suspend particles, and sediments (e.g., Fe, Mn ions see Section 6.3.2.2). Thus, oxidation-reduction reactions may reduce the concentration of chlorite ions capable of leaching into groundwater. 

Oxidation/Reduction Reactions. Reactions of chemicals via abiotic oxidation or reduction involve a transfer of electrons and result in a change in oxidation of the state of the product compared to its parent compound. As a general rule, reduction reactions are prevalent in soil sediments, while oxidation reactions are more important in surface waters and in the atmosphere.28... 

After deposition of the BIF, dolomite was deposited in a neutral environment and gradually came down to the present level. The actual picture undoubtedly was complicated by oxidation-reduction reactions and periods of clastic sedimentation, which will be considered in more detail in formulating the general model of the genesis of BIF. 

Table 2 Oxidation/reduction reactions in marine sediments.

H2S (ii) isotope effects associated with cyclic oxidation- reduction reactions and (iii) the mechanism of addition of sulfur to sediments. The last two of these factors, and possibly the first as well, are depth dependent. For this reason it is logical to divide this discussion by diagenetic zone. 

Sulfur exists naturally in several oxidation states, and its participation in oxidation-reduction reactions has important geochemical consequences. For example, when an extremely insoluble material, FeS2, is precipitated from seawater under conditions of bacterial reduction, Fe and S may be sequestered in sediments for periods of hundreds of millions of years. Sulfur can be liberated biologically or volcanically with the release of H2S or SO2 as gases. 

In the following paragraphs, oxidation/reduction reactions in sediments and authigenic mineral formation are discussed. The effects of these reactions on the sedimentary environment are illustrated using profiles of solutes in sedimentary pore waters at a near-shore location. Then, there is a brief discussion of the role that sedimentary chemical processes play in nutrient cycles and the cycling of anthropogenic contaminants. 

Several studies have shown that the more crystalline the Fe(III) and Mn(IV) oxides, the slower the rate of their reduction (reviewed by Lovley, 1991, 2004). In laboratory experiments, Ottow and Klopotek (1969, cited by Lovley, 1987) reported that the reduction capacity of select iron minerals was found to be in the order of FeP04 > Fe(OH)j > FeOOH > Fe203. Amorphous and poorly crystalline forms of Fe(III) and Mn(IV) oxides dominate wetlands that undergo frequent wet and dry cycles. These systems are dynamic, and repeated oxidation-reduction reactions involving iron and manganese will not allow time for stable crystalline forms of Fe(III) and Mn(IV) oxides to form in these systems. Strong relationships are observed between Fe(III) reduction rates and poorly crystallined forms of Fe(III) oxides (as determined by hydroxylamine extraction) in fresh and brackish water sediments (Lovley and Philips, 1987). 

Iron and manganese are widely distributed in wetland soil and sediments. Iron and manganese transformation involves various oxidation-reduction reactions mediated primarily by a range of microorganisms. 

Oxidation-reduction reactions of iron and manganese are also involved in nutrient release in flooded soil and sediments. Fe(III) and Mn(IV) serve as electron acceptors for organic matter decomposition or turnover. Organic nitrogen mineralization results in the release of nutrients such as ammonium nitrogen. Iron reduction is also coupled to phosphorous release in soils dominated by iron redox couples. 

How can oxidation-reduction reactions of iron and manganese influence nutrient release in flooded soil and sediment Under what conditions is phosphorous available ... 

Copper is an essential element, being active in many enzymes and hemocyanin. Copper is an essential nutrient element to animals and plants. However, high Cu accumulation in animals and plants can be toxic. Copper is found in three oxidation states including cupric (+2), cuprous (+1), and elemental Cu (0). Cu+ and Cu + are the most important forms and are involved in oxidation-reduction reactions in soils and sediments (Figure 12.7). Cu+ and Cu + can exist in aqueous systems, although the latter is much more dominant. Copper is widely distributed in nature in its elemental state and in the form of sulfide, arsenite, chloride, and carbonates. The earth s crust on an average contains approximately 50 ppm copper. Soil and sediment contain approximately... 

Chromium can exist in several oxidation states from Cr(0), the metallic form, to Cr(Vl). The most stable oxidation states of chromium in the environment are Cr(lll) and Cr(Vl). Besides the elemental metallic form, which is extensively used in alloys, chromium has three important valence forms. The trivalent chromic (Cr(lll)) and the tetravalent dichromate (Cr(Vl)) are the most important forms in the environmental chemistry of soils and waters. The presence of chromium (Cr(Vl)) is of particular importance because in this oxidation state Cr is water soluble and extremely toxic. The solubility and potential toxicity of chromium that enters wetlands and aquatic systems are governed to a large extent by the oxidation-reduction reactions. In addition to the oxidation status of the chromium ions, a variety of soil/sediment biogeochemical processes such as redox reactions, precipitation, sorption, and complexation to organic ligands can determine the fate of chromium entering a wetland environment. 

Most of the important chemical phenomena associated with water do not occur in solution, but rather through interaction of solutes in water with other phases. For example, the oxidation-reduction reactions catalyzed by bacteria occur in bacterial cells. Many organic hazardous wastes are carried through water as emulsions of very small particles suspended in water. Some hazardous wastes are deposited in sediments in bodies of water, from which they may later enter the water through chemical or physical processes and cause severe pollution effects. 

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