Environment oxidation-reduction, conditions

Thick sedimentary pile from middle Miocene to late Pliocene is exposed in the Oga Peninsula, northern Honshu, Japan (Fig. 1.153). Age of the sedimentary rocks has been determined by microfossil data. Thus, the sedimentary rocks in the Oga Peninsula where type localities of Miocene sedimentary rocks in northern Japan are well exposed have been studied to elucidate the paleoenvironmental change of the Japan Sea (Watanabe et al., 1994a,b). Kimura (1998) obtained geochemical features of these rocks (isotopic and chemical compositions) and found that regional tectonics (uplift of Himalayan and Tibetan region) affect paleo-oceanic environment (oxidation-reduction condition, biogenic productivity). However, in their studies, no detailed discussions on the causes for the intensity and periodicity of hydrothermal activity, and temporal relationship between hydrothermal activity, volcanism and tectonics in the Japan Sea area were discussed. They considered only the time range from ca. 14 Ma to ca. 5 Ma. 

Oxidation-reduction conditions are important in the geologic transport and deposihon of uranium. Oxidized forms of uranium (U[VI]) are relatively soluble and can be leached from the rocks to migrate in the environment. When strong reducing conditions are encountered (e.g., presence of carbonaceous materials or H2S), precipitahon of the soluble uranium will occur. 

Gambrell, R. R, R. A. Khalid, and W. H. Patrick, Jr. 1980. Chemical availability of mercury, lead, and zinc in Mobile Bay sediment suspension as affected by pH and oxidation-reduction conditions. Environ. Sci. Technol. 14 431-436. 

The last chapter in this introductory part covers the basic physical chemistry that is required for using the rest of the book. The main ideas of this chapter relate to basic thermodynamics and kinetics. The thermodynamic conditions determine whether a reaction will occur spontaneously, and if so whether the reaction releases energy and how much of the products are produced compared to the amount of reactants once the system reaches thermodynamic equilibrium. Kinetics, on the other hand, determine how fast a reaction occurs if it is thermodynamically favorable. In the natural environment, we have systems for which reactions would be thermodynamically favorable, but the kinetics are so slow that the system remains in a state of perpetual disequilibrium. A good example of one such system is our atmosphere, as is also covered later in Chapter 7. As part of the presentation of thermodynamics, a section on oxidation-reduction (redox) is included in this chapter. This is meant primarily as preparation for Chapter 16, but it is important to keep this material in mind for the rest of the book as well, since redox reactions are responsible for many of the elemental transitions in biogeochemical cycles. 

The principal abiotic processes affecting americium in water is the precipitation and complex formation. In natural waters, americium solubility is limited by the formation of hydroxyl-carbonate (AmOHC03) precipitates. Solubility is unaffected by redox condition. Increased solubility at higher temperatures may be relevant in the environment of radionuclide repositories. In environmental waters, americium occurs in the +3 oxidation state oxidation-reduction reactions are not significant (Toran 1994). 

The remaining processes, although they occur under near-surface and deep-well conditions, are less applicable to the latter. Distinct differences between the two environments, however, can lead to significant differences in how the processes affect a specific hazardous substance. Compared with the near-surface environment, the deep-well environment is characterized by higher temperatures, pressures, and salinity, and lower organic matter content and Eh (oxidation-reduction potential). 

Oxidation-reduction Partly The deep-well environment tends to be more reducing than the near-reduction surface environment, but equally reducing conditions occur in the near-surface. Some adjustments may be required for pressure/temperature effects. 

In a manner similar to pH, one can describe the availability or concentration of electrons, abbreviated as Eh, in an environment. This then is the negative log of the electron concentration. As with pH, it is really a measure of the electron activity rather than the concentration and is a measure of the oxidation-reduction potential (often referred to as redox potential) of the soil environment. Aerobic conditions represent electron-losing or oxidizing environments, and anaerobic conditions represent electron-gaining or... 

The ionization state of the coenzyme is also important. During reduction a charged pyridinium species is created while during oxidation the charge is lost. Thus, more polar environments favor reduction while more hydrophobic conditions favor oxidation [69]. Therefore the apoenzyme environment and model system scaffolds must not only enhance the reactivity of the coenzyme, but must also address these issues of equilibrium and stability. 

TT heoretical equilibrium models can be established for oxidation-reduc-- tion systems in natural waters in much the same way that acid-base or solubility models have been developed and found useful in interpreting observed concentrations of ions and other materials. To relate the theoretical models for redox processes to observed conditions and processes in the aquatic environment is, however, much more difficult and cannot be done as rigorously. Primarily this situation occurs because true oxidation-reduction equilibrium is not observed in any natural aquatic system this is partly because of the extreme slowness of most oxidation-... 

The need for biological mediation of most redox processes encountered in natural waters means that approaches to equilibrium depend strongly on the activities of the biota. Moreover, quite different oxidation-reduction levels may be established within biotic microenvironments than those prevalent in the over-all environment diffusion or dispersion of products from the microenvironment into the macroenvironment may give an erroneous view of redox conditions in the latter. Also, because many redox processes do not couple with one another readily, it is possible to have several different apparent oxidation-reduction levels in the same locale, depending upon the system that is being used as reference. 

Additional difficulties occur with attempts to measure oxidation-reduction potentials electrochemically in aquatic environments. Values obtained depend on the nature and rates of the reactions at the electrode surface and are seldom meaningfully interpretable. Even when suitable conditions for measurement are obtained, the results are significant only for those components behaving reversibly at the electrode surface. 

In natural waters, arsenic may exist as one or more dissolved species, whose chemistry would depend on the chemistry of the waters. Over time, arsenic species dissolved in water may (1) interact with biological organisms and possibly methylate or demethylate (Chapter 4), (2) undergo abiotic or biotic oxidation, reduction, or other reactions, (3) sorb onto solids, often through ion exchange, (4) precipitate, or (5) coprecipitate. This section discusses the dissolution of solid arsenic compounds in water, the chemistry of dissolved arsenic species in aqueous solutions, and how the chemistry of the dissolved species varies with water chemistry and, in particular, pH, redox conditions, and the presence of dissolved sulfides. Discussions also include introductions to sorption, ion exchange, precipitation, and coprecipitation, which have important applications with arsenic in natural environments (Chapters 3 and 6) and water treatment technologies (Chapter 7). 

Iron is the most abundant metal on earth and the commonest electron transfer agents involve iron complexes. Life is thought to have evolved in reductive conditions, in which the dominant form of iron would be as iron sulfide, not iron oxide. The simplest forms of electron transfer agents (found in plants and bacteria) involve iron with thiolate ligands. Some simple electron transfer proteins, such as rubredoxin, contain a single iron centre in an S4 donor environment within a protein (Fig. 10-7). 

More oxidized compounds, such as chlorinated benzenes, are susceptible to biologically mediated reduction in environments under anaerobic conditions, such as in lake and river sediments. It is known that highly polychlorinated biphenyl (PCB) congeners, for example, are susceptible to reductive dehalo-genation, the result of the interaction of syn-trophic microbial communities that are active under methanogenic and sulfate-reducing... 

Many, but not all, proteins are sensitive to alterations in the oxidation-reduction potential of their environment. The effect is caused in part by oxidation of sulfhydryl groups or reduction of disulfide bonds. Not all proteins are equally sensitive to such alterations, but when they are, it is critical to be aware of their sensitivity. The purification or assay of some proteins can be accomplished only by providing reducing conditions (reduced glutathione, free cysteine, dithiothreitol, or mercap-toethanol) in all buffer solutions. 

The redox conditions in the environment will determine which carbon species are thermodynamically stable carbon dioxide under oxidative conditions, and elementary carbon and hydrocarbons under reductive conditions. The oxidation state of organic substances is always between these two thermodynamically stable species. Since these organic substances are thermodynamically not stable, they are driven to transform to carbon dioxide or to elementary carbon/hydrocarbons, which processes, however, are limited by kinetic barriers. 

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