Environment oxic-anoxic

Based on the pE value, redox environments are classified as follows (a) pE > 7 indicates an oxic environment, (b) at pE values between 2 and 7, the environment is considered suboxic, and (c) pE < 2 indicates an environment considered anoxic. The occurrence of redox reactions in the subsurface environment is limited by the decomposition and reduction of water ... 

All of these reactions are partially or strongly controlled by bacterial interaction. These reactions develop a general vertical zonation of oxic, anoxic sulfidic, and anoxic methanic environments (13) within the reservoir sediment (Figure 7) that migrates with the rise and fall of the reservoir stage. This fluctuation develops a redox pump that mobilizes contaminants with each successive stage cycle. 

Cycles of manganese and iron in the oxic/anoxic environment are similar. Reduced forms of these compounds are dissolved. They diffuse upward where they oxidize and transfer to oxidized particulate forms that sink down and reduce in the hydrogen sulfide zone. 

Van Cappellen P., VioUier E., Roychoudhury A., Clark L., Ingall E., Lowe K., and DiChristina T. (1998) Biogeochemical cycles of manganese and iron at the oxic-anoxic transition of a stratified marine basin (Orca Basin, Gulf of Mexico). Environ. Sci. Technol. 32, 2931-2939. 

Microorganisms are also described by more specific substrate requirements or environmental preferences. For example, organisms that require H2 are hydrogenotrophic (i.e., H2-feeding) and those that thrive in very cold environments are psychrophilic (i.e., cold-loving ). The most important aerobic organisms in oxic-anoxic interfaces are microaerophiles, which thrive at very low O2 concentrations. 

Whereas in Chapter 4 interactions between suspended particles and dissolved constituents have been described, Chapter 5 focusses on the processes taking place within the sediment or near the sediment/water interface. These processes depend on a large extent on the position of the oxic-anoxic interface. This interface (redoxcline) is within the sediments of well-mixed waterbodies and in the water column of some stratified lakes and basins. Transport processes and changes in the chemical environment determine the behaviour of contaminants (Salomons et al., 1987). ... 

Transitional oxic-anoxic environment has Eh values of approximately 0.25 to -0.15 V. Within its bormds occurs unstable and quite drastic transition from the oxidation environment to a reducing/anoxic one. Both aerobics and anaerobics may exist there. Microorganisms capable of developing at low concentrations (lower than 0.02 mmole-1 0 are often called... 

Sulfate-reducing bacteria are the most important mercury-methylating agents in aquatic environments, with the most important site of methylation at the oxic-anoxic interface in sediments a similar pattern is documented for wetlands. In sediments, miao-bial methylation of mercury is fastest in the upper profiles where rate of sulfate reduction... 

Jorgensen BB. 1982. Ecology of the bacteria of the sulphur cycle with special reference to anoxic-oxic interface environments. Philos Trans R Soc Lond B Biol Sci 298 543-561. 

Figure 1. Schematic diagram of Fe redox cycling through biological processes. A large number of pathways are involved in dissimilatory Fe(III) reduction, as listed in Table 2. Processes that occur under oxic conditions are placed near the upper part of the diagram, and those that occur under anoxic conditions are placed in the lower part of the diagram. Major lithologic sources of Fe are noted for high and low oxygen environments.

Black shales that are formed in an anoxic environment such as the Black Sea have a Mo isotope composition nearly identical to ocean water (Barling et al. 2001 Arnold et al. 2004 Nagler et al. 2005). Organic carbon rich sediments formed in suboxic environments have variable Mo/ Mo ratios intermediate between those of ocean water and oxic sediments (Siebert et al. 2003). Thus Mo isotope values in ancient black shales can be used as a paleo-oceanographic proxy of the oxidation state of the ocean, as for example has been discussed by Arnold et al. (2004) for the Proterozoic. Figure 2.25 summarizes natural Mo isotope variations. 

Figure 3.9 Degradation pathway for the analgesic ibuprofen under anoxic and oxic environments.

Uneven Electron Density Due to Heteroatoms Oxic or Anoxic Environment... 

The solubility of Hg(II) is controlled by chemical speciation in natural waters, and the availability of ligands for complexation shifts dramatically under varying redox conditions (40). Speciation of dissolved Hg(II) in anoxic environments, such as sediments or the hypolimnion, should be strongly influenced by reactions with reduced sulfur (40, 41), whereas organic complexation is potentially important under oxic conditions (42, 43). 

Mobilization of contaminants in Milltown Reservoir can be explained by a model in which groundwater composition is controlled by successive diage-netic reactions during the transition to and from oxic and anoxic environments as the reservoir stage changes (12). Several important reactions govern the mobility of contaminants in this system ... 

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