Oxic condition

However, it has been concluded from sorption and diffusion experiments that plutonium exists largely in the tetravalent state (53) and clearly not as Pu(V), in the intermediate pH-range under oxic conditions and at low carbonate concentration. This would be representative of many groundwaters and also in agreement with the calculated curves of Figure 2. 

There has been considerable interest in the anaerobic metabolism of methane in the large reservoirs that lie beneath the seafloor, since little of this reaches the oxic conditions in the water column. Consortia of archaea that have so far resisted isolation and sulfate-reducing bacteria have been implicated (Orphan et al. 2002) ... 

The previously proposed uptake models were mathematical assumptions and had no physical or chemical basis. Millard and Hedges, on the other hand, considered the chemistry of bone-uranium interactions. With the D-A model, they proposed that U was diffusing into bone as uranyl complexes, and adsorbing to the large surface area presented by the bone mineral hydroxyapatite (Millard and Hedges 1996). Laboratory experiments showed a partition coefficient between uranyl and hydroxyapatite under oxic conditions of 10" -10, demonstrating U uptake in the U state without the need for reduction by protein decay products as proposed by Rae and Ivanovich (1986). 

Fig. 22.8. Energy yields for various anaerobic (top) and aerobic (bottom) metabolisms during mixing of a subsea hydrothermal fluid with seawater, expressed as a function of temperature, per kg of hydrothermal water. Energy yields for acetoclastic methanogenesis and acetotrophic sulfate reduction under oxic conditions are hypothetical, since microbes from these functional groups are strict anaerobes and cannot live in the presence of dioxygen.

Under typical freshwater conditions, at pH 7-9 and in presence of millimolar concentrations of carbonate, most transition metals in solution (Cu(II), Zn(II), Ni(II), Co(II), Cd(II), Fe(TII), etc.) occur predominantly as hydroxo or carbo-nato complexes. For a few metals, chloro complexes may be predominant (Ag(I), Hg(II)), if chloride is in the range 10-4—10-3 mol dm-3 or higher. Alkali and alkali-earth cations occur predominantly as free aquo metal ions [29], At lower pH values, the fraction of free aquo metal ions generally increases. Strong sulfide complexes of several transition metals have recently been shown to occur even under oxic conditions [32,33]. 

Iron is an essential cofactor of numerous enzymes, involved in, for instance, electron transfer and oxygen metabolism. It seems counterintuitive that the fourth most abundant element in the biosphere is in many instances the least bioavailable bioelement and therefore the limiting growth factor. The reason for this lies in the extremely low solubility of ferric iron (Fe3+) the prevailing form of iron under oxic conditions. Iron is precipitated as Fe(OH)3 with a solubility product of 10 39, which limits the aqueous concentration of ferric ion... 

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.

Initial efforts to apply the Mo isotope system have targeted sediments deposited rmder oxic conditions (marine ferromanganese crusts) as well as sediments deposited under redueing conditions (black shales). Results of these studies are summarized below. 

Finally, nodules may not be growing under oxic conditions. The nodules found in oxic sediments may have formed at some earlier time when the redox boundary was closer to the sediment-water interfece. Changes in the position of the redox boundary are a consequence of changes in the flux of POM and bottom-water O2 concentrations. 

The nodules that form at rates on the order of tens of millimeters per million years appear to have been produced primarily by postdepositional remobilization under oxic conditions. These nodules have relatively high copper and nickel contents. The nodules that accrete at the fastest rates (200 mm per million years) appear to have formed primarily via postdepositional remobilization under suboxic conditions. Despite these rapid accretion rates, the suboxic diagenesis -type nodules account for only half of those found in areas where biological productivity is high. The other half appear to have been formed primarily by oxic remobilization. Hydrogenous precipitation appears to play the dominant role in forming only a small percentage of the nodules. 

Landmeyer, J.E., Chapelle, F.H., Herlong, H.H., and Bradley, P.M. Methyl fert-butyl ether biodegradation by indigenous aquifer microorganisms under natural and artificial oxic conditions. Environ. Sci. TechnoL., 35(6) 1118-1126, 2001. 

Methane is produced by bacteria under anaerobic conditions in wet environments such as wetlands, swamps and rice fields. It is also produced in the stomachs of cattle and by termites. Typical anthropogenic sources are from fossil fuels such as coal mining and as a byproduct in the burning of biomass. The latter sources are considerably heavier in C than the former. Recently, Keppler et al. (2006) demonstrated that methane is formed in terrestrial plants under oxic conditions by an unknown mechanism. The size of this methane source is stiU unknown but it might play an important role for the methane cycle. 

A characteristic of the iron oxide system is the variety of possible interconversions between the different phases. Under the appropriate conditions, almost every iron oxide can be converted into at least two others. Under oxic conditions, goethite and hematite are thermodynamically the most stable compounds in this system and are, therefore, the end members of many transformation routes. The transformations which take place between the iron oxides are summarized in Table 14.1. These interconversions have an important role in corrosion processes and in processes occurring in various natural environments including rocks, soils, lakes and biota. In the latter environments, they often modify the availability and environmental impact of adsorbed or occluded elements, for example, heavy metals. Interconversions are also utilized in industry, e.g. in the blast furnace and in pigment production, and in laboratory syntheses. 

Heavy metal cations precipitate readily as hydroxides or carbonates in alkaline media. Dissolved carbonate content will be limited by calcite precipitation or by conversion of hydroxyl AFm to carbonate AFm. Hydroxide ions, on the other hand, are abundant. Here only the solids that may be present under oxic conditions will be discussed. Figure 5a shows the total dissolved heavy metal cation concentrations that would prevail if hydroxide precipitation were to be the dominant solubility-controlling process. Figure 5b shows the solubility of Ca metallate species, as these are likely to act as solubility-controlling phases for oxyanionic species. 

The chlorinated solvents, including dichloromethane, tri- and tetrachloroethene, and 1,1,1-trichloroethane (see Table 2.4), are still among the top groundwater pollutants. Under oxic conditions these compounds are quite persistent and, because they are also quite mobile in the subsurface, they can lead to the contamination of large groundwater areas. As we will discuss in detail in Chapters 14 and 17, in anoxic... 

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). 

The evidence for the reduction of Cr(VI) to Cr(III) is only indirect, because Cr(III) is not detected in solution. Cr(III) has a strong tendency to adsorb to particle surfaces and to precipitate as insoluble (hydr)oxide. Thus, Cr(III) produced within the water column by reduction is expected to bind to particles and to be found in the particulate phase. No evidence for release of Cr from sediments was found. Cr(III) is expected to be retained very strongly in sediments, so the release of Cr(III) under anoxic conditions is unlikely. Under oxic conditions the oxidation of Cr(III) by Mn oxides, for example, and release of Cr(VI) from the sediments is plausible such a mechanism in sediment pore waters is indicated in ref. 84. 

Mn2+ increased severalfold in concert with these changes. As in the water column, there was no significant difference in Mn2+ determined by ESR spectroscopy and total Mn determined by AA spectrophotometry after acidification. Similarly, there was no difference in Mn concentration between filtered and whole-water samples. All the Mn released into the water column was present as soluble-colloidal Mn2+ species. Although surprising when it occurred, the mobilization of Mn2+ under oxygen actually is in accord with the reports of other groups, who have observed accumulation of Mn2+ under oxic conditions. 

Because of the generally oxic conditions in the Indus alluvial aquifers, the mechanisms of arsenic mobilization may differ from those observed in the Ganges basin. Elevated arsenic concentrations in the shallow Quaternary aquifers of the urban Thai Doab area (Muzaffargarh district, Punjab) are thought... 

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