Anoxic condition sediments

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

Sulfides and disulfides can be produced by bacterial reactions in the marine environment. 2-Dimeth-ylthiopropionic acid is produced by algae and by the marsh grass Spartina alternifolia, and may then be metabolized in sediment slurries under anoxic conditions to dimethyl sulfide (Kiene and Taylor 1988), and by aerobic bacteria to methyl sulfide (Taylor and Gilchrist 1991). Further details are given in Chapter 11, Part 2. Methyl sulfide can also be produced by biological methylation of sulfide itself (HS ). Carbon radicals are not the initial atmospheric products from organic sulfides and disulfides, and the reactions also provide an example in which the rates of reaction with nitrate... 

In aquatic environments, Spear (1981) spotlights three research needs (1) development of analytical procedures for measurement of individual dissolved zinc species, notably the aquo ion and zinc chloride, and for nondissolved species that occur in natural waters (2) separation of natural from anthropogenic influences of sediment-water interactions on flux rates, with emphasis on anoxic conditions, the role of microorganisms, and the stability of organozinc complexes and (3) establishment of toxicity thresholds for aquatic organisms based on bioaccumulation and survival to determine the critical dose and the critical dose rate, with emphasis on aquatic communities inhabiting locales where zinc is deposited in sediments. These research needs are still valid. 

Dreier et al. [44] determined sterols in lacustrine sediments. Samples of wet lacustrine sediments were heated under anoxic conditions at 150, 175, 200 and 250°C for five days at 175°C for five days with influx of potassium hydroxide and methanol to remove sterols and at 175°C for 12, 18, 24 and 48h, after which extraction was performed. Heating the sediment increased the amounts of extractable sterols provided that the temperature did not exceed 200°C, because degradation became rapid above that temperature. The behaviour of sterol ketones was similar, but the temperature limit was slightly higher. The various levels of the sterols extracted are tabulated 4-methylsterols had a high stability towards thermal degradation under the conditions used. 

A particularly important consequence of bioirrigation and bioturbation is the introduction of relatively 02-rich bottom water into the sediments. This enhancement in O2 supply is analogous to the aeration of soil by earthworms. Bioturbation can occur as deeply as 1 m below the sediment surface, but is most intense in the top 10 cm. The depth of O2 penetration is also strongly influenced by the flux of sedimenting POM. High accumulation rates of organic-rich particles can fuel bacterially mediated aerobic respiration supporting rates of O2 removal that exceed the benthic animals abilities to reaerate the sediments. In this case, anoxic conditions result. Since animals require O2, bioturbation does not occur in anoxic sediments. Thus, the effects of bioturbation are limited to the oxic portion of the sediments. 

In coastal sediments where organic carbon concentrations are high, the redox boundary is at or near the sediment-water interfece. Under these conditions, denitrification acts as a sink for nitrate. In some settings, the rate of sedimentary denitrification is fast enough to drive a diffusive flux of nitrate from the bottom waters into the sediments. Remineralization of organic matter imder suboxic and anoxic conditions releases... 

Biological. 1,1-Dichloroethane showed significant degradation with gradual adaptation in a static-culture flask-screening test (settled domestic wastewater inoculum) conducted at 25 °C. At concentrations of 5 and 10 mg/L, percent losses after 4 wk of incubation were 91 and 83, respectively. At a substrate concentration of 5 mg/L, 19% was lost due to volatilization after 10 d (Tabak et ah, 1981). Under anoxic conditions, indigenous microbes in uncontaminated sediments produced vinyl chloride (Barrio-Lage et al, 1986). 

Soil In a methanogenic aquifer material, 1,1-dichloroethylene biodegraded to vinyl chloride (Wilson et al., 1986). Under anoxic conditions, indigenous microbes in uncontaminated sediments degraded 1,1-dichloroethylene to vinyl chloride (Barrio-Lage et al., 1986). 

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. 

The retention of zinc in the lake sediments appears to be efficient under botlToxic and anoxic conditions no indication of a release of zinc from the sediments into the water column was found. In a similar way, the retention of chromium in the sediments appears to be efficient. Under anoxic conditions Cr(III) is formed, which is strongly bound to particles and is thus retained in the sediments.The occurrence of anoxic conditions favors the retention in the sediments of chromium and of zinc, in contrast to the release of manganese and of iron. 

Redox Processes in Sediments Deposited under Anoxic or almost Anoxic Conditions... 

Anoxic or almost anoxic conditions are found in stagnant basins such as the Black Sea or in basins with sill depths in the oxygen minimum layer like the Santa Barbara or San Pedro basins off California (16). Under such conditions anoxic conditions may develop in the water under the sill depth or in the sediment just below the sediment-water interface. 

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