Redox interface

Chemical Structure of Pelagic Redox Interfaces Observation and Modeling... 

Uranium is a trace constituent in most ground waters. In typical aquifers with oxidizing ground water, uranium s concentration in solution appears to be limited by its abundance in source rocks. The uranium in ground water is removed from solution and deposited at a redox interface between sediments with reducing minerals and sediments without. In the resultant roll-front deposit, uranium is concentrated in the sediment, and its ground-water concentration remains low because of the low solubility of the uranium minerals that compose the deposit. Consequently, it is possible to have localized high concentrations of uranium in the earth s crust that are stable. 

Keywords Biogeochemical structure Black Sea Nutrients Redox interface ... 

Quantification of Trace Metal Fluxes across the Redox Interface Caused by Vertical Turbulent Mixing... 

The vertical transport of Cd, Cu, Pb, Zn, and Co across the redox interface is a result of their adsorption to the oxidized species of Mn(I V) and Fe(lll), their adsorption to particles of biogenic and lithogenic origin, and by turbulent mixing and difftision of the dissolved species. 

Internal particulate trace metal fluxes were obtained from an experimental sediment trap flux study (Pohl et al., 2004) (Section 13.4), and internal dissolved trace metal fluxes across the redox interface were considered as estimated above. 

About 25% of Cd, 12% of Cu, and 8% of Zn, which were supplied by rivers and atmosphere, were eliminated by vertical turbulent mixing across the oxic-anoxic interface and subsequent sulfide precipitation. For Pbjiss, the flux at the redox interface was negligible. Comparing the sum of the internal fluxes (dissolved and particulate) in relation to the total... 

The diffusive flux of Fediss across the redox interface has been calculated n.Tpmol/m day (Pohl and Hennings, 2005). How far this dissolved Fe(ll) from deepwater sources crosses the stable halocline and reaches the euphotic zone and contributes to the fertilization of cyanobacteria is unknown. 

Groundwater aquifers in older carbonate and sandstone rocks may have been leached of any reactive organic matter and of Fe(II) and sulfide minerals through time. In such systems, the paucity of reductants may create a redox interface that spans hundreds of meters to several kilometers. For example, Langmuir and Whittemore (1971) found that Eh , gradually drops from 250 mV to -100 mV over a flow distance of about 6 km in the Potomac-Magothy-Raritan aquifer of coastal plain New... 

What is a redox interface and what determines its scale (inches or miles) in a stream- or lake-bottom mud versus a sandy aquifer ... 

The polysulfides, thiosulfate and sulfite, which are generally metastable relative to sulfate, are most abundant near groundwater redox interfaces where they can complex with borderline or soft metal cations (cf. Barnes 1979 Daskalakis and Helz 1992). Many sedimentary and hydrothermal ore deposits are found at redox interfaces where the breakdown and formation of such complexes may have locally caused metal precipitation and remobilization (cf. Granger and Warren 1969 Boulegue and Michard 1979 Barnes 1979). Similar metal-sulfur species mobilization may also occur in the vicinity of some toxic metal-organic waste sites. 

Redox-sensitive elements such as As, Mo, Se, and V, are soluble along with U(VI) in oxidized groundwaters where they occur as oxyanions. However, in the reduced waters at the redox interface of a roll front or other sedimentary U deposit, they are precipitated nearby along with U(IV) in insoluble minerals (cf. Wanty et al. 1987). 

Figure 13.15 Schematic cross-section of an idealized uranium roll-front orebody showing the zonation of elements and primary hydrologic and geochemical features. Oxidized groundwaters flow from left to right. The roll front and associated redox interface moves in the same direction. After Larson (1978).

Vertical distributions of dissolved Ba and total (dissolved+particulate) Pu, Am and Th in Framvaren Fjord all show increased concentrations with depth (Falkner etal., 1993 Roos etal., 1993). Ba cycling was dominated by its uptake into particulate matter associated with productivity in surface waters, followed by its regeneration at depth or in the sediments. Microbiological activity near the redox interface likely promotes the breakdown of settling particulate matter and the release of barite just above the 02/H2S interface (Falkner etal., 1993). Complex formation with dissolved organic carbon (DOC) is believed to be the main cause for the observed behavior of Pu, Am and Th (Roos etal., 1993). The distributions of these elements were not examined within the regions near the 02/H2S interface and the associated microbial layer. 

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