Sediments redox potentials

In addition to effects on the concentration of anions, the redox potential can affect the oxidation state and solubility of the metal ion directly. The most important examples of this are the dissolution of iron and manganese under reducing conditions. The oxidized forms of these elements (Fe(III) and Mn(IV)) form very insoluble oxides and hydroxides, while the reduced forms (Fe(II) and Mn(II)) are orders of magnitude more soluble (in the absence of S( — II)). The oxidation or reduction of the metals, which can occur fairly rapidly at oxic-anoxic interfaces, has an important "domino" effect on the distribution of many other metals in the system due to the importance of iron and manganese oxides in adsorption reactions. In an interesting example of this, it has been suggested that arsenate accumulates in the upper, oxidized layers of some sediments by diffusion of As(III), Fe(II), and Mn(II) from the deeper, reduced zones. In the aerobic zone, the cations are oxidized by oxygen, and precipitate. The solids can then oxidize, as As(III) to As(V), which is subsequently immobilized by sorption onto other Fe or Mn oxyhydroxide particles (Takamatsu et al, 1985). 

Tolley MD, DeLaune RD, Patrick WH. The effect of sediment redox potential and soil acidity on nitrogen uptake, anaerobic root respiration, and growth of rice (Oryza saliva). Plant Soil. 1986 93 323-331. 

In Limnodrilus sp., an oligochaete worm, copper bioavailability from surhcial freshwater sediments is associated with the amount of copper present in the manganese oxide fraction of the sediment. The redox potential and pH in the gut of Limnodrilus allows the dissolution of the manganese oxide coating, making copper and other metals available for uptake (Diks and Allen 1983). 

Aquatic plants can sequester As from soils, sediments and directly from water. Temperature, pH, redox potential and nutrient availability affect this sequestration (Robinson et al. 2006), but aquatic plants can control the local conditions. Arsenic is adsorbed to the surface of plant roots via physiochemical reactions. A positive correlation between As and Fe concentrations is consistent with As being incorporated into HFO on the surface of plants. Plant roots at NBM generally have >1000 mg/kg dw As. Plant roots contain 4-5 orders of magnitude more As than surface water or sediments at the same location. 

The abiotic stress affecting microbial activity and growth in an interfacial microenvironment include factors such as light, moisture, temperature, pH, soil/sediment grain size, carbon/nitrogen content, and redox potential [40-43, 46,47,49-51,56-58]. 

Gambrell, R.P., Taylor, B.A., Reddy, K.S., and Patrick, W.H., Jr. Fate of selected toxic compounds nnder controlled redox potential and pH conditions in soil and sediment-water systems, U.S. EPA Report 600/3-83-018, 1984. 

Sorption of pharmaceuticals onto the surface of particulate matter or their distribution between two phases (water and either sludge, sediment or soil) depends on many factors, the most important being liquid phase pH and redox potential, the stereochemical structure and chemical nature of both the pharmaceutical compound and the sorbent, the lipophilicity of the sorbed molecules (excellent sorption at log Kov > 4, low sorption at log < 2.4), the sludge-water distribution coefficient Kd Kd > 2 L g SS good sorption, < 0.3 L g SS low sorption), the extent of neutral and ioiuc species present in the wastewater and the characteristics of the suspended particles. Moreover, the presence of humic and fulvic substances may alter the surface properties of the sludge, as well as the number of sites available for sorption and reactions, thereby enhancing or suppressing sorption of PhCs [38, 55, 61]. 

In most natural water, phosphine is very unstable and oxidizes even under anoxic conditions. Depending upon the redox potential of water, the oxidation products are diphosphine (P2H4), phosphorus, hypophosphorus acid, phosphorus acid, and phosphoric acid (Kumar et al. 1985). Based on soil studies (Berck and Gunther 1970 Hilton and Robison 1972), small amounts of phosphine may also be adsorbed (reversible sorption) or chemisorbed (irreversible sorption) to suspended solid and sediments in water. However, based on the estimated Henry s law constant (H) of 0.09 atm-m3/mol (see Table 3-3) and the expected volatility associated with various ranges of H, volatilization is expected to be the most important loss process for phosphine in water. 

Figure 10.9 Correlation between concentrations (nmol cm-3) of NH4+ (triangles), N03- (squares), and N2O (circles) and redox potential (mV) in Rhizophora mangle sediments. (Modified from Bauza et al., 2002.)...

Redox potential discontinuity layer a distinct layer in sediments associated with distinct coloration, indicative of difference between oxic and suboxic conditions. 

As introduced above, the interaction between the hydrosphere and the lithosphere frequently results in mass exchange. In this way rocks get dissolved, sediments build-up, stalactites and stalagmites form, materials get transported, and so on. The pH of water and of sediments determines the mobility and solubility of different elements, which may in turn modify the redox potential of the aqueous medium. For example, aluminum, calcium, magnesium, iron, manganese, and other metals become more soluble at low pH if pH increases, their solubility decreases and precipitation occurs. 

ZoBell, C. E. Studies on redox potential of marine sediments. Bull. Am. Assoc. Petrol. 

Experimental. An uncontaminated sediment sample was collected from the Menominee River. The sediment had no distinctive odor, appeared to be a mixture of sand and clay, and was anaerobic (i.e., the pore waters had a redox potential of approximately -115 mV). The sample was stored in a plastic bag at 4°C until use. 

Thus in highly reducing conditions iron can migrate in a wide pH range (from 0 to 6) and precipitates as sediment in the form of oxides and hydroxides only in neutral environments. The acidity of the environment, as a natural geochemical barrier governing the precipitation of iron, is appreciably reduced. Variation in the redox potential as a result of the overall evolution of the atmosphere, hydrosphere, and biosphere plays a large role. 

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