Oxygen in soil

Temperature Water content Oxygen in soil air Toxic substances Ammonia... 

Consider an unsaturated soil (i.e., a soil that contains both air and water in the pores between soil grains). Suppose the concentration of dissolved oxygen in soil water at equilibrium with soil air is 100 /xmol/liter (/xM). Given a Henry s law constant of 26 (dimensionless) for oxygen at 20°C, what is the corresponding oxygen concentration in soil air What is the Henry s law constant in units of atm m3/mol at 20°C ... 

The concentration of oxygen in soil pore waters is generally lower than that in the atmosphere above the soil, so that the net movement is downward, that is, from the atmosphere to soil. In wetlands, this net movement is prevented by the presence of floodwater within the pore space. However, when the water table is below the soil surface, the oxygen movement into soil is similar to uplands. 

What factors wonld snpport the rapid consnmption of oxygen in soil ... 

Nonmineral nutrients Carbon, hydrogen, and oxygen in soil Mineral nutrients Plant nutrients absorbed through roots as solutes in water... 

Besides the above-discussed Ti02 forms, there are several Fe-Ti mixed oxides that deserve attention, included in Table 9.4. The most important is ilmenite, the second-most-abundant Ti mineral in soils it has the ideal formula Fe(II)Ti03 (52.65% TiOj and 47.35% w/w FeO), with a crystal structure similar to hematite (Figure 9.8b), with half of the Fe atoms replaced by Ti. The ideal composition is observed only in the absence of oxygen in soils, ilmenite is found to contain a mix of FeO and Fe203,... 

Graphite has an electron conductivity of about 200 to 700 d cm is relatively cheap, and forms gaseous anodic reaction products. The material is, however, mechanically weak and can only be loaded by low current densities for economical material consumption. Material consumption for graphite anodes initially decreases with increased loading [4, 5] and in soil amounts to about 1 to 1.5 kg A a at current densities of 20 A m (see Fig. 7-1). The consumption of graphite is less in seawater than in fresh water or brackish water because in this case the graphite carbon does not react with oxygen as in Eq. (7-1),... 

The demands on insulating materials in soil and fresh water are relatively low. Anodically evolved oxygen makes the use of aging-resistant insulating materials necessary. These consist of special types of rubber (neoprene) and stabilized plastics of polyethylene, and polyvinylchloride, as well as cast resins such as acrylate, epoxy, polyester resin and many others. 

In addition to its presence as the free element in the atmosphere and dissolved in surface waters, oxygen occurs in combined form both as water, and a constituent of most rocks, minerals, and soils. The estimated abundance of oxygen in the crustal rocks of the earth is 455 000 ppm (i.e. 45.5% by weight) see silicates, p. 347 aluminosilicates, p. 347 carbonates, p. 109 phosphates, p. 475, etc. 

As mentioned earlier, there is an inverse relationship between water volumes and oxygen concentration in soil. As soils dry, conditions become more aerobic and oxygen diffusion rates become higher. The wet-dry or anaerobic-aerobic alternation, either temporal or spatial, leads to higher corrosion rates than would be obtained within a constant environment. Oxygen-concentration-cell formation is enhanced. This same fluctuation in water and air relations also leads to greater variation in biological activity within the soil. 

This is a simplified treatment but it serves to illustrate the electrochemical nature of rusting and the essential parts played by moisture and oxygen. The kinetics of the process are influenced by a number of factors, which will be discussed later. Although the presence of oxygen is usually essential, severe corrosion may occur under anaerobic conditions in the presence of sulphate-reducing bacteria Desulphovibrio desulphuricans) which are present in soils and water. The anodic reaction is the same, i.e. the formation of ferrous ions. The cathodic reaction is complex but it results in the reduction of inorganic sulphates to sulphides and the eventual formation of rust and ferrous sulphide (FeS). 

The corrosion of iron and steel in soil is generally electrochemical in character but the conditions are such that the corrosion products usually remain in contact with the metal. Moreover, the rate of oxygen supply is often low in comparison with that in air or in water. This is probably the main reason... 

There are several environmentally significant mercury species. In the lithosphere, mercury is present primarily in the +II oxidation state as the very insoluble mineral cirmabar (HgS), as a minor constituent in other sulfide ores, bound to the surfaces of other minerals such as oxides, or bound to organic matter. In soil, biological reduction apparently is primarily responsible for the formation of mercury metal, which can then be volatilized. Metallic mercury is also thought to be the primary form emitted in high-temperature industrial processes. The insolubility of cinnabar probably limits the direct mobilization of mercury where this mineral occurs, but oxidation of the sulfide in oxygenated water can allow mercury to become available and participate in other reactions, including bacterial transformations. 

Results from other studies support the rapid degradation of methyl parathion in soils with a high water (i.e., low oxygen) content (Adhya et al. 1981, 1987 Brahmaprakash et al. 1987). Experiments in flooded and nonflooded soils showed that the redox potential affected both the rate of degradation and the transformation products of methyl parathion (Adhya et al. 1981, 1987). Transformation to volatile products was suggested by Brahmaprakash et al. (1987) as the reason that significant amounts of " C from labeled methyl parathion could not be accounted for, especially in flooded soils. 

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