Oxidation of Reduced Soil

When a spadeful of wet, anaerobic soil is brought to the surface and allowed to dry, air enters through drying cracks and the soil tends to become uniformly oxidized and turn a uniform brown. Whereas when oxidation occurs without drying—as, for example, near a root releasing O2 into wet soil—it is far less uniform and reddish-brown ferric oxide deposits form on and near the oxidizing source. The difference depends on the relative rates of movement of O2 into the soil and of ferrous iron and other reductants in the opposite direction, and the rates of reaction. 

The reaction between Fe + and O2 to form insoluble ferric hydroxide can be written 

Equation (4.36) shows that two H+ ions are produced for each mole of Fe + oxidized, i.e. the reaction is accompanied by acidification. In aqueous solution, the rate is found to be very sensitive to pH and at near neutral pH the reaction is accelerated 100-fold if the pH is raised by one unit. The following empirical rate law applies in the pH range 5-8 (Stumm and Lee, 1961 Wehrli, 1990) 

As discussed in Section 4.1, most redox reactions reach equilibrium only slowly if they are not catalysed. Oxidation of Fe + is catalysed by adsorption of Fe + onto Fe(OH)3 formed in the reaction, so Equation (4.36) only holds for the initial rates of reaction. Tamura et al. (1976) studied the oxidation of a solution of Fe + at different controlled pHs near neutral and with varying additions of Fe(OH)3. The reaction obeyed the rate law 

They also found that the oxidation of Fe + when sorbed on a mixture of soil exchange and Fe(OH)3 sites was much slower than on Fe(OH)3 in the absence of soil, described by Equations (4.38) and (4.38a). 

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Reduced Cr(III) may be re-oxidized into Cr(VI) by Mn oxides in soils. Chung et al. (2001) reported that native Cr(III) in subsurface materials of arid and semi-arid regions can be oxidized to soluble chromate by native manganese oxides. If subsurface materials contain a low content of Mn oxides, the re-oxidization of reduced Cr(III) is not significant. 

Water regime, particularly where there is mid-season drainage allowing escape of entrapped CH4 but also oxidation of the soil. The water regime is also affected by soil texture as this affects percolation rates high percolation rates tend to decrease emissions because less reducing conditions are maintained in the soil. Texture may also affect gas entrapment and ebullition. 

Laboratory studies by Ratering and Conrad (1998) and Kliiber and Conrad (1998) demonstrated that short-term drying allows air oxidation of reduced nitrogen, sulfur, and iron species in paddy soils. This oxidized reservoir must be reduced before methanogenesis can resume, resulting in a lag before methane emission is evident. 

The use of microwaves in the acceleration of digestion and colour development in the determination of total Kjeldahl nitrogen in soil has been examined [1]. Visual method based on the colour development of indothymol blue formed between ammonia and thymol has recently been developed for the determination of ammonia-nitrogen in environmental waters [2]. UV photo-oxidation of reduced forms of nitrogen to nitrate by peroxodisulfate can make a basis for the determination of total nitrogen in urban and industrial waste waters [3]. 

Oxidation of reduced forms of S and N can also acidify the soil. Insoluble sulfides in the soil matrix react when exposed to air ... 

The nitrifying bacteria, universally found in aerobic soil and aquatic environments, derive energy from the oxidation of reduced inorganic nitrogen compounds (ammonia and nitrite). As do autotrophic bacteria, they obtain carbon from carbon dioxide in the atmosphere. 

Yields of oats were higher as a result of creatinine and stannous chloride treatments as compared with the manganese sulfate treatment. Soluble manganese determinations established that the beneficial eflFect of the free sulfur and the reducing chemicals was to solubilize the manganese oxides of the soil. 

The redox reactions of iron are involved in several important phenomena occurring in natural waters and water treatment systems. The oxidation of reduced iron minerals, such as pyrite (FeS2(s)), produces acidic waters and the problem of acid mine drainage. The oxidation/reduction of iron in soil and groundwaters determines the iron content of these waters. Redox reactions are intimately involved in the removal of iron from waters. As we have already seen in Section 7-6, the oxidation of metallic iron is an important corrosion reaction. 

FIGURE 6.21 Schematic showing oxidation of reduced compounds in the aerobic soil layer. 

In biological systems, oxygen serves as an electron acceptor during respiration by bacteria and as a reactant in certain biochemical reactions. In addition, oxygen can be involved in chemical oxidation of reduced species in wetland soils. 

Metals complexed with insoluble high-molecular-weight humic compounds are effectively immobilized. There is some evidence that these metals are less effectively immobilized if reduced soils are oxidized. The long-term oxidation of wetland soils will result in the significant release of Cd and Zn (Gambrell and Patrick, 1988 Gambrell et al., 1991). 

The electrokinetic remediation of Hg from contaminated soils is notoriously very difficult due to its low solubility, as stated in the previous chapter. Moreover, the electrokinetics of Hg mixed with heavy metals has not been extensively studied. The most efficient removal of Hg in soils was conducted by the oxidation of reduced insoluble Hg(l) to Hg(II) using I2 (Cox, Shoesmith, and Ghosh, 1996). Here, an anionic complex is formed, where Hg(II) ions are available to migrate through the soil toward the anode. In a recent investigation of the decontamination of mixed heavy metals from contaminated field soils, only Hg was observed to have a different removal property from more than the 10 other metal contaminants (Reddy and Ala, 2005). The system where EDTA solution was applied as the electrolyte was... 

The concern over ecological consequences of the atmospheric input of oxidized and reduced nitrogen centres on soil acidification by the oxidized nitrogen directly, and by the reduced nitrogen following its transformation in the soil into NO/ or due to its uptake by vegetation (Figure 5). ... 

Most phytoactive compounds do not persist in soil in a free and active form for very long, yet they have been plausibly implicated, for example, in a mechanism of infection or nutrient acquisition therefore some suitable explanation must be found. The right set of circumstances was invoked by Uren and Reisenauer (17) to explain how labile reducing agents may be protected physically from Oi and be directed toward insoluble oxides of Mn. The right set of circumstances may have relevance in other situations, and some po.ssibilities are discussed later in this chapter. 

Contact reduction was proposed to explain how plants obtain Mn from soils of neutral and alkaline pH (91). The evidence that sterile sunflower roots could directly reduce in.soluble reactive oxides of Mn strengthened the theory (92). Nevertheless, the idea of the right set of circumstances to explain how labile... 

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