Soil Oxidation-Reduction

Redox reactions in the soil are mostly the result of a cycle started by photosynthesis. One part of the reaction is 

In these reactions O2- is the electron donor, and C4 is the electron acceptor. Equations 4.1 and 4.2 are called half-reactions because they describe only half of the reaction. Although half-reactions appear to imply that free electrons exist, half-reactions imply only that the other half of the reaction is unspecified. The overall reaction of photosynthesis is the sum of the half-reactions  

Respiration (oxidation) in plants and animals and oxidation in soils complete the photosynthetic cycle by utilizing the energy stored in the carbohydrates and organic compounds derived from the carbohydrates, by disposing of organic wastes, and by producing the C02 needed for more photosynthesis by the reaction  

To obtain the energy and complete the reaction, organisms must find an electron acceptor to accept the electrons. If oxygen is available, the half-reaction of aerobic electron acceptance is the reverse of Eq. 4,2  

Equation 4.4 summarizes the many steps of the intricate Krebs or citric acid cycle that organisms utilize to obtain die energy in a useful form. Equation 4.5 also oversimplifies the intricate mechanism of electron acceptance by oxygen in living organisms. 

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The mobility of metals in soil solutions is controlled by several processes (1) desorption or dissolution (rate depends on the solubility of metal-mineral form) (2) diffusion (depends on speciation of metal, soil oxidation/reduction potential, and pH) (3) sorption or precipitation (depends on soil solution concentration and rhi-zosphere effects) and (4) translocation in the plants (depends on plant species, soil solution concentration, and competing ions) (McBride... 

An indicator of soil corrosivity is the value of soil oxidation-reduction (ORP) or redox potential. It is calculated from the potential difference measured with a probe that contains an inert platinirm (Pt) electrode and a saturated calomel electrode (Hg/HgjClj/KCl, +0.241 V versus SHE) as a reference electrode. The value of this soil redox piotential depends on the dissolved oxygen content in the piore water and provides some information on the conditions under which sulfate-reducing bacteria could grow. The use of redox potentials to predict soil corrosivity is presented in Table 4. 

Oxidation-reduction potential Because of the interest in bacterial corrosion under anaerobic conditions, the oxidation-reduction situation in the soil was suggested as an indication of expected corrosion rates. The work of Starkey and Wight , McVey , and others led to the development and testing of the so-called redox probe. The probe with platinum electrodes and copper sulphate reference cells has been described as difficult to clean. Hence, results are difficult to reproduce. At the present time this procedure does not seem adapted to use in field tests. Of more importance is the fact that the data obtained by the redox method simply indicate anaerobic situations in the soil. Such data would be effective in predicting anaerobic corrosion by sulphate-reducing bacteria, but would fail to give any information regarding other types of corrosion. 

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. 

James, B.R. and R.J. Bartlett. 1983b. Behavior of chromium in soils. VI. Interactions between oxidation-reduction and organic complexation. Jour. Environ. Qual. 12 173-176. 

Soil samples were collected along a traverse over the Honerat kimberlite and extended off the kimberlite approximately 75 m SE and 225 m NW from the pipe s centre (Fig. 1). Although it is common practice to collect samples from upper B-horizon soil (Levinson 1980 Bajc 1998 Mann et al. 2005) our samples were collected from C-horizon soil because GAGI samplers were placed at a depth of 60 cm (well below the B horizon). Within 8 hours of sampling, a portion of each soil sample was mixed with Milli-Q water (1 1) to create a slurry. The values of pH and oxidation-reduction potential (ORP) were determined in each slurry. Ammonia acetate leach of the soil samples were performed at Acme Analytical Laboratories, Vancouver, where 20 ml of ammonium acetate was mixed with 1 g soil sample and elements were determined by inductively coupled plasma-mass spectrometry. The GAGI samplers installed at Unknown were placed in piezometers and submerged in water at a depth of approximately 1 m below ground surface. 

The species of components present will also be affected by oxidation-reduction, and pH. For example, iron is primarily in the Fe3+ (oxidized) or the Fe2+ (reduced) state depending on the oxidation-reduction potential of the soil. Speciation, which depends, in part, on the oxygen status of soil, is of environmental concern because some species are more soluble, such as Fe2+, and are thus more biologically available than others. The occurrence of a specific species is related to the chemistry occurring in a soil, which is related to its features. Thus, large features must be taken into consideration when studying soil chemistry and when developing analytical and instrumental methods. 

The color of soil gives an indication of its oxidation-reduction conditions and the amount of OM present. Well-aerated soils will be under oxidizing conditions iron will be in the Fe3+ state, less soluble and thus less available for chemical reaction. Under water-saturated conditions, soil will be under reducing conditions as indicated by increased yellow colorings, gleying, and mottling. Iron will be in the Fe2+ state, which is more soluble and thus more available for chemical reaction. Under these conditions, reduced species such as methane (CH4), hydrogen, (H2), and sulfides will be found. 

The movement of animals through soil and their deposition of organic matter can dramatically affect the soil s structure. As explained in Chapter 2, pushing together soil separates results in the formation of peds, increasing air and water movement through soil and changing the oxidation-reduction... 

In addition to the physical effects previously described, animals can change the soil s biological and chemical reactions. For instance, animal paths become devoid of plants and compacted, thereby changing water infiltration and percolation and oxidation-reduction reactions, particularly when there is continual use of the paths [1-4]. 

In addition to oxidation-reduction reactions occurring under various conditions, increased carbon dioxide will also affect soil pH. Carbon dioxide... 

Bartlett RJ. Oxidation-reduction status of aerobic soils. In Dowdy RH (ed.), Chemistry in the Soil Environment. Madison, WI American Society of Agronomy, Soil Science Society of America 1981, pp. 77-102. 

Most commonly, iron is discussed as being in either the ferrous (Fe2+) or ferric (Fe3+) state. Changes between these two depend on the soil s pH and Eh (where Eh is a measure of the oxidation-reduction potential of soil) as discussed in Chapter 9. Add conditions and low Eh values tend to lead to the production of ferrous ion, while high pH and high Eh values result in the predominance of ferric ion. It should be noted that the ferrous ion is more soluble than the ferric ion and, thus, it will be more available to plants. 

Not everyone has access to a lysimeter, which means that one often needs to obtain the soil solution from a soil sample that has been collected in the field. It is easiest to remove the soil solution when the soil is saturated. Thus, experiments are sometimes carried out using soil suspensions. In these cases, questions regarding oxidation-reduction potential and its effect on the species or compound under investigation come in to play. For example, are reduced species that are observed under these conditions common in the field and do they play a significant role in the chemistry of that particular soil, or are they artifacts of the experimental conditions This question must be both asked and answered for the results to be useful. 

In a manner similar to pH, one can describe the availability or concentration of electrons, abbreviated as Eh, in an environment. This then is the negative log of the electron concentration. As with pH, it is really a measure of the electron activity rather than the concentration and is a measure of the oxidation-reduction potential (often referred to as redox potential) of the soil environment. Aerobic conditions represent electron-losing or oxidizing environments, and anaerobic conditions represent electron-gaining or... 

One easily demonstrated electrical characteristic of moist soil is seen in the production of electricity when two different metals, namely, copper and zinc, are inserted into it. This is not unexpected because any salt-containing solution adsorbed in media, such as paper or cloth, and placed between these same two electrodes will cause a spontaneous reaction that produces electricity. The source of this flow of electrons is an oxidation-reduction reaction, represented as two half-reactions as shown in Figure 9.1 for copper and zinc. 

In soil analysis, pH, specific ion, oxidation-reduction (redox), electrical conductivity (EC) cells, and oxygen electrodes are commonly used. For each of these measurements, a different specific electrode along with a separate or integral reference electrode is needed. In some cases, with extended use or long exposure to soil or soil-water suspensions, electrodes may become polarized. When this happens, erroneous results will be obtained and depolarization will need to be carried out using the electrode manufacturers directions [3],... 

The standard potentials of practically all oxidation and reduction reactions, especially those common in the environment and soil, are known or can easily be determined. Because of the specificity and relative ease of conducting voltammetric measurements, they might seem well suited to soil analysis. There is only one major flaw in the determination of soil constituents by voltammetric analysis and that is that in any soil or soil extract, there is a vast array of different oxidation-reduction reactions possible, and separating them is difficult. Also, it is not possible to start an investigation with the assumption or knowledge that all of the species of interest will be either oxidized or reduced. 

In well-aerated soil, it is expected that all species will be in their highest oxidation states. However, this does not happen for reasons elucidated in previous chapters. In well-aerated soil, both ferrous and ferric iron can exist along with elemental iron.3 Zinc, copper, and especially manganese can apparently exist in a mixture of oxidation states simultaneously in soil. Add to this a multitude of organic species that are also capable of oxidation-reduction reactions and the result is truly a complex voltammetric system [12,13],... 

Zhang TC, Pang H. Applications of microelectrode techniques to measure pH and oxidation-reduction potential in rhizosphere. Soil Environ. Sci. Technol. 1999 33 1293-1299. 

Most oxidation reactions are between specific metal cations or metal oxy-anions and cations. The problem that arises when applying oxidation-reduction reactions to soils is that all soils contain a complex mixture of oxidizable and reducible cations, anions, and organic matter, which means that it is impossible to determine which is being titrated. An exception to this is the oxidation of organic matter where an oxidation-reduction titration is routinely carried out. Organic matter determination will be discussed in Section 10.3. 

An almost infinite variety of chemical reactions is possible among soil, additives, and organic contaminate. However, at the moisture, temperature, and pressure conditions present at most sites, only a few reactions are responsible for most stabilization processes. Aside from such processes as absorption, volatilization, and biodegradation, chemical reactions include processes such as hydrolysis, oxidation, reduction, compound formation, and fixation on an insoluble substrate. 

Bouldin DR. 1966. Speculations on the oxidation-reduction status of the rhizosphere of rice roots in submerged soils. In IAEA Technical Report 655. Vienna IAEA, 128-139. 

De Gee JC. 1950. Preliminary oxidation-reduction potential determination in a Sawah profile near Bogor (Java). In Transactions of the 4th International Congress of Soil Science. Amsterdam International Society of Soil Science, 300-303. 

Patrick WH, Ir. 1966. Apparatus for controlling the oxidation-reduction potential of waterlogged soil. Nature 212 1278-1279. 

Pearsall WH, Mortimer CH. 1939. Oxidation-reduction potentials in waterlogged soils, natural waters and muds. Journal of Ecology 27 483-501. 

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