Redox reactions soil conditions

While not stated explicitly, in this discussion so far, it has been assumed that all the systems were well defined, at equilibrium, and at a constant 25°C. None of these conditions occur in soil in the environment. Soil is not a pure system and, often, all the components affecting redox reactions are not known, defined, or understood, and a host of different redox couples are likely to be present. Unless it is possible to take into account all couples present, it is not possible to describe the exact redox conditions in a soil without measuring it. 

Changes in the oxidation state of trace metals can occur depending on the redox condition of the environment. Redox reactions are thus important in influencing the chemical speciation of a number of metals and metalloids, notably Hg, As, Se, Cr, Pu, Co, Pb, Ni, and Cu (Oscarson et al., 1981 Bartlett and James, 1993 Alloway, 1995 Myneni et al., 1997 Huang, 2000 James and Bartlett, 2000 Adriano, 2001 Sparks, 2003). Redox reactions also exert a great influence in the transformation and reactivity of Fe and Mn oxides in soils tliat have an enormous capacity to adsorb metals and metalloids (Huang and Germida, 2002). Furthermore, reduction of sulfate to sulfide in an anerobic environment also affects... 

The importance of pH as a master variable controlling chemical reactions in soils has been stressed in previous chapters. However, soils subjected to fluctuations in water content come under the influence of another master variable the reduction-oxidation (or redox) potential Under conditions of water saturation, the lack of molecular oxygen can result in a sequence of redox reactions that changes the soil pH. In this sense the redox state of the soil exerts control over the pH. The nature of redox reactions will be discussed in this chapter, as these reactions profoundly influence metal ion solubility and the chemical form of ions and molecules dissolved in soil solution. The reader is referred to section 1.2f in Chapter 1 for a review of the basic chemical principles necessary for the understanding of redox reactions. 

In aerobic soils, in fact, the only stable forms of carbon are CO2, HCO3", and CO3" all soil organic matter is potentially susceptible to oxidation by O2. While the persistence of humus in soils for years, even centuries, may seem to belie this statement, the redox reaction moves slowly but inexorably in the direction of equilibrium. The reduced forms of carbon in the soD organic matter provide the energy (and electrons) that drives the engine of chemical reduction under water-saturated conditions. 

The positive value of suggests that the oxidation of hydroquinone by soluble Fe should occur spontaneously xn6sr standard-state conAitxom. However, the standard-state activities of dissolved ions (in this case Fe ", Fe, and H" ) of unity correspond to concentrations that are absurdly high for soil solution (on the order of 1 molar). The standard-state potential, then, has little use in predicting the favorability of the reaction under conditions likely to prevail in soil solutions. It is necessary to use the Nemst equation (see Chapter 7) to calculate the adjusted redox potential, E, for more realistic reaction conditions ... 

Ponnamperuma (1972) reviewed the chemistry of flooded soils. This research has understandably concentrated on the soil conditions of paddy rice agriculture. Only some generalizations are mentioned here. The behavior of C, N, S, Fe, and Mn generally follows that shown in Table 4.3. When rice paddies are drained before harvest, redox potentials rise, Fe2 1 and Mn2+ concentrations decrease, and C, N, and S oxidize. When the soils are flooded again, the reactions reverse. 

The kinetics and mechanisms of chemical reactions in soils have been broadly studied, and comprehensive mathematical models for the particular soil conditions have been presented (Bolt 1979, Huang 2000, Sauve 2001, Schmitt and Sticher 1991, Sparks 1999, Tan 1998). The diversity of ionic species of trace elements and their various affinities to complex inorganic and organic ligands make possible the dissolution of each element over a relatively wide range of pH and Eh. In most soil conditions the effect of pH on the solubility of trace cations is more significant than that of redox potential (Chuang et al. 1996). However, redox potentials of soils also have a crucial impact on the behavior of trace elements (Bartlett 1999). 

As(V)) and arsenite(As(III)) are the most abundant forms of arsenic (Smith et ah, 1998). In soils and water systems, As(V) is dominant under aerobic condition and As(III) under anoxic and anaerobic conditions. But, because the redox reactions between As(V) and As(III) are relatively slow, both oxidation forms are also found in soils regardless of the pH and Eh (Masscheleyn et al., 1992). Reducing soil conditions (Eh < 0 mV) greatly enhances the solubility of arsenic, and the majority of soluble arsenic is present as As(III). 

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. 

Effects of Flooding and Redox Conditions on OClAIC. Reductive dissolution reactions of the sort indicated in Figures 2.6 and 2.7 will affect the amount of a solute in diffusible forms in the soil and the distribution of the diffusible forms between the soil solid and solution. These processes are discussed in detail in Chapter 3. 1 here exemplify their effects by reference to a study of phosphate diffusion in a soil under different water regimes. 

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