Redox reactions reduced soil oxidation

Redox reactions in soils are affected by a number of parameters, including temperature, pH (see Chapter 7), and microbes. Microbes catalyze many redox reactions in soils and use a variety of compounds as electron acceptors or electron donors. For example, aerobic heterotrophic soil bacteria may metabolize readily available organic carbon using NO3, NOj, N20, Mn-oxides, Fe-oxides and compounds such as arsenate (As04 ) and selenate (Se04 ) as electron acceptors. Similarly, microbes may use reduced compounds or ions as electron donors, for example, NH4, Mn2+, Fe2+, arsenite (AsCXj), and selenite (SeO ). 

Co2+ to Co3+, but also from a direct exchange of Co2+ for Mn2+ produced during the redox reaction. The cobalt in arid soils, as indicated by Han et al. (2002b), mainly occurs in the residual and the Mn oxide (easily reducible oxide) fractions. Furthermore, after water saturation, the Co is transferred mainly from the Mn oxide fraction into the carbonate and exchangeable fraction. This will be discussed in detail in the next chapter. 

The firm thermodynamic status of log KR for reduction half-reactions permits the use of these parameters in the normal way (see Section 1.2 and Special Topic 1) to evaluate equilibrium activities of oxidized and reduced species and to compare the stabilities of reactants and products in redox reactions. As an example of a stability comparison, consider the possible reduction of N(V) to N(0) through the oxidation of C(0) to C(IV) in a soil solution.13 The reduction half-reaction for denitrification is implicit in Eq. 2.20 that for C oxidation is... 

Natural processes involving redox reactions are very frequent. Examples include the oxidation of aqueous ions [like Fe(II) to Fe(III)], oxidation of solids (like pyrite, FeS2 to SO2-), corrosion of metals, production of H2S by sulfate-reducing bacteria, photoredox processes in the atmosphere, water, and soil, etc. Therefore, it is important to understand the principles underlying redox chemistry and to organize them in the form of tables and diagrams such as those discussed below. 

Many adsorbates and/or adsorbents are redox sensitive. The specific adsorption in such systems depends on the redox potential, which is very difficult to measure or control, thus, systematic studies in this direction are rare. On the other hand some practical implications are well known, e.g. the uptake of chromates by soils and sediments in enhanced on addition of Fe(II) salts [27] as an effect of a redox reaction, in which Cr(VI) is reduced to Cr(III). A few examples of redox reactions accompanying sorption processes are reported in the column results". The changes of oxidation state in the sorption process are probably more common than it is apparent from literature reports, but they are often overlooked, namely, analytical methods must be specially tailored to observe these changes. 

The O2—H2O couple is the redox pair controlling reactions in aerated solutions, so that reaeration of anoxic soils drives reduced species (e.g., Fe " ) toward the oxidized state. The range of redox potentials over which Fe ", and NH4 have been found to oxidize and disappear on aeration of a reduced soil are denoted by the open boxes in Figure 7.5. Nitrate reappearance on aeration is also depicted by an open box. The measured redox potentials that follow re-aeration do not directly reflect the 02—H20 equilibrium state but rather the status of redox couples having faster electron exchange rates. Furthermore, while each redox couple would be expected (in theory) to undergo complete conversion to the reduced form (in flooded soils) or to the oxidized form (in re-aerated soils) before the adjacent redox couple on the Eh scale became active, actual behavior in soils is much less ideal. Several redox reactions are typically active simultaneously. This may reflect spatial variability in the aeration (and redox potential) of soil aggregates, caused by slow diffusion processes in micropores. 

In soils, pc values lie in the range of —6 (strongly reduced) to +12 (strongly oxidized). However, these pc values (and values as well) are negatively correlated with pH. Such a correlation is expected from coimderation of the individual redox reactions occurring in soils (see Figure 7.1). 

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. 

Redox reactions describe the coupling of reduction reactions to oxidation reactions in free soil. Electrons are transferred from reductors (e donors or reducing agents) to oxidants (e acceptors or oxidising agents) (James Bartlett 2000). 

An oxidation-reduction (redox) reaction is a chemical reaction in which electrons are transferred completely from one chemical species to another. The chemical that loses electrons is oxidised while the one that gains electrons is reduced. Redox reactions can be applied to soil remediation to achieve a reduction of toxicity or a reduction in solubility. 

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. 

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). 

The reaction of a redox couple that controls the system will directly cause a change in Eh. pH can affect the redox reactions by determining the concentrations of members of the redox couple in the soil solution. Decrease in soil pH will increase the solubility of trivalent iron and of other oxidized transition metal species, but will have a smaller effect on the solubility of the reduced species of these metals. The redox reactivity of a xenobiotic in soil is dependent on the pH. Altering the pH of the soil can affect redox reactions of toxic organics, just as it affects other pH-dependent reactions such as hydrolysis. 

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