Redox reactions soils

Immobilization and stabilization render contaminants insoluble and prevent leaching of the contaminants from the soil and their movement from the contamination area. The techniques used for immobilization are precipitation, chelation, redox reaction, and polymerization. 

Concentrations of trace elements in soil solution may be controlled by the solubility of certain solid phases via dissolution/(co-)precipitation or by other physicochemical and biological processes such as adsorption-desorption, complexation, and redox reactions. 

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. 

Cr(VI).Other remediation processes for Cr(VI) contaminated soils include H2S injection, aqueous Fe(II) injection, and the use of reduced Fe solids. Aqueous-phase Cr(VI)-Fe(II) redox reactions may be significant if Fe2+ concentrations are in equilibrium with relatively soluble, ferric hydroxide-like phases (Tokunaga et al., 2003). The overall interactions involving microbial activity, organic carbon degradation, Fe2+, and mineral surfaces control the net rates of Cr(VI) reactions in soils. 

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. 

Redox Reactions of Iron and Manganese at the Oxic Anoxic Boundary in Waters and Soils... 

Adsorption may influence precipitation by means other than the processes mentioned above. Davies (Chapter 23) discusses the role of the surface as a catalyst for oxidation of adsorbed Mnz+. Redox reactions may contribute substantially to the formation of manganese oxide coatings on mineral surfaces in soils and sediments. 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

Most redox reactions in vitro reach equilibrium only extremely slowly with half times of the order of months or years, even though they may be highly favoured thermodynamically. This is illustrated by the persistence of N2 in oxic systems even though its oxidation to NOs is strongly favoured (Table 4.1). However, microbes in soil and water are capable of catalysing particular reactions from which they obtain energy for metabolism. The half times of such microbially catalysed reactions are of the order of hours or days. 

In the environment, Fe " oxides may help detoxify pollutants through a range of redox reactions. Chromate (Cr ) is a toxic form of Cr, whereas Cr " is not. Reduction of Cr to Cr " is, thus, a detoxifying process and takes place in soils and sediments... 

The role of transition metal oxide/hydroxide minerals such as Fe and Mn oxides in redox reactions in soils and aqueous sediments is pronounced (Stumm and Morgan, 1980 Oscarson et al., 1981a). These oxides occur widely as suspended particles in surface waters and as coatings on soils and sediments (Taylor and McKenzie, 1966). 

Amacher, M.L., and Baker, D, E. (1982). Redox Reactions Involving Chromium, Plutonium, and Manganese in Soils, DOE/DP/OY515.1. Inst. Res. Land and Water Resour. Pennsylvania State University, University Park. 

In soil solutions the most important chemical elements that undergo redox reactions are C, N, O, S, Mn, and Fe. For contaminated soils the elements As, Se, Cr, Hg, and Pb could be added. Table 2.4 lists reduction half-reactions (most of which are heterogeneous) and their equilibrium constants at 298.15 K under 1 atm pressure for the six principal elements involved in soil redox phenomena. Although the reactions listed in the table are not full redox reactions, their equilibrium constants have thermodynamic significance and may he calculated with the help of Standard-State chemical potentials in the manner... 

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... 

Of course, redox reactions do not occur in isolation but are coupled through complexation reactions to other species. For example, Eq. 2.30 could include terms for nitrate and amine complexes, in addition to those for free nitrate, nitrite, and ammonium ions, if a typical soil solution were under consideration. The calculation of nitrogen speciation then would proceed just as described above. Indeed, redox reactions introduce no new mathematical elements into a speciation computation, any more than would the consideration of, for example, C02 reactions. The only new item brought in is an additional variable, the pE value, which, like the partial pressure of C02(g), must be specified in order to solve mole balance equations for distribution coefficients. 

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