Soils under reduced
conditions have a high Eh value, and oxidized soils have a low Eh value. Reduced
conditions often occur in water-logged soils while aerated soils are under oxidizing
conditions. Oxidation and reduction (redox) reactions occur simultaneously in the soil-water system. A redox reaction can be considered a pair of coupled half-reactions, namely a half-reaction of oxidation and a half-reaction of reduction. Both half reactions involve the transfer of electrons between the chemical species in the system. The half-reaction of oxidation results in a net loss of electrons from one species, and the half-reaction of reduction a net gain of the same number of electrons by another species. The species losing electrons in the half-reaction of oxidation is the reducing agent since it causes reduction of the other species. Similarly, the species gaining electrons in the half-reaction of reduction is the oxidizing agent since it causes oxidation of the other species. For example in the two half-reactions:
Zn(s) => Zn2+(aq) + 2e-
Cu2+(aq) + 2e- => Cu(s)
Zn(s) is oxidized and causes reduction of Cu2+(aq) and therefore is the reducing agent. Cu2+(aq) is reduced and causes oxidation of Zn(s) and therefore is the oxidizing agent. It is clear that if these half-reactions were reversed these roles would be reversed.
The readiness with which a species gains electrons in a half-reaction of reduction is measured by the Standard Reduction Potential (Eh°) in volts. The higher the Eh° value the more susceptible the species is to being reduced. A half-reaction of reduction with a given Eh° value can therefore be coupled to another half-reaction of reduction with a lower Eh° value to form a redox pair. The Eh of the redox pair is the difference of the Eh° values of the two half-reactions of reduction. For example, for
Cu2+(aq) + 2e- => Cu(s) , Eh° = +0.34 V
Zn2+(aq) + 2e- => Zn(s) , Eh° = -0.76 V
The Eh of the redox couple is +0.34 V - (-0.76 V) = + 1.10 V. The positive value indicates that this reaction will occur spontaneously.
Elements often exist in more than one oxidation state in soils. The oxidation state of these elements depends on the redox status in the soil or the amount of electrons available in the soil.
Soils under reduced
conditions have a high Eh value, and oxidized soils have a low Eh value. Reduced
conditions often occur in water-logged soils while aerated soils are under oxidizing
conditions.
Fixation of elements in the soil depends on the Eh and pH of the soil. Eh-pH diagrams can provide information on the potential fixation of elements in the soil. From these diagrams one can estimate if soil conditions are conducive to fixation. Caution must be taken when apply these diagrams directly to a soil system. The Eh that is observed in the soil is also influenced by water content of the soil, oxygen concentration in the soil, and soil pH. It may be necessary to create diagrams that directly relate to a specific soil system.
Figure 1: Eh-pH Diagram for Chromium-Water System at Standard State Conditions. Source: Dragun
Redox reactions can be important in altering the mobility and toxicity of inorganic and organic contaminants.
In saturated soils, oxygen is quickly depleted and microbes must utilize the next most favorable electron acceptor. Under these conditions, Mn and Fe-oxides are reduced to Mn2+ and Fe2+. As favorable electron acceptors are depleted, sulfate, SO42- is reduced to sulfide. Pollutants, especially heavy metals will precipitate out with sulfide causing them to no longer be soluble.
Chromium (Cr) can exist in the soil as Cr(III) which is stable and innocuous and Cr(VI) which is mobile and harmful. Mn(III/IV) naturally oxidizes Cr(III) to the toxic Cr(VI). Ferrous iron (Fe(II)), FeS and soil organic matter all have the capability to reduce Cr(VI) to the more stable Cr(III).
Plutonium exists in the +3 and +6 oxidation states. Pu(III) can be oxidized to the toxic and mobile Pu(VI) by Mn-oxides.
Mn-oxides can oxidize As(III) to the less toxic As(V).
Mn-oxides may also catalyze the oxidation of Co2+, Co3+, Cu2+, Ni2+, Ni3+, and Pb2+.
Organic contaminants can be naturally degraded by redox reactions. Mn and Fe-oxides can oxidize aromatic pollutants. The degradation depends on the electron donating power of the functional groups found on the aromatic ring. The greater the electron donating power, the easier they are oxidized.