Redox Conditions in Soils

This topic has a long history of research (Harrison and Aiyer, 1920 Sturgis, 1936 Pearsall and Mortimer, 1939 Shioiri, 1943 De Gee, 1950 Takai, 1952 Ponnamperuma, 1955 Baas-Becking et al., 1960 Jeffrey, 1961 Patrick, 1966 Ponnamperuma, 1972 Yu, 1985 Kyuma, 2003). The following factors result in conditions differing from those in simple aquatic systems  

In this section the redox conditions developing in soils following submergence 

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Mihelcic, J. R., and R. G. Luthy, Degradation of polycyclic aromatic hydrocarbon compounds under various redox conditions in soil-water systems. Appl. Environ. Microbiol., 54, 1182-1887 (1988). 

Redox conditions in soils vary widely over short distances because O2 must diffuse through pores of various sizes and water-filled pores. In aerobic soils the interior of soil aggregates may be partially anaerobic. The change from oxygen sufficiency to deficiency can occur within a few millimeters. In wet soils, only the largest pores are open to gas diffusion from the atmosphere. 

Both iron and manganese change their oxidation status depending on redox conditions in soils and sediments. The ecological and environmental significance of iron and manganese oxidation and reduction reactions can be summarized as follows ... 

A major control for the occurrence of either reaction is the oxygen availability or redox condition in soils. Nitrogen reduction increases with increasing oxygen limitation. Thus, with decreasing aeration, denitrification shifts more and more towards N2 as the final product. Globally, denitrification returns 54-115 Tg N to the atmosphere as NON2O-I-N2. 

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. 

The identities of the solid phases that form remain a mystery. Direct identification is difficult because Fe(II) and Mn(II) solid phases are readily oxidized by O2 and it is therefore necessary to maintain scrupulously anoxic conditions to ensure that the material examined actually represents that in anoxic soil. An alternative is to make indirect assessments through measurements of pe, pH and [Fe +] in solution, but these too are difficult (see section on measurement of redox potential in soil). 

Gambrell, R.P., Taylor, B.A., Reddy, K.S., and Patrick, W.H., Jr. Fate of selected toxic compounds nnder controlled redox potential and pH conditions in soil and sediment-water systems, U.S. EPA Report 600/3-83-018, 1984. 

Constructed wetlands (CWs) can promote removal of PhCs through a number of different mechanisms, including photolysis, plant uptake, microbial degradation and sorption to the soil. The main benefits of horizontal and vertical subsurface flow systems are the existence of aerobic, anaerobic and anoxic redox conditions in proximity to plant rhizomes this provides an ideal environment for reducing... 

The cations in the interlayer space can spontaneously transform to other chemical species they can undergo oxidation or reduction, and thus they can exert an effect on the redox conditions of soils and rocks. The new species can react further, forming precipitates and nanolayers in the interlayer space. In some cases, the basal spacing (d001) remains the same, and the thickness of the interlayer does not change. The transformation processes of interlayer cations will be shown in Section 2.10.1. 

Plants also vary with respect to their uptake of Se and their capacity to accommodate large, potentially toxic concentrations (Rosenfeld and Beath, 1964 Sors et al., 2005). Wetland plants have been reported to volatilize Se (Pilon-Smits et al., 1999 Lin and Terry, 2003 Wu, 2004), and factors influencing volatilization have been reviewed by Terry and Zayed (1994). As stated above, it has been suggested tlrat changes in redox conditions in the rhizosphere of some plants may influence Se speciation in soil. Some species accumulate Se in Se-rich soils, but not to the extent reported for hyperaccumulators of toxic metals such as Cd and Ni (Banuelos et al., 1997 Prasad and Freitas, 2003 Srivastava et al., 2005). There are few recent studies of Se uptake from soils with normal or deficient levels of Se, as would be the probable situation in the case of radio-Se pollution, but the data have been reviewed comprehensively by Coughtrey et al. (1983) and Ihnat (1989). 

Sulfate is reduced to sulfide when the redox potential in soil drops to 0 to — 0.15 V. Both organic and inorganic sulfate are biochemically reduced by airaerobic bacteria (Starkey, 1966). Upon returning to oxidizing conditions, sulfide may be oxidized to H2SO4, producing very acidic conditions. Further... 

The quantitative capability of the Nernst equation to predict the activity of chemical species is valid only under equilibrium conditions. Most of the redox couples are not in equilibrium, except in highly reduced soils steady-state condition may result in pseudoequilibrium conditions. In soils, redox equilibrium is probably never reached because of the continuous addition of electron donors and acceptors. Biological systems add and remove electrons continuously. Thus, redox potential measurements cannot be used to accurately predict the activity of specific reductant and oxidant of the system. 

The importance of these gas conduits in aquatic macrophytes of natural wetland areas has been well documented. Methane flux to atmosphere by vascular transport is related to soil redox conditions in which the plant grows (Figure 16.3). 

Bourg, a. C. M Loch, J. P. G. 1995. Mobilisation of Heavy Metals as Affected by pH and Redox Conditions. In Salomans, W. Forstner, A. U. (eds) Biogeodynamics of Pollutants in Soils and Sediments Risk Assessment of Delayed and Non-Linear Responses. Springer, Berlin, 213-238. 

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