Redox Potentials in Soils

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). 

Radius ratio of Al3+ and Si4, 118 Rates of reactions, 274 Redox potential in soils, 258 Redox reactions, 229-231 pe-pH diagrams, 245-251... 

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... 

Redox potential measurements have been widely used to characterize wetland soils and sediments. The value of these measurements depends on their interpretation with due recognition to its theoretical and practical limitations. Redox potentials in soils are measured in (i) soil pore water, (ii) soil slurry,... 

Redox potentials in soils are affected by a number of factors ... 

Because it is difficult to make a precise measurement of redox potential in soil, especially in well-aerated soil, reliable data on redox potential in the rhizosphere in this study are lacking. The results presented in Fig. 5 may be inaccurate however, a trend of change toward negative Eh in the rhizosphere soil is inferred. Rappart et al. (1987) indicated that greater amounts of exchangeable copper have been detected in soils at low rather than high redox potential. It is assumed that in the maize rhizosphere in this study, the reduced redox potential was favorable to metal mobilization. 

Typical range of pH and redox potentials in soils, sediments, and dredged materials... 

Table 2.21 Redox potentials of soils in relation to corrosiveness...

These factors rather constrain the nsefnlness of Uh measnrements in soil solutions. Inferences about the thermodynamics of redox processes in soils that rely heavily on measurements of redox potential shonld be treated with caution. Nonetheless soil h measnrements provide a ready measnre of redox status, for example in experiments in which constant and pH are reqnired (Patrick et al, 1973). 

Soil Boesten et al. (1992) investigated the transformation of [ C]l,2-dichloropropane under laboratory conditions of three subsoils collected from the Netherlands (Wassenaar low-humic sand, Kibbelveen peat, Noord-Sleen humic sand podsoil). The groundwater saturated soils were incubated in the dark at 9.5-10.5 °C. In the Wassenaar soil, no transformation of 1,2-dichloropropane was observed after 156 d of incubation. After 608 and 712 d, however, >90% degraded to nonhalogenated volatile compounds, which were detected in the headspace above the soil. These investigators postulated that these compounds can be propylene and propane in a ratio of 8 1. Degradation of 1,2-dichloropropane in the Kibbelveen peat and Noord-Sleen humic sand podsoil was not observed, possibly because the soil redox potentials in both soils (50-180 and 650-670 mV, respectively) were higher than the redox potential in the Wassenaar soil (10-20 mV). 

In water logged soils radial oxygen loss from the root raises the redox potential in the rhizosphere as a consequence of which iron oxide plaques are seen to develop on root surfaces. Bacha and Hossner (1977) removed the coatings on rice roots growing under anaerobic conditions. The coatings were composed primarily of the iron oxide mineral lepidocrocite (y-FeOOH) as the only crystalline component. St-Cyr and Crowder (1990) studied the iron oxide plaque on roots of Phragmites and detected both Fe and Mn. The Fe Mn ratio of the plaque resembled the ratio of Fe Mn in substrate carbonates. The plaque material also contained Cu. 

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. 

The E% values for a number of indicators useful in measuring redox potentials of soil solutions are listed in Table 7.2. The EX value (adjusted for pH) represents the approximate solution Ef, range for which that indicator is diagnostic. Outside of its range, any particular indicator is fully in the reduced or oxidized state for the Q— HQ indicator, this would mean that the last term in equation 7.13 could not be quantified. In that event, the solution Eh could not be measiued by this single indicator, it would be possible to state only that the Eh is above the E% value (if the indicator is fiilly oxidized) or below the E% value (if the indicator is fully reduced). 

Aomine, S. (1962) A review of research on redox potentials in paddy soils in Japan. Soil Science, 94,6-13. 

Table 4.3 shows, however, that redox potentials often differ greatly from electrode potentials. Ion activities are only qualitatively related to redox potentials, except in rare circumstances. One reason is that the Nemst equation applies only to equilibrium. Redox reactions in soils are noiiequilibria, though in some cases for highly reduced soils, a steady state may be reached approximating equilibrium. Then only a few redox couples in the soil affect the platinum electrode and the result may approach a pseudo-equilibrium. 

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). 

Arsenic. More than 300 arsenate and associated minerals have been identified (Escobar-Gonzalez and Monhemius 1988). Inevitably, some of the arsenic contained in these minerals enters any industrial circuit, and concentrations of As in soils and waters can become elevated due to mineral dissolution. The original National Priority List (USA) identified approximately 1000 sites in the United States (USA) that posed environmental health risks (Nriagu 1994 Allen et al. 1995) with arsenic cited as the second most common inorganic constituent after lead (Database 2001). The more common oxidation states of arsenic are III and V, and the predominant form is influenced by pH and redox potential. In aqueous solutions of neutral pH, arsenate is present... 

FIGURE 4.2 Schematic showing the range in redox potential in wetland soils. 

The redox potential of soils is also inflnenced by freqnent additions of organic snbstrates, or soils high in native organic matter. In these soils Eh valnes approach -100 mV within a few days after flooding. For example, the Eh valnes of organic soils decrease rapidly to <-200 mV within a few days after flooding, as compared to mineral soils. From an oxidation-reduction point of view, wetland soils are usnally limited in oxidants (electron acceptors) and contain an nnlimited snpply of reductants, such as organic matter (electron donors). In contrast, drained/npland soils are usually limited in rednctants (electron donors) and contain an unlimited snpply of oxidants (electron acceptors). 

Stable electrode potential reading can be obtained if the electrode used is truly inert and reversible and a redox couple is established at the platinum surface. Rapid equilibration can be obtained in a soil system containing one redox couple at a relatively high concentration. A platinum electrode responds to all redox couples present in the soil, and provides an average mixed potential, which may not thermodynamically represent any one redox couple. In soils and sediments, redox equilib-rinm can never be reached due to continuous addition of electron donors and acceptors. 

FIGURE 4.9 Changes in redox potential of soil as affected by aerobic and anaerobic conditions. (Redrawn... 

---