Soil redox potentials

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). 

Uchida, S Sato,T. Okuwaki, A. (1993) Synthesis of monodispersed micaceous iron oxide by the oxidation of iron with oxygen under hydrothermal conditions. J. Chem. Techn. Biotechn. 57 221-227 Ugwuegbu, B.J. Prasher, S.O. Ahmad, D. Dutilleul, P. (2001) Bioremediation of residual fertilizer nitrate II. Soil redox potential and soluble iron as indicators of soil health during treatment. J. Environ. Qual. 30 Ills... 

Soil redox potential is also critical in controlling elemental mobility. Some elements are much more soluble and mobile in one oxidation state than another (examples include Cr, Mn, Se, and others). The elements classified as chalcophiles (e.g., Hg, Cu, Pb, Cd, Zn, As, Se) form insoluble sulfide minerals in reducing environments where sulfide (S ) is generated from sulfate reduction (see Chapters 4 and 7). Mobility for chalcophiles is then extremely low unless oxidizing conditions are restored in the soil. Those elements that, in the sulfide form, have the very lowest solubility products (notably mercury, copper, lead, and cadmium) are the most Ukely to become highly immobile and unavailable in reduced soils. ... 

Mn, despite being the most weakly complexing transition metal, bonds with oiganic matter, oxides, and silicates and its solubility decreases. Small changes in the soil redox potential or pH can shift the Mn —Mn oxide reaction. Low pH or low Eft (see Chapter 7) favors the reduction of insoluble Mn oxides and an increased sol-uDility of Mn. As a result, Mn solubility within any particular soil can fluctuate tremendously over time, sometimes ranging from deficient to toxic levels. 

Selenium, a chalcophile, tends to be associated with sulfide minerals in rocks. Weathering processes in the soil oxidize these very insoluble reduced forms, including elemental Se (Se ), the selenides (Se ), and selenium sulfides, to the more soluble sel-enites (SeO ) and selenates (SeC ). With numerous oxidation states possible for Se in soils, redox potential is a critical factor in Se behavior. 

Farrell, R.E., Swerhone, G.D.W. van Kessel, C. (1991) Construction and evaluation of a reference electrode assembly for use in monitoring in situ soil redox potentials. Commun. In Soil Sci. Plant Anal., 22, 1059-1068. 

Under aerobic conditions the redox potential deviates widely from the potentials of soil redox couples. In anaerobic soils, redox potentials may be more quantitatively related to ion activities. The Fe24" and perhaps Mn2+ concentrations are high and tend to dominate the redox potential. The range of redox potentials that have been measured in soils is shown in Fig. 4.7. The envelope around those data was considered by the investigators to be the extreme limits of likely redox potentials and pH values in soils and natural waters. Redox potentials can closely approach the H+-H2 potential, because it is nearly reversible at the platinum electrode. 

FIGURE 4.10 Influence of electron donor (soil organic matter) on soil redox potential. 

In controlled laboratory mesocosms, Spartina patens, a dominant brackish marsh species found along the U.S. Gulf coast, and rice (0. sativd) showed a decrease in net photosynthesis in response to reduced soil redox potentials. Net photosynthesis decreased when soil redox potential or Eh was below -100 mV (Kludze and DeLaune, 1995b). A similar reduction in photosynthetic rates was observed in 0. sativa with increase in intensity of reduction (Figure 7.31). However, wetland plants... 

FIGURE 7.32 Root elongation of Spartina patens in response to changing soil redox potential (Eh), (a) Response to high Eh (aerobic) followed by low Eh (anaerobic) conditions, (b) Reverse procedure. (Modified from Pezeshki and DeLaune, 1990.)... 

FIGURE 7.33 Recovery of root elongation in Spartina patens after an increase in soil redox potential (Eh) to +500 mV. The increase in Eh followed anaerobic conditions in which soil Eh was reduced for 5 days to a range of -180 to -200 mV (closed circles), -100 to 150 mV (x s), and 0 to -100 mV (open circles). (Erom Pezeshki and DeLaune, 1990.)... 

FIGURE 7.36 Relationship of soil redox potential to total uptake (whole plant) and leaf uptake in cherrybark oak and overcup oak. (From DeLaune et al., 1998.)... 

Intensity of soil reduction promotes increase in root porosity and ROL from root to rhizosphere. There is an increasingly higher rate of oxygen loss as soil redox potential becomes more reduced. 

FIGURE 8.19 Mineralization of organic nitrogen as influenced by soil redox potential. (Redrawn from McLatchy and Reddy, 1998.)... 

FIGURE 11.8 Effect of soil redox potential on sulfate reduction or sulfide production. (From Connell and Patrick, 1968.)... 

Dissolved oxygen can oxidize Cr(III) to Cr(VI) however, the oxidation is slow at normal temperature in aqueous environments. Due to the slow oxidation, Cr(lll) is involved in faster sorption and precipitation reactions. Similar results were obtained by Masscheleyn et al. (1992) investigating Cr(III) oxidation as affected by the soil redox potential. In this reported soil study, Cr(III) was... 

Speciation and solubility of chromium in wetlands and aquatic systems is governed by the competition among chromium oxidation states, adsorption/desorption mechanism, and soil/sediment redox-pH conditions. Chromium (VI) is reduced to chromium (HI) at approximately +350 mV in soils and sediment. Reduced Cr(III) can be rapidly oxidized to the tetravalent chromate and dichromate forms by manganese compounds. Cr(III) is much less soluble in natural system than the hexavalent form and has a much lower toxicity. Chromium is less likely to be a problem in wetlands than in nonwetlands because the reducing conditions cause its reduction or conversion to the more insoluble Cr(III) form. This is depicted in Figure 12.15, which shows changes in water-soluble chromium as affected by the soil redox potential. 

Soil redox potential (Eh) and the pH parameters are closely related. Production of carbon dioxide, an end product of the reduction of oxygen, has considerable influence on the soil s pH. When a reducing wetland soil system becomes oxidized, its pH may decrease drastically due to the oxidation of iron to Fe(lll) and the subsequent hydrolysis of the iron or the oxidation of sulfite to sulfate, which is accompanied by the release of protons. Lowering of the Eh of the soil due to flooding will result in a rise of pH, because many reduction reactions (such as the reduction of sulfate to sulfide, Ee to Fe, and Mn + to Mn +) involve the uptake of protons or the release of hydroxyls. 

FIGURE 16.3 Rates of CH4 emission from rice plants as a function of soil redox potential after 50 days of soil incubation. (Data from Kludze et al., 1993.)... 

DeLaune, R. D., S. R. Pezeshki, and C. W. Lindau. 1998. Influence of soil redox potential on nitrogen uptake and growth of wetland oak seedlings. J. Plant Nutr. 21 757-768. 

Chiu et al. (53) reported that lowering of soil redox potential increased the ratio of As(III) and promoted arsenic methylation. Methylation of arsenic compounds by yeast and bacteria under oxic conditions plays a significant role, whereas methanogenic bacteria are important under anoxic conditions in releasing volatile arsenic from the soil to the atmosphere (39,41). Woolson and Kearney... 

Soil resistance is more dmn 2000 ohm.cm and soil redox potential is more than 400 mV (0.4) ... 

TABLE 4—Soil redox potentials as an indicator for soil corrosivity. 

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