Ferrous iron reduced soil

The remediation of chromium-contaminated sites requires knowledge of the processes that control migration and transformation of chromium. Chromium(VI) can be reduced to chromium(III) in the presence of ferrous iron, reduced sulfur compounds, or organic matter in soil. However, chromiu-m(III) also can be oxidized by manganese dioxide, a common mineral found in many soils (Bartlett 1991 Palmer and Wittbrodt 1991 Pandey etal. 2003). Usually, Part of any chromium(VI) added to a soil or sediment will be reduced very rapidly, especially under acid conditions. On the other hand, excess chromium(VI) may persist for years in soils or lagoons without reduction (Bolan et al. 2003). The addition of organic amendments such as manure enhanced the rate of reduction of chromium(VI) to chromium(III) in soils low in organic matter (Bolan et al. 2003). 

Iron in the ferrous state is more soluble than iron in the ferric state. Indeed, in the ferric state, it is insoluble under most soil conditions. Under reducing conditions, ferrous iron may be leached out of soil, leaving it gray in color. This is the origin of the term gleying. 

In continually submerged soils, there is no oxygen. Thus, the entire environment is anaerobic and reducing. Under these conditions, there will be a predominance of the reduced forms mentioned earlier, namely, methane, hydrogen sulfide, ferrous iron, and so on. 

Figure 6.15 Profiles of ferrous and ferric iron and pH in blocks of two reduced soils in contact with planar layers of rice roots for indicated times in Iloilo soil. Iloilo soil is a highly weathered sandy loam, org C = 1.2%, aerobic pH = 3.4, reducible Fe = 80 mmol kg (Begg et al., 1994). Reproduced by permission of Blackwell Publishing...

Under well drained conditions, iron in soil, unless chelated, is insoluble and tends to be concentrated through weathering as the more soluble ions are removed. In poorly drained soils, iron is reduced to the soluble ferrous state and is either removed from the soil or precipitated as sulfide, phosphate, or carbonate minerals. Other processes, such as chelation, are also effective in mobilizing the iron in the soil profile. The bulk of secondary iron in soils is in the oxide forms. 

Absorbance of the partly oxidized TF+-citrate solution is measured at 527 nm on a U 7V1S spectrophotometer. Released oxygen on a whole-plant basis is determined by extrapolation of the measured absorbance to a standard calibration curve. These hydroponic studies using artificial redox buffer may be useful in screening related response of wetland plants to reduced soil conditions. However, the conditions do not truly mimic the conditions in wetland soils. Oxidation of other reductants such as DOC, ferrous iron, and ammonium can also be used to estimate oxygen transport capacity of wetland plants (Reddy et al., 1990 Burgoon and Reddy, 1996). 

We know that under anaerobic soil conditions, oxidized forms of Fe function as electron acceptor and are reduced to ferrous iron. 

Sequential reduction of electron acceptors can have a significant effect on soluble phosphorus release. After a soil is flooded, it is expected that the amount of soluble P will increase. This is attributed to the anaerobic conditions occurring in the flooded soil and the various mechanisms of releasing phosphorus under those conditions. As shown in Figure 9.58, the amount of soluble phosphorus starts increasing after the third day of inundation, when almost the entire nitrate pool has been reduced, and consequently the reduction of manganese and iron contained in oxide minerals is already in process. On reduction of ferric oxide minerals, water-soluble and exchangeable concentrations of ferrous iron increase markedly. Thus, the dissolution of iron minerals is accompanied by increases in concentrations of both adsorbed and water-soluble phosphorus. Some of the ferrous ions react with the released phosphorus and precipitate to form new ferrous phosphate minerals. As the soil continues to be under anaerobic conditions, ferric ions are soon depleted and the reduction... 

In contrast to Cr(III), the soil redox condition strongly influences sorption of Cr(VI). Under oxidized and moderately reduced (+500 to +100 mV) soil conditions, chromium behavior is dominated by Cr(VI) sorption and reduction of Cr(VI) to Cr(III) (DeLaune et al., 1998). Under more reduced soil redox levels (<+100 mV), chromium chemistry and solubility is controlled by the reduction of Cr(VI) by soluble ferrous iron. 

Localized spots of decomposing oigauicmattei aie important in reducing small but important quantities of iron to ferrous form and manganese to divalent form so that they become available to plants. Moderately to highly alkaline soils sometimes have inadequate activity of the reduced forms of iron and manganese, particularly in the absence of sufficient organic matter. 

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