Ferric iron reduced soil

Achtnich C, Schuhmann A, Wind T, Conrad R. 1995. Role of interspecies H2 transfer to sulfate and ferric iron-reducing bacteria in acetate consumption in anoxic paddy soil. FEMS Microbiology Ecology 16 61-69. 

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...

High moisture contents in alkaline soils have been shown to increase the availability of Mo (Nayyar, 1972). Poonamperuma (1972) observed an increase in soluble-Mo concentration under wetland rice conditions. Flooding of soils increases the availability of Mo. Soils in the tropics that remain flooded for considerable periods have relatively high contents of Mo (Lopes, 1980), because a high moisture content in the soil reduces ferric iron to the ferrous form, which favors fixation of Mo (Jones, 1956). 

Alternatively, iron-rich sacrificial electrodes, which dissolve under acidic conditions generated at the anode by the application of electric field, may be used. The dissolved iron, in cationic form, migrates toward the cathode and then precipitates as iron-rich mineral phases (ferric iron oxyhydroxides, hematite, goethite, magnetite, and ZVl) near the cathode due to high-pH conditions. Contaminants such as Cr(Vl) can react with this iron and reduce into Cr(III). Cr(VI) transport may be limited by high sorption under low-pH conditions therefore, alkaline solution may be injected from the anode to increase the soil pH, and thereby reduce sorption and increase transport of Cr(Vl) to react with iron. 

In kaolin (clay) processing, sulfur dioxide reduces colored impurities, eg, iron compounds. In the bromine industry, sulfur dioxide is used as an antioxidant in spent brine to be reinjected underground. In agriculture, especially in California, sulfur dioxide is used to increase water penetration and the avadabiHty of soil nutrients by virtue of its abiHty to acidulate saline—alkaH soils (327). It is also usefiil for cleaning ferric and manganese oxide deposits from tile drains (328). 

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. 

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