Reduced soil, oxidation

Cobalt is strongly adsorbed by Mn oxides. There are close relationships between Co and the easily reducible fraction of Mn (Mn oxides) in soils (Jarvis, 1984) and will be in detail discussed in next chapter. Cobalt is frequently accumulated in Mn nodules in soils (Mckenzie, 1975). It was suggested that the Co2+ ion was first sorbed, then slowly oxidized to Co3+ and became incorporated into the surface layers of the crystal lattice, releasing the Mn2+ ion into the solution (Bums, 1976 Mckenzie, 1975). X-ray photoelectron spectroscopy showed that Co3+ was present on the surface of bimessite after the sorption of the Co2+ ions took place (Murray and Dillard, 1979). Traina and Donor (1985) suggested that the Mn release during Co2+ sorption resulted not only from the oxidation of Co2+ to Co3+, but also... 

Soil pH affects the transformation of Cr between Cr(III) and Cr(VI) in soils. Since Cr(VI) has greater bioavailability and mobility in soils than Cr(III), which is strongly bound by soil solid matrix (Han and Banin, 1997). Cr(III) can be oxidized by soil manganese oxides into Cr(VI), while Cr(VI) can be reduced by organic matter, Fe(II) and microorganisms in soils. Reduction of Cr(VI) has been found to occur much slower in alkaline soils compared to acid soils (Cary et al., 1997). 

Reduced Cr(III) may be re-oxidized into Cr(VI) by Mn oxides in soils. Chung et al. (2001) reported that native Cr(III) in subsurface materials of arid and semi-arid regions can be oxidized to soluble chromate by native manganese oxides. If subsurface materials contain a low content of Mn oxides, the re-oxidization of reduced Cr(III) is not significant. 

Sulfate poorly prevents arsenate sorption onto metal oxides and soils (Wu et al. 2001 Inskeep et al. 2002 Violante et al. 2005b). Violante et al. (2005b) found that high concentrations of sulfate (sulfate/arsenate molar ratio (rf) 4-10) retarded but not prevented arsenate sorption onto ferrihydrite (see their Fig. 15.10) or other metal oxides. Roy et al. (1986) showed that the sorption of arsenate by two soils (an Ultisol and a Typic Apludults) was reduced in the presence of molybdate. 

When a reduced soil is re-oxidized, Fe " " changes into Fe(OH)3. The original Fe oxides are thus distributed differently, generally with a higher specific surface and activity. In high-activity clay soils, this may increase the stability of the structure established just before flooding. In low-activity clay soils the effects of alternate reduction and oxidation are less clearly beneficial, partly because of leaching of nutrients. 

There is evidence that mixed Fe(II)-Fe(III) hydroxides are formed. These can be produced easily in vitro by partial oxidation of pure Fe(II) hydroxy salts and they have some of the observed properties of the solid phase Fe(II) found in reduced soils, including the grayish-green colours characteristic of reducing conditions in soils. This material is green rust and has the general formula Fe(II)6Fe(III)2(OH)i8 with Al + partly substituted for Fe + and Cl, S04 and C03 substituted for OH . 

Calcium hydroxide is a cheap industrial alkali (Figure 13.9). It is used in large quantities to make bleaching powder, by some farmers to reduce soil acidity, for neutralising acidic industrial waste products, in the manufacture of whitewash, in glass manufacture and in water purification. Calcium hydroxide, in its white powder form, is produced by adding an equal amount of water to calcium oxide in a carefully controlled reaction. The control is needed because it is a very exothermic reaction. 

The effect of amending soil with other types of organic-rich material has also been investigated by sequential extraction. These materials include chicken manure and cowpea leaves (Li et al, 1997) spent mushroom compost, commercial humic acid and poultry litter (Shuman, 1998) and cow manure, pig manure and peat soil (Narwal and Singh, 1998). The mechanisms by which inorganic additives (zeolite, apatite and iron oxide) reduce uptake of Cd and Pb by crops have also been studied (Chlopecka and Adriano, 1997). 

Presence of jarosite (iron sulfate mineral) due to oxidation of pyrite in previously reduced soils... 

The oxidation states of arsenic in rainwater vary according to source but are likely to be dominantly as As(III) when derived from smelters, coal burning, or volcanic sources. Organic arsenic species may be derived by volatilization from soils, and arsine (As(— III)H3) may be produced in landhlls and reducing soils such as paddy soils and peats. Arsenate may be derived from marine aerosols. Reduced forms undergo oxidation in the atmosphere and reactions with atmospheric SO2 or O3 are likely (Cullen and Reimer, 1989). 

Kd,Za was reduced by half. Removing amorphous hydrous oxides reduced Kd cu 100-fold and Kd zn by 20-fold as compared to natural soil. The metal sorption sites in the amorphous hydrous oxides and organic matter were more selective for Cu than for Zn. 

Oxides in soils, as the end product of weathering of natural rocks, have dense, three-dimensional crystal structures and are reduced to very fine particle size. Ion exchange on oxides has long been demonstrated to be essentially confined to only the very first hydrated surface layers (Davis and leckie, 1978). 

Oscarson et al. (1981a) reported that birnessite, one of the most common Mn oxides in soils and sediments, is a very effective oxidant with respect to As(in). The appearance of As(V) in solution after adding solutions of various concentrations of As(III) to Mn(IV) oxide (Table 8-1) shows that As(III) is converted to As(V) by Mn(IV). In a control experiment, no detectable As(III) is oxidized in the absence of Mn(IV) oxide. Manganese (II) is more soluble than Mn(IV) (Stumm and Morgan, 1980), and the high concentrations of Mn in solution in the As(III)-Mn(IV) oxide systems relative to the As(V)-Mn(IV) oxide system is, thus, further evidence that Mn(IV) is reduced to Mn(II) by As(III). The decrease in the Mn concentration in solu-... 

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. 

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. ... 

The most probable oxidation states of As in soil environments are +3 and +5, although the — 3 and 0 oxidation states are at least possible in strongly reduced soils and sediments. Arsenite (+3), which takes various forms such as As(OH)3, As(OH)7, As02(0H) , and AsOj", is the reduced state of As that is most likely to be found in anaerobic soils. Arsenate ( + 5), AsO ", the oxidized state, is stable in aerobic soils. 

Copper occurs in soil solids and solutions almost exclusively as the divalent cation Cu ". However, reduction of Cu " (cupric) to Cu (cuprous) and Cu (metallic copper) is possible under reducing conditions, especially if halide or sulfide ions ( soft bases) are present to stabilize Cu" (a soft acid). Copper is classified as a chalcophile, owing to its tendency to associate with sulfide in the very insoluble minerals, CU2S and CuS. In reduced soils, then, copper has very low mobility. Most of the colloidal material of soils (oxides of Mn, Al, and Fe, silicate clays, and humus) adsorb strongly, and increasingly so as the pH is raised. For soils with high Cu accumula-... 

In reducing soil solutions, Sb is likely to have the form of the uncharged Sb(OH)3 molecule, except at very acid and alkaline pH where Sb(OH)3 converts into the Sb(OH)J cation and Sb(OH) anion, respectively. The oxide of Sb ", Sb203, is too soluble to limit solubility of the element except in highly polluted soils. The oxide becomes more soluble below pH 3 and above pH 10. 

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