FeIIFeIII

The products are silicic acid (H4SiO4; see below) and colloidal hydrated iron oxide (Fe(OH)3), a weak alkali, which dehydrates to yield a variety of iron oxides, for example Fe2O3 (haematite—dull red colour) and FeOOH (goethite and lepi-docrocite—yellow or orange-brown colour). The common occurrence of these iron oxides reflects their insolubility under oxidizing Earth surface conditions.

The water in equation 4.5 acts as an intermediary to speed up oxidation, as shown by the everyday experience of metallic iron oxidation (rusting). In this role water acts as a kind of catalyst (Box 4.4). The oxidation potential of equation 4.5 depends on the partial pressure of gaseous oxygen and the acidity of the solution. At pH 7, water exposed to air has an Eh of 810 mV (Box 4.3), an oxidation potential well above that necessary to oxidize ferrous iron.

Oxidation of organic matter

The oxidation of reduced organic matter in soils is catalysed (Box. 4.4) by heterotrophic microorganisms. Bacterially mediated oxidation of organic matter to CO2 is important because it generates acidity. In biologically active (biotic) soils, CO2 may be concentrated by 10-100 times the amount expected from equilibrium with atmospheric CO2, yielding carbonic acid (H2CO3) and H+ via dissociation; to simplify the equations organic matter is represented by the generalized formula for carbohydrate, CH2O (see Section 2.7.2):

These reactions may lower soil water pH from 5.6 (its equilibrium value with atmospheric CO2; see Box 3.5) to 4-5. This is a simplification since soil organic matter (humus) is not often completely degraded to CO2. The partial breakdown products, however, possess carboxyl (COOH) or phenolic functional groups (see Section 2.7.1), which dissociate (Box 4.5) to yield H+ ions that lower the pH even more:

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