An Introduction to Environmental Chemistry

eqn. 4.5

The products are silicic acid (H 4 SiO 4 ; see below) and colloidal hydrated iron oxide
(Fe(OH) 3 ), a weak alkali, which dehydrates to yield a variety of iron oxides, for
example Fe 2 O 3 (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 poten-
tial 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 het-
erotrophic microorganisms. Bacterially mediated oxidation of organic matter to
CO 2 is important because it generates acidity. In biologically active (biotic) soils,
CO 2 may be concentrated by 10–100 times the amount expected from equilib-
rium with atmospheric CO 2 , yielding carbonic acid (H 2 CO 3 ) and H+via dissoci-
ation; to simplify the equations organic matter is represented by the generalized
formula for carbohydrate, CH 2 O (see Section 2.7.2):

eqn. 4.6

eqn. 4.7

eqn. 4.8

These reactions may lower soil water pH from 5.6 (its equilibrium value with
atmospheric CO 2 ; see Box 3.5) to 4–5. This is a simplification since soil organic
matter (humus) is not often completely degraded to CO 2. 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:

eqn. 4.9

eqn. 4.10

R

OH

R

O–

+ H+

Phenol Phenate

RCOOH()saÆ+RCOO()-qH()+aq

H CO 23 ()aq ªH+()aq+HCO 3 - ()aq

CO 22 ()gl+HO()ªHCO 23 ()aq

CH 2 O(s) + O2(g) CO2(g) + H 2 O(l)

Fe SiO

fayalite

Fe II

OHOFeOH

Fe III

24 ()sg^1222 ()^52 ()l 3 ()s H SiO 44 ()aq

()

()()

++ Æ()

()()

+

The Chemistry of Continental Solids 79