Chemistry Reference
In-Depth Information
stable elements and the dozen or so natural radio-
active elements together contribute the remaining
1.5%. The surface of the crust that forms the inter-
face with the atmosphere and terrestrial waters is a
complex biogeochemical mixture that we call soil.
as Gibbsite (gAl(OH)
3
). These oxides have pH-
dependent surface charges, being positively charged
under acidic and negatively charged under alkaline
conditions. Other soil minerals include quartz (SiO
2
),
which confers acidity, and calcite (CaCO
3
), which
buffers the soil pH to between 7 and 8.5.
The amount of organic matter in soil varies from
less than 1% in arid regions to 10% in humid, tem-
perate regions. It comprises humic compounds (col-
loidal polymerised products of plant decomposition)
and non-humic material (undecomposed and par-
tially decomposed dead plant and soil organisms).
Humic compounds have a high pH-dependent capac-
ity for complexing cations and adsorbing non-polar
organic molecules.
The most important parameter affecting ionic con-
taminant behaviour in soils is acidity. The pH of soil
normally lies in the range 4-8.5, although extremes
of 2 and 10.5 have been measured. Aridity leads to
higher pH and humidity leads to lower pH. Micro-
bial activity is highest at pH 6-8 and metal mobility
is greatest at low pH.
Redox conditions in soils reflect the oxygen avail-
ability. They can affect plant growth and determine
the microorganisms present. The balance of oxida-
tion and reduction also determines the speciation of
the elements, particularly C, O, S, N, Fe and Mn.
Soil chemistry
Soil differs substantially in chemical as well as phys-
ical and biological properties from the underlying
rock. It is a matrix of mineral particles, organic
material, aqueous solutions and gases and is popu-
lated by micro- and macroorganisms. The mineral
component is made up of sub-2-mm particles of clay
with other minerals, principally iron, aluminium and
to a lesser extent manganese oxides and hydroxides.
Clays are secondary minerals produced from igneous
rocks by weathering and glaciation. They comprise
combinations of sheets of Si-O tetrahedra and
Al-OH octahedra linked by hydrogen bonds. There
are two dominant patterns of layer combination.
The first has alternating tetrahedral and octahedral
layers, e.g. crystolite (Mg
4
Si
4
O
10
(OH)
8
) and kaolinite
(Al
4
Si
4
O
10
(OH)
8
) (Fig. 3.4). The second has repeating
units of tetrahedral, octahedral and tetrahedral
layers and includes illite (K
1.6
Si
6.4
Al
5.6
O
20
(OH)
4
)
and vermiculite (Mg
6.6
Si
6.8
Al
1.2
O
20
(OH)
4
). Isomor-
phous substitution of Al
3+
for Si
4+
and Mg
2+
for Al
3+
in the layers of some clays gives rise to excess
negative charges on their surfaces and confers the
ability to absorb and exchange cations from aque-
ous solution.
The main form of iron in soils is ferrihydrite
(Fe
2
O
3
·2FeOOH·nH
2
O) and the main form of alu-
minium is the amorphous hydroxide that crystallizes
3.2 Pollution of the land
'Such prosperity as we have known it up to the
present is the consequence of rapidly spending
the planet's irreplaceable capital.'
Aldous Huxley (1894-1963)
Two views of kaolinite structure
Fig. 3.4
Idealised structure of kaolinite
(two views).
Oxygen
Silicon
Aluminium
