Page 102 - Soil and water contamination, 2nd edition

Solid phase constituents 89

2+
of the bulk solution. In general, calcium (Ca ) ions dominate the CEC, except in acidic
+
and saline soils. In acidic soils, the exchange complex is largely occupied by protons (H )
+
+
3+
and aluminium species (Al(OH) and/or Al ). In saline soils, sodium (Na ) occupies
2
+
2+
2+
+
a considerable part of the CEC. The exchangeable Ca , Mg , Na , and K ions are often
called the exchangeable bases, because soils with the CEC fully saturated with these cations
+
react as a base by accepting H and so neutralise the pH of the bulk solution. The proportion
of these cations on the CEC is called the base saturation . The base saturation of typical soils
in the Netherlands and other parts of the world is indicated in Table 4.3.
EXERCISES
1. Define the following terms:
a. Colloidal particles
b. Nanomaterials
c. Asbestos
d. Sesquioxides
e. Diffuse layer
f. CEC
g. Base saturation
h Humic substance
i. DOM
j. Chelate
k. Biofilm
l. BOD

2. Name the two principal mechanisms responsible for the surface charge of solid particles.

3. Given the PZC of the following materials:
Material PZC
9.1
Al 2 O 3
5.0
Al(OH) 3
6.7
Fe 2 O 3
8.5
Fe(OH) 3
Quartz (SiO 2 ), feldspars 2.0–2.4
Clay minerals: kaolinite 4.6
m ontmorillonite 2.5
a. Indicate whether the surface charge of the above minerals is negative or positive at
pH = 5.4.
b. What will happen if the pH increases to 6.8?
4. Why is the CEC of quartz and feldspars much smaller than the CEC of sesquioxides and
clay minerals?
5. Describe the differences in structure and physico-chemical properties of 1:1 and 2:1 clay
minerals.
6. Briefly describe the process of K fixation in illitic clay minerals.

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Soil and Water.indd 101