Question

The range of Cation exchange capacity of kaolinite clay mineral is :

• A. 3-15 cmol (\(\text{p}^+\))/kg
• B. 20-45 cmol (\(\text{p}^+\))/kg
• C. 80-120 cmol (\(\text{p}^+\))/kg
• D. 120-150 cmol (\(\text{p}^+\))/kg

Hint:

- 1:1 Non-expanding clay (Kaolinite): lowest CEC (3-15 cmol (\(\text{p}^+\))/kg).
- 2:1 Non-expanding clay (Illite): medium CEC (20-40 cmol (\(\text{p}^+\))/kg).
- 2:1 Expanding clay (Montmorillonite): high CEC (80-150 cmol (\(\text{p}^+\))/kg).
- Organic matter (Humus): extremely high CEC (100-300 cmol (\(\text{p}^+\))/kg).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Cation exchange capacity (CEC) is a measure of the soil's ability to hold and exchange cations, which is a key indicator of soil fertility and nutrient retention.
This capacity depends heavily on the type and amount of clay minerals present in the soil.
Clay minerals are classified based on their crystal layer structure, such as 1:1 or 2:1 type minerals.

Step 2: Detailed Explanation:
Kaolinite is a classic 1:1 type silicate clay mineral consisting of alternating tetrahedral sheets of silicon and octahedral sheets of aluminum.
Because the layers are held tightly together by hydrogen bonding, kaolinite does not expand when wetted (it is non-expanding).
Consequently, there is very little internal surface area available for cation adsorption.
Furthermore, kaolinite exhibits minimal isomorphous substitution of aluminum for silicon in the tetrahedral sheet, meaning its permanent negative charge is extremely low.
Most of the negative charge on kaolinite is pH-dependent, arising from the deprotonation of hydroxyl (\(\text{OH}\)) groups at the broken edges of the crystal lattice.
Due to these structural characteristics, kaolinite has a very low cation exchange capacity compared to 2:1 expanding clays like montmorillonite.
The standard CEC range for kaolinite is typically 3 to 15 cmol (\(\text{p}^+\))/kg (or meq/100g).
For comparison, illite (a non-expanding 2:1 clay) has a CEC of 20 to 40 cmol (\(\text{p}^+\))/kg, and montmorillonite (an expanding 2:1 clay) ranges from 80 to 120 cmol (\(\text{p}^+\))/kg.

Step 3: Final Answer:
The range of Cation exchange capacity of kaolinite clay mineral is 3-15 cmol (\(\text{p}^+\))/kg (Option A).