Question

Given below are two statements:
Statement I: Dominance of $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ on the clay exchange complex decrease the zeta potential that results in the flocculation of clay particles.
Statement II: High $\text{Na}^+$ saturation in the soil increases the zeta potential resulting in dispersion of clay particles.
In light of the above statements, choose the correct answer from the options given below:

• A. Both Statement I and Statement II are true
• B. Both Statement I and Statement II are false
• C. Statement I is true but Statement II is false
• D. Statement I is false but Statement II is true

Hint:

Divalent cations ($\text{Ca}^{2+}$, $\text{Mg}^{2+}$) compress the double layer, decrease zeta potential, and promote flocculation.
Monovalent cations with large hydrated radii ($\text{Na}^+$) expand the double layer, increase zeta potential, and promote dispersion.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Clay particles carry a negative surface charge, creating an electrostatic double layer in the soil solution.
The balance between electrostatic repulsive forces (characterized by the zeta potential) and attractive van der Waals forces determines whether clay particles will flocculate or disperse.

Step 2: Detailed Explanation:
The charge and size of exchangeable cations regulate the thickness of the diffuse double layer and the magnitude of the zeta potential:
1. Divalent cations like calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) have a high charge density and a relatively thin hydration shell.
They compress the diffuse double layer surrounding clay particles, lowering the electrostatic repulsion (zeta potential) below a critical threshold.
This compression allows attractive van der Waals forces to bring the clay particles together, resulting in flocculation (aggregation). Thus, Statement I is true.
2. Monovalent sodium ($\text{Na}^+$) has a lower charge density and a large hydrated ionic radius.
When sodium dominates the exchange complex, it expands the diffuse double layer and increases the zeta potential.
This increase in electrostatic repulsion prevents clay particles from coming close enough for van der Waals forces to act, leading to the dispersion of clay particles and the degradation of soil structure. Thus, Statement II is true.

Step 3: Final Answer:
Both Statement I and Statement II are true, which corresponds to Option (A).