Question

Which ion is the dominant ion adsorbed on clay surfaces under very acidic conditions?

• A. $\text{H}^+$
• B. $\text{Al}^{3+}$
• C. $\text{Ca}^{2+}$
• D. $\text{Mg}^{2+}$

Hint:

Remember the lyotropic series for cation adsorption strength on clay surfaces:
\[ \text{Al}^{3+} > \text{Ca}^{2+} > \text{Mg}^{2+} > \text{K}^+ \approx \text{NH}_4^+ > \text{Na}^+ \]
Because trivalent aluminum has the highest charge among these common cations, it is adsorbed most strongly and dominates the exchange complex in acidic environments.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Soil acidity is determined by the concentration of hydrogen ($\text{H}^+$) and aluminum ($\text{Al}^{3+}$) ions present in the soil solution and adsorbed on the soil exchange complex.
As the pH of the soil decreases, the solubility and behavior of aluminum ions undergo significant shifts.

Step 2: Detailed Explanation:
In highly acidic soils (typically where the pH drops below $5.0$ or $4.5$), clay minerals begin to break down.
During this chemical weathering process, aluminum is released from the crystal lattice of silicate clays.
Once released, the trivalent aluminum ion ($\text{Al}^{3+}$) acts as a highly active acid cation.
Due to its high positive charge ($+3$) and relatively small ionic radius, $\text{Al}^{3+}$ is held much more tightly by the negatively charged clay surfaces than monovalent hydrogen ($\text{H}^+$) or divalent calcium ($\text{Ca}^{2+}$) and magnesium ($\text{Mg}^{2+}$) ions.
Therefore, under very acidic conditions, $\text{Al}^{3+}$ becomes the dominant exchangeable cation occupying the cation exchange sites of the soil.
Furthermore, these adsorbed $\text{Al}^{3+}$ ions undergo hydrolysis in the soil solution, releasing additional $\text{H}^+$ ions and further contributing to active soil acidity according to the reaction:
\[ \text{Al}^{3+} + \text{H}_2\text{O} \rightleftharpoons \text{Al(OH)}^{2+} + \text{H}^+ \]

Step 3: Final Answer:
Under highly acidic conditions, $\text{Al}^{3+}$ is the dominant cation adsorbed on the clay surface, corresponding to option (B).