Question

Which of the following is NOT related to anion exchange capacity?

• A. Dissociation of carboxyl and phenolic group on broken crystal edges
• B. Dissociation of OH ions from Al-OH group on the broken crystal edges and addition of H ion on these sites
• C. Dissociation of OH ion from Al-OH group on the broken crystal edges and addition of H ion on these sites
• D. Addition of H ion on humus micelle

Hint:

Remember: Protonation ($+ H^+$) creates Positive charge (AEC).
Dissociation ($- H^+$) creates Negative charge (CEC).
AEC is most prominent in highly weathered acidic soils rich in Kaolinite and Fe/Al oxides.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the process that does not contribute to the development of Anion Exchange Capacity (AEC) in soil.
AEC is the result of positive charges on soil colloids that attract and retain anions.
Detailed Explanation:

• Sources of AEC:
Anion Exchange Capacity arises from pH-dependent positive charges.
At low pH (acidic conditions), hydroxyl groups ($OH$) on the broken edges of silicate clays or on the surfaces of hydrous oxides of iron and aluminum can accept a proton ($H^+$).
The reaction is: $Al-OH + H^+ \rightarrow Al-OH_2^+$.
This creates a positive site that can attract anions like $SO_4^{2-}$, $Cl^-$, or $NO_3^-$.
Statements (B) and (C) describe this protonation process correctly.

• AEC on Organic Matter (Statement D):
At very low pH, even certain sites on humus (organic matter) can become protonated, though this is less common than in mineral colloids.

• Cation Exchange Capacity (Statement A):
The dissociation of carboxyl ($-COOH \rightarrow -COO^- + H^+$) and phenolic ($-OH \rightarrow -O^- + H^+$) groups releases a hydrogen ion into the soil solution.
This leaves behind a negative charge on the colloid.
Negative charges are responsible for Cation Exchange Capacity (CEC), not Anion Exchange Capacity.
Therefore, Statement A is the process related to CEC and is NOT related to AEC.

Step 2: Final Answer:
The dissociation of carboxyl and phenolic groups creates negative charge sites; thus, it is related to CEC, making statement (A) the correct answer.