Question

A river water sample has pH of 4 and suspended solids concentration of 100 mg/L. If alum is chosen as a coagulant, what will be the coagulation mechanism?

• A. Ionic layer compression only
• B. Sweep coagulation and Polymer bridging
• C. Polymer bridging only
• D. Charge neutralization, surface adsorption, ionic layer compression

Hint:

At low pH, alum mainly provides Al\(^{3+}\) ions that compress the electrical double layer—other mechanisms require hydroxide formation and do not occur.

Answer 1

The Correct Option is D

At pH 4, alum (Al\(_2\)(SO\(_4\))\(_3\)) dissociates to form Al\(^{3+}\) ions.
At such low pH:
- Hydrolysis products of alum (like Al(OH)\(_3\)) do not form, as hydroxide concentration is too low.
- Thus, no sweep flocculation and no polymer bridging occur.
- Charge neutralization is minimal due to strong acidity and strong positive charge on particles.
The only mechanism that dominates is compression of the electrical double layer (ionic layer compression) caused by high ionic strength.
Hence, (A) is the correct option.
Final Answer: Ionic layer compression only