Regeneration in Zeolite Softening — After exhaustion, which regenerant solution is used to restore sodium-form zeolite in a domestic/industrial softener?

Introduction:
Ion-exchange softeners eventually “exhaust” as their exchange sites become occupied by calcium and magnesium. Regeneration reverses the exchange, restoring the sodium form so the unit can continue removing hardness. Choosing the correct regenerant is essential for practical, economical operation.

Given Data / Assumptions:

• Softening resin/zeolite initially in Na-form (Na-Z).
• Exhausted resin becomes Ca-Z and Mg-Z during service.
• Regeneration uses a concentrated brine to displace Ca2+/Mg2+ from the matrix.

Concept / Approach:

High-strength NaCl brine provides a large excess of Na+ that drives the equilibrium back to Na-Z, releasing Ca2+/Mg2+ into the waste stream. Alternative salts with Ca2+ or Mg2+ cannot regenerate a Na-form softener—indeed, they would further load the bed with hardness ions rather than remove them.

Step-by-Step Solution:

Verification / Alternative check:

Standard operating procedures for domestic and industrial softeners specify NaCl brine regeneration; the chemistry and economics are well established.

Why Other Options Are Wrong:

A–C would not regenerate a Na-form softener and would contaminate with hardness ions. E is not used for regeneration because it lacks the necessary driving force and can alter alkalinity.

Common Pitfalls:

Confusing regeneration (restoring capacity) with backwashing (re-stratifying the bed). Both are necessary steps but serve different purposes.

Final Answer:

10% sodium chloride (NaCl) solution (brine)