Water softening by the zeolite (sodium aluminosilicate) method: after exhaustion, the used zeolite bed is regenerated by flushing with a solution of which salt?

Introduction:
Zeolite softeners exchange sodium ions for hardness-causing calcium and magnesium. Regeneration restores the sodium form of the zeolite for continued service—an essential concept in water treatment operations.

Given Data / Assumptions:

• Zeolite (Na-form) exchanges Na+ for Ca2+/Mg2+.
• After exhaustion, zeolite is in Ca/Mg form.
• Regeneration uses a concentrated brine.

Concept / Approach:
Flushing with concentrated NaCl displaces Ca2+ and Mg2+ from the zeolite structure, reinstating Na+ on the exchange sites. Other salts listed either add hardness or do not effectively drive the exchange back to sodium form.

Step-by-Step Solution:
Identify exhausted state: zeolite-Ca/Mg.Apply regeneration: high [Na+] from brine (NaCl) shifts equilibrium to zeolite-Na.Rinse to remove displaced Ca/Mg chloride brine.

Verification / Alternative check:
Standard water softener operation manuals specify saturated or near-saturated NaCl brine for regeneration cycles.

Why Other Options Are Wrong:

• Calcium sulphate / Magnesium chloride: would load more hardness.
• Sodium sulphate: weaker driving force and not standard practice for regeneration.

Common Pitfalls:
Confusing zeolite regeneration (NaCl) with ion-exchange deionization (acid/caustic) or lime–soda softening chemistry.

Final Answer:
Sodium chloride