Step 1: Understanding the Question:
The question is related to the demineralization or deionization process of water softening using ion-exchange resins.
Specifically, it asks for the reagent used to regenerate an anion-exchange resin that has lost its capacity to remove anions from water.
Step 2: Detailed Explanation:
Anion-Exchange Process:
Anion-exchange resins are high-molecular-weight organic polymers containing basic functional groups like quaternary ammonium (\(-NR_3^+OH^-\)) or amino groups.
When hard water passes through these resins, the anions present in the water (such as \(Cl^-\), \(SO_4^{2-}\), \(NO_3^-\)) are exchanged for the hydroxyl (\(OH^-\)) ions of the resin.
The general reaction is: \(R-OH + Cl^- \rightarrow R-Cl + OH^-\).
Exhaustion of the Resin:
Over time, all the available \(OH^-\) ions on the resin are replaced by other anions from the water. At this point, the resin can no longer remove anions and is said to be "exhausted."
Regeneration Mechanism:
To restore the resin's capacity, it must be treated with a solution that can replace the accumulated anions with \(OH^-\) ions.
This is achieved by passing a dilute solution of a strong base, typically Sodium Hydroxide (\(NaOH\)), through the resin bed.
The high concentration of \(OH^-\) ions in the \(NaOH\) solution forces the reverse reaction, displacing the trapped anions: \(R-Cl + NaOH \rightarrow R-OH + NaCl\).
The displaced anions are washed away as waste, and the resin is returned to its active hydroxide form (\(R-OH\)).
Note: Cation-exchange resins are regenerated with dilute acids like \(HCl\) or \(H_2SO_4\) to restore \(H^+\) ions.
Step 3: Final Answer:
An exhausted anion-exchange resin is regenerated using a dilute solution of \(NaOH\) to restock the resin with hydroxyl (\(OH^-\)) ions.