What happens when the brine solution is fed into the resin?

• A. Iron Cations Are Exchanged For Calcium And Manganese Cations
• B. Sodium Cations Are Exchanged For Calcium And Magnesium Cations
• C. Bicarbonate Anions Are Exchanged For Chloride And Silicate Anions
• D. Sodium Cations Are Exchanged For Bicarbonate And Silicate Anions

Answer: (B)

The main idea is regenerating a cation-exchange resin used for water softening with a brine solution. In this process, the resin, which is in the sodium form to start, has hard ions calcium and magnesium attached after softening water. To recharge the resin, a brine solution rich in sodium ions passes through. The high concentration of Na+ in the brine drives an exchange: sodium ions replace the calcium and magnesium on the resin, releasing Ca2+ and Mg2+ into the brine waste. This restores the resin to the Na+ form for another softening cycle. The other options describe exchanges that aren’t part of this regeneration step, such as exchanging iron or exchanging anions, which occur in different parts of treatment or with different resin types.