What substance is used to regenerate exhausted natural zeolites in water softening?

The substance used to regenerate exhausted natural zeolites in water softening is sodium chloride. In the context of water treatment, natural zeolites act as ion exchangers, primarily exchanging sodium ions for hardness ions such as calcium and magnesium found in water. When zeolites become saturated with hardness ions, they need to be regenerated to restore their softening capability.

Sodium chloride (common salt) serves as the regenerating solution because it provides sodium ions that can effectively replace the hardness ions that have been collected by the zeolite during the softening process. The regeneration is typically done by circulating a brine solution (high concentration of sodium chloride) through the zeolite, allowing the exchange process to reverse, thus flushing out the hardness ions and replenishing the zeolite with sodium ions.

Other substances listed, while they may have their own purposes in various chemical processes, are not effective for this specific regeneration purpose. For instance, magnesium sulfate, calcium carbonate, and potassium nitrate do not provide the necessary sodium ions for effective regeneration of zeolites in the context of water softening. Hence, sodium chloride is the correct and essential choice for this process.