Ion-exchange resins

ion-exchange resins are used for water purification to remove copper and heavy meatals such as lead aluminum, and water hardness ions from water and replacing them with more harmless ions like sodium and potassium.

And for water softening, ion-exchange resins are used to replace magnesium and calcium ions that are in hard water with sodium ions.

When the ion-exchange resin is fresh it has sodium ions at its active sites when it is in contact with magnesium and calcium ions, the magnesium and calcium ions are removed out of the water to the active site on the resin.

Being replaced in solution by sodium ions. In return the adjustment is a much lower concentration of magnesium and calcium ions than what was started with.