Ion exchange resins exchange one ion for another...and are regenerable.
From the Betz Handbook on Industrial Water Conditioning, 7th edition, 1976, page 50:
"Today's ion exchange resins consist of a hydrocarbon network to which ionizeable functional groups are attached."
"The ionizeable groups within the beads give the resin the characteristics of a concentrated water solution in that they contain both positively charged cations and negatively charged anions. Unlike a water solution, however, only one ionic species is mobile; the other is attached to the bead structure."
"Ion exchange occurs when raw water ions diffuse into the bead structure and the displaced ions from the bead structure diffuse back into the water solution."
You can have both anion and cation exchange resins. Anion exchange resins would be regenerated with something like caustic soda, and cation exhange resins would be regenerated with, say sulphuric acid.
