UW-Madison researchers have developed a method of using an ion-storage electrode to electrochemically extract Ca2+ and Mg2+ from feed water and recover them in a separate cell as useful chemicals. They have shown that 5,7,12,14-pentacenetetrone (PT) can preferentially extract Ca2+ and Mg2+ over Na+ while maintaining a stable capacity over a wide pH range (pH 3-12). Due to PT’s stability in basic media, it is possible to recover the extracted Ca2+ as CaCO3 in basic media where Na+ is highly soluble. As a result, although PT's divalent ion selectivity is modest, the co-extracted and co-released Na+ did not interfere with the separation and collection of CaCO3. They have also developed polyimide electrodes that show even higher selectivities for Ca2+ and Mg2+ removal/recovery than the PT electrodes. Furthermore, both PT and polyimide electrodes may selectively extract trivalent ions like rare earth ions (i.e., lanthanide ions) with the preference of trivalent ions > divalent ions > monovalent ions. This ability may be used to separately extract and recover divalent and trivalent ions generated from various mining, industrial, and water-treatment processes.