Iron is normally considered as a divalent or trivalent cation that would be easily picked up by standard cation exchange resins, including DOWEX™ G 26 (H) Resin. While this is true, one of the most useful applications of ion exchange for iron removal employs anion exchange resin such as DOWEX 21K XLT Resin to capture Fe3+ (ferric) ions as an anionic complex. Similar to zinc, this is possible because these metals form multivalent anionic complexes (42KB PDF) in the presence of excess chloride ion:
FeCl3+ MCl is at equilibrium with M+FeCl4-
Purification of concentrated HCl is commonly carried out on a commercial scale. Interestingly, the resin is eluted with plain water. In the absence of excess halide ion the equilibrium shifts back to the left and the neutral salt "falls off of the resin". For more information, see page 6 of DOWEX™ Ion Exchange Resins: Powerful Chemical Processing Tools.
Iron removal from water via ion exchange is common in chemical and petrochemical processing, oil refining, semiconductor manufacturing and power utility plants.
In waters with low levels of dissolved salts, a strongly acidic cation exchange resin such as DOWEX™ G 26 (H) Resin is employed.
Weak acid cation exchange resins such as DOWEX MAC 3 Resin often provide high capacity and good multivalent ion selectivity.
In more brackish waters, however, chelating resins may be required. Chelating resins have reactive units dispersed along their polymer matrix, each of which has multiple metal binding sites. Hence, chelating resins, such as AMBERLITE™ IRC748i Resin, are selective for multivalent cations over monovalent cations like sodium and potassium.