Iron complexes with organic matter are anionic and are not removed by which type of exchange?

• A. Cation exchange
• B. Anion exchange
• C. Mixed bed resins
• D. Ion exchange

Answer: (A)

Iron complexes with organic matter carry a negative charge, so they behave as anions in solution. Cation exchange resins exchange positive ions for other positive ions and do not capture negatively charged species. Therefore, these anionic iron–organic complexes would not be removed by cation exchange. In contrast, anion exchange resins are designed to exchange negative ions and would remove such anionic complexes; mixed-bed resins contain both cation and anion exchangers and can remove both types; and the broader term ion exchange includes both processes. The key idea is that the resin’s charged sites determine what gets exchanged, so negatively charged species are not removed by processes that only exchange cations.