Ion exchange units are known primarily as water softeners. But they can also remove nitrates, sulfates, and various toxic metals from water. Understanding how ion exchange works and what contaminants different units will remove can help you make informed decisions about water treatment by ion
How Ion Exchange Works
Ions are atoms, small particles that are the building blocks for molecules. Ions have a weak electrical charge. The charge may be positive (for cations) or negative (for anions). This positive and negative
charge is similar to the north or south pole of a magnet or the positive or negative terminal of a car battery. Positively charged sodium ions are commonly used to coat cation exchange resins. Negatively
charged chloride or hydroxide ions are commonly used to coat anion exchange resins. Mixed bed resins combine both positive and negative ions. Ion exchange units actually exchange ions from the resins with those in the water. When water to be treated passes through the ion exchange unit, ions in the water are attracted by either a positive or a negative charge to the ions in the resin bed. Since the ions from the water are usually held more tightly by the resins than they were held in the water, they are, in effect, removed from the water in the exchange process.
What Cation Exchange Removes. Water softeners exchange calcium and magnesium with sodium.
Cation exchange resins also remove barium, cadmium, copper, iron, manganese, radium, zinc, and
other metallic, positively-charged ions.
How Anion Exchange Works. Anion exchange units have a resin that exchanges chloride or hydroxide for the anions (the negatively charged atoms) that they remove. Most use chloride, which increases the chloride content of water and may cause a salty taste.
What Anion Exchange Removes. Anion exchange units can remove nitrate, sulfate, and other negatively
charged atoms called anions. Researchers are developing resins to selectively remove nitrate more efficiently than can now be done.
Mixed Bed Ion Exchange
Often called demineralizers, mixed bed ion exchange units combine resins for removal of both positive and negative ions. They do not remove organic chemicals and they produce water similar to distilled water. Mixed bed units for industrial use are renewed with hydrogen ions from an acidic solution for cation exchange and with hydroxyl ions from an alkaline solution for anion exchange. Mixed bed units for homes are usually not rechargeable. They must be discarded after a certain volume of water has passed through them.