An ion exchange resin is a polymer (normally styrene) with electrically charged sites at which one ion may replace another. Natural soils contain solids with charged sites that exchange ions, and certain minerals called zeolites are quite good exchangers. Ion exchange also takes place in living materials because cell walls, cell membranes, and other structures have charges. In natural waters and in wastewaters, there are often undesirable ions and some of them may be worth recovering. For example, cadmium ion is dangerous to health but is not present usually at concentrations that would justify recovery. On the other hand, silver ion in photographic wastes is not a serious hazard, but its value is quite high. In either case, it makes sense to substitute an ion such as sodium for the ion in the wastewater.

Synthetic ion exchange resins are usually cast as porous beads with considerable external and pore surface where ions can attach. Whenever there is a great surface area, adsorption plays a role. If a substance is adsorbed to an ion exchange resin, no ion is liberated. Testing for ions in the effluent will distinguish between removal by adsorption and removal by ion exchange. Of course, both mechanisms may be significant in certain cases, and mass balances comparing moles removed with moles of ions liberated will quantify the amounts of adsorption and ion exchange.
 

While there are numerous functional groups that have charge, only a few are commonly used for man-made ion exchange resins. These are:

• -COOH which is weakly ionized to -COO¯
• -SO₃H which is strongly ionized to -SO₃¯
• -NH₂ that weakly attracts protons to form NH₃⁺
• -secondary and tertiary amines that also attract protons weakly
• -NR₃⁺ that has a strong, permanent charge. (R stands for some organic group)

The Freundlich and Langmuir equations used for adsorption will also describe ion exchange. However, the BET equation is not applicable because multiple layers do not form; electrical charge must balance. The Donnan membrane equilibrium discussed in the section about membranes is also relevant to ion exchange because the charges on the resin backbone are localized in an situation analogous to charged proteins than cannot pass through membranes.

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