Ion Exchange resins
An ion-exchange resin is an insoluble matrix (or support structure) normally in the form of small (1–2 mm diameter) beads, usually white or yellowish, fabricated from an organic polymer substrate. The material has a highly developed structure of pores on the surface of which are sites with easily trapped and released ions. The trapping of ions takes place only with simultaneous releasing of other ions; thus the process is called ion-exchange. There are multiple different types of ion-exchange resin which are fabricated to selectively prefer one or several different types of ions.
Ion-exchange resins are widely used in different separation, purification, and decontamination processes. The most common examples are water softening and water purification.
Four main types differing in their functional groups
1. Strongly acidic (typically, sulfonic acid groups, e.g. sodium polystyrene sulfonate or polyAMPS)
2. Strongly basic (quaternary amino groups, for example, trimethylammonium groups, e.g. polyAPTAC)
3. Weakly acidic (mostly, carboxylic acid groups)
4. Weakly basic (primary, secondary, and/or ternary amino groups, e.g. polyethylene amine)
• Water softening
• Water purification
• Production of high purity water
• Ion-exchange in metal separation
• Juice purification
• Sugar manufacturing
In this application, ion-exchange resins are used to replace the magnesium and calcium ions found in hard water with sodium ions. When the resin is fresh, it contains sodium ions at its active sites. When in contact with a solution containing magnesium and calcium ions (but a low concentration of sodium ions), the magnesium and calcium ions preferentially migrate out of solution to the active sites on the resin, being replaced in solution by sodium ions. This process reaches equilibrium with a much lower concentration of magnesium and calcium ions in solution than was started with.
The resin can be recharged by washing it with a solution containing a high concentration of sodium ions (e.g. it has large amounts of common salt (NaCl) dissolved in it). The calcium and magnesium ions migrate off the resin, being replaced by sodium ions from the solution until a new equilibrium is reached. The salt is used to recharge an ion-exchange resin which itself is used to soften the water.
In this application, ion-exchange resins are used to remove poisonous (e.g. copper) and heavy metal (e.g. lead or cadmium) ions from solution, replacing them with more innocuous ions, such as sodium and potassium.
Few ion-exchange resins remove chlorine or organic contaminants from water – this is usually done by using an activated charcoal filter mixed in with the resin. There are some ion-exchange resins that do remove organic ions, such as MIEX (magnetic ion-exchange) resins. Domestic water purification resin is not usually recharged – the resin is discarded when it can no longer be used.
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