Mechanics of Ion-Exchange System Operation
By William F. Harfst
In-exchange (IX) resins selectively remove cations (positively charged) and anions (negatively charged) from water by replacing the ion located at the resin exchange site for the ion dissolved in the water. A cation resin exchanges cations, and an anion resin exchanges anions. The exchange sites are active molecular nodes located along the styrene divinyl benzene backbone of the resin. The functional units for cation resins are sulfonic and carboxylic acids. For anion resins, itﾒs a quaternary amine group. In both cases, the functional sites serve as the source of the exchangeable ions. For example, a strongly acidic cation resin in the sodium form has a sodium ion (a cation) located on the sulfonic acid exchange site. As water flows past the exchange site, sodium is exchanged for other dissolved and dissociated cations such as calcium and magnesium hardness. Iron, another cation, is also exchanged for sodium. Sodium, of course, is not removed by a sodium-form IX column. If the cation resin is in the hydrogen form, meaning that hydrogen is located on the exchange site, the cations will be exchanged for hydrogen. Calcium magnesium, iron, and sodium will be exchanged at this site. Anion resins are used in either the chloride form or the hydroxide form. Here the dissolved anions are exchanged for either chloride or hydroxide. Strongly basic anion resins in the hydroxide form exchange hydroxide (OH) for strongly dissociated anions like sulfate and chloride. Other weakly dissociated anions like carbonate, bicarbonate, and silica are also removed by strongly basic anion resins. Weakly basic anion resins, however, are unable to remove weakly dissociated anions like silica.
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