White paper - Ion Exchange Theory

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Courtesy of W2 Systems

Ion exchange is the removal of unwanted “charged” particles, called ions, from raw water. In this process the ions are transferred to a solid particle, called an ion exchanger, which accepts these ions while returning an equal number of desirable ions. The exchangers are made of porous plastic resin, which gives them many “exchange sites”. In the basic reaction, an ion diffuses through a pore to a site, the site “holds” the ion while releasing another.

Where Na+ is an unwanted sodium ion, H+ is a desired hydrogen ion, and X is the exchange site. This case is called the “hydrogen cycle” of a cation (+ charged ion) exchanger. It is used to “soften” water by removing such cations as sodium, calcium, and potassium. Anion (- charged ion) exchangers work the same as their cation counterparts.

Where bicarbonate ion, HCO -, is exchanged for hydroxide ion, OH-, on A which is the exchanged site. Anion and cation resins are used in conjunction to create DI (deionization) water systems. The most common DI systems are the two bed and the mixed bed. In the two bed system, raw water is pumped into a pressure vessel containing cation resin which removes “hardness” (Na, Ca, K) , and then it is transferred to another pressure vessel containing anion resin to complete deionization. In a mixed bed system, untreated water is sent to a vessel containing a mixture of anion and cation resins. The reactions are the same as in the two bed, but a mixed bed system produces water of higher purity because the reactions take place simultaneously.

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