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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.7

Each ion pair has a unique selectivity value for each ion exchange resin. The higher the
selectivity coefficient, the higher the relative affinity of the ion for the resin. The higher
the affinity, the easier it is to load the ion, and conversely the more difficult it is to re-
move during regeneration. The operating performance, capacity, and leakage data for
ion exchange resins for the common ions found in water are usually provided by the
resin supplier.
In general, the ion exchange resin is used in and regenerated to an ionic form, which
will exchange ions that are acceptable in the treated water. As the untreated water passes
through the resin, the undesirable ion or ions exchange for the unobjectionable ion on the
resin. For example, a cation exchange resin is regenerated with sodium chloride and op-
erated in the sodium cycle. The resin exchanges sodium ions for all the positively charged
ions (cations), including hardness-causing calcium and magnesium ions, and the water is
thereby softened. Likewise, if an anion resin is regenerated with sodium chloride, it op-
erates in the chloride cycle. It will exchange chlorides for the anions, i.e., bicarbonates,
sulfates, nitrates, etc., and the effluent water will have a chloride concentration equiva-
lent to the total concentration of the anions in the raw water.
Although salt exchanges are common, so are other ion exchanges that involve acids
and bases. In the process of demineralization, for example, the cation resin is operated in
the hydrogen form and exchanges hydrogen ions. This converts all incoming salts to their
equivalent acids. After the cation exchange, the water passes through a hydroxide form
anion exchanger where anions are exchanged for hydroxides, which converts the acids to
water molecules. In this process the net result is that an equivalent amount of water mol-
ecules is added to the water in exchange for the salts that are removed.
The weakly acidic and weakly basic ion exchange resins are generally incapable of
converting salts to acids and bases and are generally limited in use to neutralization re-
actions. The hydrogen form weakly acidic cation resins are usually used to neutralize al-
kalinity, and weakly basic resins are likewise used to neutralize acidity.
Ion exchange is a dynamic equilibrium-driven process, and regeneration is never 100%
complete. There is always some, albeit trace, level of the undesirable ionic constituent left
on the resin and in the product water. The most complete ion exchange reactions are those
in which the resulting products disappear, such as by H-- and OH- neutralization which
drives the reaction to completion by the formation of water molecules.
When the equilibrium is favorable, the exchange of ions occurs in a narrow band within
the resin bed, and a large portion of the resin bed can be used before significant leakage
occurs due to kinetic slippage. When the equilibrium is not favorable, the exchange zone
ion is bigger and more diffuse. It can be as long as or longer than (in terms of the amount
needed to reach the desired level of purification) the entire resin bed. The leakage of the
undesirable ion is almost immediate and gradually increases throughout the service run.
When the exchange zone is larger than the entire bed, the desired quality cannot be
achieved, even in the initial portion of the service cycle.

Resin Capacity and Regeneration
Ion exchange resins have a finite capacity. When this capacity is used up, the resins are
exhausted and leakage of the unwanted ions increases. The exhausted resin can be re-
generated with a salt, acid, or base solution containing the ion whose "form" the resin
will be operated in. This is passed through the resin bed in sufficient quantity and at a
sufficiently high concentration to reverse the exchange, desorb, and replace the previously
exchanged ions from the resin with the ions from the regenerant solution. The high con-
centration of the regenerant minimizes waste volumes and changes the equilibrium rela-
tionships to make the regeneration more efficient.