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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.5

form of a strong acid cation exchange resin for calcium and magnesium (cation soften-
ing) and also the exchange of chlorides for other anions by the chloride form of a strong
base anion exchange resin (anion softening). Cation softening is so much more common
than anion softening that the term softening by itself refers only to the cation softening
process.
Cation softening:
4R - Na + Ca 2+ + Mg 2+ ~ R2Ca + R2Mg + 4Na +
Anion softening:
3R - C1 + SO42- + HCO3- ~ R2804 + RHCO3 + 3C1-
As a result of these two ion exchange processes, all the salts in the raw water would be
changed by the ion exchange resins to an equivalent amount of sodium chloride.

The Nature of Resin Degradation
Ion exchange resins degrade both physically and chemically. In practical terms, the degra-
dation of a resin can be defined as any change in the physical or chemical properties that
adversely affects the operating performance or the ability of the resin to attain desired re-
sults. A change in the resin properties is not automatic grounds for replacement. Degra-
dation must be evaluated in light of the way the resin is used. However, it is safe to say
that most resin replacements occur as a result of degradation. A physically degraded ion
exchange resin that continues to perform well chemically without high pressure loss or
sporadic quality due to flow channeling may be acceptable for certain low-flow-rate ap-
plications. However, the same resin would not be acceptable in high-flow-rate applica-
tions such as condensate polishing. A strong base anion resin that has chemically degraded
may give good results in separate bed applications on certain kinds of waters, but the same
resin would retain only a small fraction of its original operating capacity in a mixed-bed
demineralizer operating on the same water.

Physical. Ion exchange resins degrade physically by breaking. Usually the resin devel-
ops cracks, which then fracture. The result of this fragmentation is a reduced average par-
ticle size. The void volume (space between beads) also decreases as the irregularly shaped
fragments fill spaces between the beads, which increases pressure loss and can lead to
channeling (irregular flow distribution) and clogged flow distributors. New resins typi-
cally have more than 90% whole, perfect uncracked beads. Over time, physical and os-
motic stresses will gradually crack and break beads.
Once placed in service, resins are often exposed to oxidants such as dissolved oxygen
and chlorine. The chemical bond between the DVB cross-linker and the copolymer is the
primary place where oxidative attack occurs. Cleavage of the divinylbenzene copolymer
bond reduces the level of cross-linking. As the cross-linkage is reduced, the resin becomes
more hydrated and the beads swell.

Oxidative. Oxidation occurs first on the outside of the beads and works its way inward.
This results in an uneven distribution of cross-linking and swelling, which creates an os-
motic stress. This is a major cause of bead breakage in cation resins used in domestic wa-
ter softeners. Both macroporous and gel types are subject to oxidative degradation.
Chlorine in potable water is present as the OC1- anion. The negatively charged an-
ions are excluded from the interior of the cation resin due to charge repulsion by the ion
exchange groups. The lower the DVB level, the higher the water retention and the more