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12.2 CHAPTER TWELVE

cesses and for product purification and recovery for specialty separations, such as chro-
matographic separations by size, valance charge, and as catalysts. A full discussion of ion
exchange process theory can be found in Water Quality and Treatment.
Ion exchange, as its name implies, is the exchange of ions from one phase to another.
In water treatment, the exchange of ions occurs between the solid phase of the ion ex-
changer and influent water. For example, in a water softener, a cation exchange resin op-
erating in the sodium form exchanges sodium ions for an equivalent amount of calcium
and magnesium ions in the influent raw water. The result is that most of or all the cal-
cium and magnesium ions are removed from the raw water by the resin, and an equiva-
lent number of sodium ions are added to the water from the resin.
Ions, such as calcium, magnesium, barium, copper, lead, zinc, strontium, radium, am-
monium, fluorides, nitrates, humates, arsenates, chromates, uranium, anionic metallic
complexes, phosphates, perchlorates, hydrogen sulfide, bicarbonates, sulfates, and many oth-
ers, are routinely removed from water by ion exchange resins. Proper certifications are re-
quired for resins used in potable waters. The original regulations that applied to ion ex-
change resins are stated in paragraph 21CFR173.125 of the Food Additives Regulations of
the Food and Drug Administration (FDA). The common cation exchange resins used in wa-
ter softening and their strongly basic anion exchange resins counterparts, as well as other
commercially available resins at the time the regulations were issued, are listed by compo-
sition in the CFR. The listed types of resins are therefore acceptable for use in potable
water applications. However, the CFR requirement specifies that the end user test the resins
for compliance with maximum allowable extractables in order to be in compliance.
Several new classes of resins have been developed since the FDA-sponsored CFR was
issued. However standards and protocols for resins that are used in potable water appli-
cations are available through the American National Standards Institute (ANSI). These
include toxicology review of the ingredients used to manufacture the resin, operating ca-
pacity and effluent quality testing, and extraction tests under simulated start-up conditions
to make sure the effluent will be safe and the resin will perform as stated. Third-party
certification against the ANSI standards for resins used for potable water treatment is of-
ten mandated. Organizations such as NSF International and Water Quality Association
Gold Seal are certified to issue certifications against the ANSI adopted standards. Certi-
fication seals are issued with the name of the certifying organization and the number of
the ANSI adopted standard. Several more organizations are expected to be certified to
perform this service in the near future.
It is impossible to cover the subject adequately in a single chapter. In writing this chap-
ter, the authors hope to provide the reader with a general overview of the ion exchange
process, general information about how ion exchange systems are designed and built, and
impart enough information that the reader will be able to decide whether ion exchange is
an appropriate technology to solve a specific problem and to estimate the size and scope
of the equipment required. At the end of this chapter is a comprehensive Bibliography or-
ganized by topic for those wanting more details.

Chemical Structure of Ion Exchange Polymers
Almost all modern day ion exchange resins consist of an organic polymer, chemically
bonded to an acidic or basic functional group. Most ion exchange polymers are poly-
merized polystyrene cross-linked with divinylbenzene (DVB). Other polymers are used,
for example, acrylic or methacrylic acids, phenol formaldehyde, epoxy polyamine, and
pyridine-based polymers. The acrylics and epoxies are the most widely used in this group,
which, in total, comprise only a small percentage of the ion exchangers produced
commercially.