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ION EXCHANGE APPLICATIONS IN WATER TREATMENT 12.3

In general, the polymerization reaction is carried out by suspending the reacting
monomers in a nonmiscible liquid containing a dispersant. The mixture is stirred, and the
monomers polymerize into cross-linked polymers in the form of spherical droplets, which
later become solid spherical hydrophobic copolymer beads. The beads are screened to the
proper size range and then converted to hydrophilic ion exchange resins by adding func-
tional groups through additional chemical reaction steps. Most ion exchange resins are
hydrated to the extent that chemically bound water comprises one-half the weight of the
final product after all surface moisture has been drained away.
Cation resins are created by attaching negatively charged functional groups to the
copolymer structure. Strongly ionized (strongly acidic) cation exchange resins have sul-
fate functional groups. These sulfate groups have a permanent negative charge which is
strongly attracted to the positively charged cations in water. A second group of cationic
exchangers is made by attaching carboxylic acid functional groups. These are not fully
ionized and are limited to specific applications. Another broad group of cation exchange
resins have functional groups that act as chelants; some examples are those based on imin-
odiacetate, aminophosphonic, and thiol groups.
Anion resins are functionalized by attaching chemical groups with positive charges to
the copolymer. While there are several chemicals that can be used to functionalize anion
resin, only a few are actually used in commercial products, and these are all based on
amines.
Type I (strong base) anion resins are functionalized with trimethylamine. Type II
(strong base) anion resins are functionalized with dimethylethanolamine. The resulting
structures of the amines in type I and type II anion resins are quaternary. These are fully
ionized. These resins are referred to as being strongly basic due to their ability to react
with neutral salts such as NaC1 to create NaOH (salt splitting). Less strongly ionized resins
have tertiary, secondary, and primary amine functional groups. These are referred to as
weakly basic. The weakly basic anion resins are generally limited to neutralization reac-
tions as they are unable to react effectively with salts. Most weakly basic resins exhibit
some strong base characteristics. Those with appreciable strong base capacity are some-
times called intermediate basic resins.
There is also a group of anionic chelating resins of the weakly basic type. Some ex-
amples are those based on picolylamine, thiouronium, and methylglucamine. These per-
form effectively at low pH and exhibit high selectivity for certain metals or metal
containing anionic complexes.

Size of Resin Beads
Ion exchange resins are homogeneous solids; most of the exchange groups are below the
surface of the resin bead. The exchange of ions from the liquid to the solid phase occurs
at the surface of the beads: The ions then diffuse through the solvated polymer from one
ion exchange site to another toward the center of the bead. To create a balance between
pressure loss and kinetics, the size of resins chosen for most water treatment applications
is a range between 0.3 and 1.2 mm in diameter. This size range has been proved to pro-
vide the best combination of flow equalization, pressure drop, and kinetics. Resins smaller
than 0.3 mm are in used in specialty applications. They have enhanced kinetic properties
that make them superior to larger resins for certain types of slow chemical reactions. How-
ever, their small size makes them more expensive to produce, requires special contain-
ment equipment, and creates higher pressure drops.
Gel versus Macroporous Resins. The physical structure of the commonly available ion
exchange resins is classified as macroporous or gelular. In the gelular types, the pores are