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12.4 CHAPTER TWELVE

in the copolymer structure and are due to molecular spacing. The macroporous resins have
actual physical pores in the resin bead. The macroporous types have high surface area and
a higher surface-to-volume ratio than the gel types. Macroporous resins are currently made
in two different ways. The original (first-generation) method agglomerated microspheres
into large spherical beads. A second process (second generation) is based on extraction
technology in which an inert substance is added to the reacting monomers mixture. After
polymerization the inert substance is removed from the resulting copolymer beads, leav-
ing behind discrete pores.
Historically, macroporous resins were originally made to provide physical stability to
very highly cross-linked cation exchange resins. By their nature, macroporous resins are
more resistant to stresses brought on by changes in operating conditions. Highly cross-
linked macroporous resins are basically better able to withstand severe and rapid changes
in operating environments such as sudden and very high flow rates, sudden temperature
and pressure changes, very high temperature and oxidative environments. Increasing cross-
linkage in gel resins to gain thermal and oxidative resistance also decreases kinetics and
increases brittleness, which leads to reduced physical stability. These effects can be re-
duced by introducing macroporosity. The volume of the discrete pores provides stress re-
lief but also reduces the amount of copolymer within a bead and therefore reduces the
volumetric capacity of the resin. This reduced volumetric capacity makes macroporous
resins less desirable in most bulk water treatment applications.
However, macroporosity has been proved superior for the high-capacity weakly ion-
ized resins, due to the macroporous resins' ability to accommodate the very significant
size change that these resins undergo between the exhausted and regenerated ionic forms.
Inorganic Ion Exchangers and Zeolites. Inorganic zeolites are no longer commonly
used in softening or in bulk deionization. However, they exhibit high affinities for cer-
tain substances which makes them ideal to remove specific contaminants. Usually these
products are not stable at all pH values, so pH control is required. The major categories
of commercially available inorganic exchanges and zeolites are green sand, activated alu-
mina, and aluminosilicates. They are pH-sensitive.
Green sand and some aluminosilicates are manufactured from mined deposits. Alu-
minosilicates can also be synthesized. Green sand is primarily used for removal of iron
and manganese. Some of the aluminosilicates such as clinoptilolite and chabasite are used
for the removal of ammonia from wastewater and in the nuclear power industry for re-
moval of specific radio nuclides such as cesium and rubidium. Activated alumina is a
highly processed form of aluminum oxide. It has a high selectivity for fluoride, arsenic,
and lead.

Adsorbents. In certain applications, the nonfunctional polymers, especially those with
macroporous structures, are used as adsorbents. They generally have limited or no ion ex-
change functionality. Instead, they have certain characteristics such as static charge,
copolymer structure, or pore size that make them useful for some types of chemical sep-
arations. They are not widely used in water treatment.
Nature of the Ion Exchange Process. When salts dissolve in water, they separate into
charged ions. Cations carry positive charges, and anions carry negative charges. With this
in mind, there are two classes of ion exchange resins: those that exchange cations and
those that exchange anions. Each of these classes of exchangers is further divided into
strongly ionized and weakly ionized, according to the nature of the functional groups. Al-
most all ion exchange materials can be classified in this manner.
The general ion exchange reactions shown below are for cation softening and anion
softening. The following equations show the exchange of sodium ions from the sodium