WATER TREATMENT PROCESSES
DOE-HDBK-1015/2-93
Principles of Water Treatment
CH-04
Rev. 0
Page 4
There are two general types of ion exchange resins: those that exchange positive ions, called
cation resins, and those that exchange negative ions, called anion resins. A cation is an ion with
a positive charge. Common cations include Ca , Mg , Fe , and H . A cation resin is one that
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exchanges positive ions. An anion is an ion with a negative charge. Common anions include
Cl , SO , and OH . An anion resin is one that exchanges negative ions. Chemically, both types
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are similar and belong to a group of compounds called polymers, which are extremely large
molecules that are formed by the combination of many molecules of one or two compounds in
a repeating structure that produces long chains.
A mixed-bed demineralizer is a vessel, usually with a volume of several cubic feet, that contains
the resin. Physically, ion exchange resins are formed in the shape of very small beads, called
resin beads, with an average diameter of about 0.005 millimeters. Wet resin has the appearance
of damp, transparent, amber sand and is insoluble in water, acids, and bases. Retention elements
or other suitable devices in the top and bottom have openings smaller than the diameter of the
resin beads. The resin itself is a uniform mixture of cation and anion resins in a specific volume
ratio depending on their specific gravities. The ratio is normally 2 parts cation resin to 3 parts
anion resin.
In some cases, there may be chemical bonds formed between individual chain molecules at
various points along the chain. Such polymers are said to be cross-linked. This type of polymer
constitutes the basic structure of ion exchange resins. In particular, cross-linked polystyrene is
the polymer commonly used in ion exchange resins. However, chemical treatment of polystyrene
is required to give it ion exchange capability, and this treatment varies depending on whether the
final product is to be an anion resin or a cation resin.
The chemical processes involved in producing anion and cation resins are outlined in Figure 1
and Figure 2, beginning with the formation of cross-linked polystyrene. The polymer itself is a
covalent compound. By the chemical reactions indicated in Figure 2, hydrogen atoms covalently
bonded to the original polymer at certain sites are replaced by functional groups (called radicals)
such as SO H (sulfonic acid) and CH N(CH ) Cl (quaternary ammonium). Each such group is
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covalently bonded to the polymer, but each also contains an atom that is bonded to the radical
group by a predominantly ionic bond. In the two examples above, H in SO H and Cl in
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CH N(CH ) Cl are the ionically-bonded atoms. Sometimes these are written as SO H and
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CH N(CH ) Cl to emphasize their ionic characters. These ions (H and Cl ) are replaceable by
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other ions. That is, H will exchange with other cations in a solution, and Cl will exchange with
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other anions.
In its final form, an ion exchange resin contains a huge, but finite, number of sites occupied by
an exchangeable ion. All of the resin, except the exchangeable ion, is inert in the exchange
process. Thus, it is customary to use a notation such as R-Cl or H-R for ion exchange resins.
R indicates the inert polymeric base structure and the part of the substituted radical that does not
participate in exchange reactions. The term R is inexact because it is used to represent the inert
portion of both cation and anion resins, which are slightly different. Also, the structure
represented by R contains many sites of exchange, although only one is shown by the notation,
such as R-Cl. Despite these drawbacks, the term R is used for simplicity.
