Principles of Water Treatment
DOE-HDBK-1015/2-93
WATER TREATMENT PROCESSES
Rev. 0
CH-04
Page 7
A particular resin may be prepared in different forms according to the identity of the
exchangeable ion attached. It is usually named according to the ion present on the active sites.
For example, the resin represented by R-Cl is said to be the chloride form of the anion resin, or
simply the chloride form resin. Other common forms are the ammonium form (NH -R), hydroxyl
4
form (R-OH), lithium form (Li-R), and hydrogen form (H-R).
The mechanics of the ion exchange process are somewhat complicated, but the essential features
can be understood on the basis of equilibrium concepts discussed in Module 1 and recognition
that the strength of the ionic bond between the resin and an ion varies with the particular ion.
That is, for a particular resin, different ions experience different attractions to the resin. The
term affinity is often used to describe the attraction between a resin and a given ion. This affinity
can be described quantitatively by experimental determination of a parameter called the relative
affinity coefficient. For a qualitative discussion, it suffices to note the relative affinities between
a resin and different ions.
In order of decreasing strength, the relative affinities between a cation resin and various cations
are as follows.
Ba > Sr > Ca > Co > Ni
> Cu
> Mg > Be
+2
+2
+2
+2
+2
+2
+2
+2
Ag > Cs > Rb > K NH > Na > H > Li
+
+
+
+
+
+
+
+
Similarly, the relative affinities between an anion resin and various anions are as follows.
SO
> I > NO > Br > HSO > Cl > OH > HCO > F
4
3
3
3
-2
-
-
-
-
-
-
-
-
The physical arrangement of one type of ion exchange vessel for purifying water is shown in
Figure 3. The ion exchange resin is contained in a vessel with a volume of several cubic feet.
Retention elements at the top and bottom consist of screens, slotted cylinders, or other suitable
devices with openings smaller than the resin beads to prevent the resin from escaping from the
vessel. The resin bed is a uniform mixture of cation and anion resins in a volume ratio of 2 parts
cation resin to 3 parts anion resin. This arrangement is called a mixed-bed resin, as opposed to
an arrangement of cation and anion resins in discrete layers or separate vessels. The use of
different volumes of the two types of resins is due to the difference in exchange capacity between
cation and anion resins. Exchange capacity is the amount of impurity that a given amount of
resin is capable of removing, and it has units of moles/ml, equivalents/ml, or moles/gm. The
anion resin is less dense than the cation resin; thus, it has a smaller exchange capacity, and a
larger volume is needed for anion resins than for the cation resins to obtain equal total exchange
capabilities.
