H R
R OH
Ca
SO4
Ca R
R SO4
H
OH
H R
R OH
Na
Cl
Na R
R Cl
H
OH
WATER TREATMENT PROCESSES
DOE-HDBK-1015/2-93
Principles of Water Treatment
CH-04
Rev. 0
Page 12
Consider a situation in which the entering impurities are calcium sulfate (CaSO ) and sodium
4
chloride (NaCl), and the ion exchanger is a mixture of both hydrogen and hydroxyl resins
(mixed-bed).
(4-3)
(4-4)
In the reaction with NaCl, both Na and Cl ions are removed from solution and replaced by H
+
-
+
and OH ions, respectively (the CaSO reaction has the same result). If the initial solution
-
4
contained only NaCl, then the concentrations of Na and Cl ions were equal. Because both are
+
-
removed with 100 percent efficiency, the concentrations of H and OH ions added to the
+
-
solution are equal; thus, the solution is neutral. In solution, H and OH ions must obey the
+
-
relationship for the ionization of water (refer to Module 1).
K = 10
= [H ] [OH ]
w
-14
+
-
Because of the very small value of the dissociation constant K , the great majority of H and
w
+
OH ions supplied by the resin must combine to form water by the following reaction.
-
H + OH H O
(4-5)
+
-
2
By this process, the original NaCl solution becomes pure water.
The preceding examples involve hydrogen and hydroxyl resins. The use of other resins,
especially cation resins, is very common. For instance, suppose a solution containing Na ions
+
is passed through a lithium resin. Again referring to the relative affinities, Na is attracted to
+
the resin more strongly than is Li ; thus, Na ions will displace Li from the resin.
+
+
+
