Although not desalination, the exchange of sodium ions
Some naturally occurring and
for divalent cations produces a change in the type of
synthetic materials can exchange one type of ion for
salinity. This change in the salinity reduces the levels of
another dissolved ion of like charge, e.g., one positive
the calcium and magnesium ions, such that the
ion for another positive ion. The number of charges on
concentration of other ions in the reject or blowdown
the ions removed from solution must equal the number
stream can be increased in the desalination process with
of charges on the ions exchanged from the material.
a resultant increase in water recovery. Saturation of
The major types of synthetic ion-exchange resins that
scale-forming materials, such as calcium carbonate,
have been developed are as follows: strong acid cation
calcium sulfate, and magnesium hydroxide, is still
resins, weak acid cation resins, strong base anion
reached, but the total concentration of salts in the reject
resins, and weak base anion resins. Strong acid and
attains a higher overall level than would be possible
weak acid cation resins exchange hydrogen ions (H+)
for other cations. Strong acid cation resins may also
exchange monovalent sodium ions (Na+) for such
processes can best utilize dealkalization-softening as a
divalent cations as calcium (Ca+ +) and magnesium
pretreatment method when the raw water contains high
(Mg+ +). Strong base anion resins exchange hydroxyl
levels of alkalinity, bicarbonate and carbonate ions, and
(OH-) or bicarbonate (HCO3) ions for other anions.
high levels of hardness, calcium and magnesium ions.
Weak base anion resins adsorb acidic ionic materials,
A properly operated pretreatment that utilizes a weak
such as hydrochloric acid, sulfuric acid, and carbonic
acid cation resin will produce water with an alkalinity
acid from solutions. Once adsorbed on the weak base
level of no more than 10 parts per million (as CaCO3)
anion resin, the anion part of the acid may be
and a residual hardness approximately equal to the
exchanged for other anions. These exchanges occur
original raw water non-carbonate hardness.
during the service cycle when treated water is produced.
(1) Treatment process. The major benefit of
When the capacities of resins have been used up or
dealkalization-softening using carboxylic (weak acid
exhausted, they are regenerated with acid or base or
cation) resins lies in the actual reduction of the
salt to restore the resin to the original ionic state.
dissolved solids content of the water. Hydrogen (H+)
Illustrations of the strong acid cation resin hydrogen ion-
ions from the resin exchange with the divalent calcium
cation exchange and the strong base anion hydroxyl ion-
and magnesium ions in the water. This exchange
anion exchange that occur in the complete
occurs only if the anions of weak acid salts, such as
demineralization of water are shown in figure 8-1.
bicarbonate or carbonate ions, are present. Carbonic
Pretreatment. Ion exchange can be used as a
acid is formed when the hydrogen and bicarbonate ions
pretreatment method in the desalination process to
react. The carbonic acid is weakly ionized and reverts
reduce the levels of sparingly soluble salts. A strong
to its basic constituents of carbon dioxide and water.
acid cation resin in the sodium form and a weak acid
The dissolved carbon dioxide can be removed by using
cation resin in the hydrogen form can be used. In both
degasification methods, as discussed in paragraph 5-3.
processes the levels of alkaline earth metal cations,
The combination of the weak acid cation exchange with
such as calcium (Ca+ +) and magnesium (MG+ + ), are
degasification reduces both the calcium and magnesium
reduced. The use of the strong acid cation resin in the
levels as well as the alkalinity level in the raw water.
sodium form is called water softening, and the use of
(2) Dealkalization-softening uses.
the weak acid cation resin in the hydrogen form in
pretreatment should be investigated when pH
conjunction with a carbon dioxide degasifier is called
adjustment of the raw water by an acid addition is
indicated for the desalination process. Weak acid resins
Water softening by sodium ion
use about 10-percent more acid than that required for
exchange can be used as a pretreatment method in a
pH adjustment alone and will reduce the calcium and
During water softening,
magnesium concentration as an additional advantage.
monovalent sodium ions on the strong acid cation resin
are exchanged for the divalent calcium and magnesium
in the water.