Civil Engineering Reference
In-Depth Information
2. For ions of the same valence, the extent of the exchange reaction increases with
decreasing hydrated radius and increasing atomic number, for example:
Ca
2
Mg
2
Be
2
and
K
Na
Li
This type of response is a result of swelling pressure within the resin. Ions of
larger hydrated radius increase the swelling pressure within the resin and de-
crease the affinity of the resin for such ion.
3. For a solution with a high total ionic concentration, the extent of the exchange
reaction follows no general rule and is often reversed. This type of response is
the basis for the reversibility of regeneration.
4. The relationship between the degree of cross-linking and the size of the hydrated
ion may affect the extent of the exchange reaction. If the resin has a high degree
of cross-linking, the ion may be too large to penetrate into the matrix of the
resin.
In general, it is advantageous to use a resin with a high affinity for the ion to be
exchanged. High affinities improve the kinetics of adsorption and allow higher hy-
draulic loading rates. Also, the greater the affinity, the sharper the breakthrough curve,
and hence the shorter the ion-exchange column. The only disadvantage of a high
affinity is that a higher regenerant concentration will be required.
Types of Ion Exchange Resins
The first ion-exchange materials were naturally occurring zeolite clays. Today, most
ion-exchange resins are synthetic materials made up of a polymeric matrix (generally
polystyrene chains held together by divinylbenzene cross-links), with soluble ionic
functional groups attached to the polymer chain. The number and kind of functional
groups determine the exchange capacity and ion selectivity, whereas the polymer ma-
trix determines the durability and toughness of the resin. Resins are granular in nature
and may have either a spherical or irregular shape. Although spherically shaped resins
are generally used, the irregularly shaped form provides a larger surface, and often a
lower void volume, which increases the exchange capacity per unit of volume.
Ion-exchange resins are usually classified in the following manner:
Cation-Exchange Resins (contain exchangeable cations):
Strong-acid exchange resins
•
Weak-acid exchange resins
•
Anion-Exchange resins (contain exchangeable anion):
Strong-base exchange resins
•
Weak-base exchange resins
•
Strong-acid exchange resins contain functional groups derived from a strong acid (nor-
mally sulfuric acid). Their degree of ionization is analogous to that of a strong acid
(low pKa), which permits the hydrogen to be dissociated and ready for exchange over
a wide pH range. Weak-acid exchange resins, on the other hand, contain functional
