Page 336 - Water and wastewater engineering

ION EXCHANGE 8-5

Properties of Ion Exchange Resins
Exchange Capacity. One of the major considerations in selecting an ion exchange resin is the

quantity of counterions that can be exchanged onto the resin. This quantity is called the exchange
capacity of the resin. The total capacity is dependent on the quantity of functional groups on a resin
bead. The exchange capacity may be reported as milliequivalents per gram of dry resin (meq/g) or
as milliequivalents per milliliter of wet resin (meq/mL). The typical dry-weight capacity of a strong
acid cation exchange resin falls in the range of 3.6 to 5.5 meq/g. Typical wet-volume capacity is
1.8 to 2.0 meq CaCO 3 /mL.

Selectivity. Ion exchange resins have a variable preference or affinity for the ions in solution.
This preference or affinity is called selectivity. Quantitatively, for a binary exchange, selectivity
i
may be expressed as a selectivity coefficient ( K j ). For the generalized cation reaction

n
n[]RA + B n + [ R ]B n + nA + (8-4)
the equilibrium expression is

+ n n++
[ A ] {RB }
A (8-5)
K =
B n n+
{RA } [B ]

A
where K apparent equilibrium constant or selectivity coefficient for cation (or
B
anion) A exchanging with ion B onto resin

[A ] aqueous-phase concentration of presaturant ion, eq/L
n
[B ] aqueous-phase concentration of counterion, eq/L
{ RA }, { RB n } activities of resin-phase presaturant ion and counterion, respectively
n valence of the exchanging ion
A similar expression can be written for anions. Although the activity terms are a function of
ionic strength, concentrations are used in practice because they are measured more easily than
activities. Thus, Equation 8-5 is conveniently expressed as

( C j ) n q i
i
K (8-6)
j
Cq() n
i j
i
where K equilibrium constant for cation (or anion) i exchanging with ion j
j

C j aqueous phase concentration of presaturant ion j, meq/L
q i resin phase concentration of counterion i, meq/L
C i aqueous phase concentration of counterion i, meq/L
q j resin phase concentration of presaturant ion j, meq/L
n valence of the exchanging ion