Environmental Engineering Reference
In-Depth Information
ION-EXCHANGE RESIN
FLUID PHASE
PORE
1
2
4
3
+
+
A
B
Figure 8.1
Schematic diagram of a cation-exchange resin.
An alternative equation is:
B
+
+
A
+
↔
A
+
+
B
+
.
(8.2)
Ion exchange will continue until the equilibrium described by Equation (8.1) is reached.
Note that equilibrium does not imply equal concentrations of each ion in the resin and
fluid phase. Also, ion diffusion is coupled with charge neutrality and not solely due to
concentration differences.
Like adsorption, the mass-separating agent is the resin material. The transport steps that
take place during ion exchange are also similar to adsorption.
1 Transport of the exchanging ions to and from the bulk solution to the surface film
(boundary layer) surrounding the resin;
2 transport of the exchanging ions through the surface film (or boundary layer) at the
external surface of the particle;
3 interstitial (pore) transport of the exchanging ions to the sites of active exchange; and
4 kinetics of the exchange process.
Again, as with adsorption, Steps 2 and 3 are typically the slowest and rate controlling. The
nature of the rate-determining step can be predicted by use of the simple dimensionless
criterion given by Helfferich [2, 3]:
C
D
δ
CDr
0
(5
+
2
α
AB
)
1
pore transport
(8.3)
C
D
δ
CDr
0
(5
+
2
α
AB
)
1
boundary layer
,
(8.4)
where
is the boundary-
layer thickness of th
e fl
uid adjacent to the resin particle surface,
C
is the resin-phase
concentration of ions,
D
is the diffusion coefficient in the resin phase,
C
is the concentration
of ions in the solution phase, and
D
is the diffusion coefficient in the solution phase.
If film diffusion is much faster than diffusion within the ion-exchange particles, then
concentration differences in the liquid are very small.
α
is the separation factor,
r
0
is the radius of an ion-exchange be
ad
,
δ
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