Ion exchange - Principles of Chemical Separations with Environmental Applications - page 216

Concentration gradients can exist within the resin pore structure. Ion diffusion is com-
plex since the resin porosity is low and this leads to steric hindrance effects and tortuous
diffusion paths. Also, ion diffusion is coupled to the fixed ionic groups and the mobility of
each ion within the resin due to charge balance. This coupled diffusion is present in both
boundary layer and pore transport. Also, the forward and reverse rates of ion exchange
can be affected by the different mobilities of the ions.

8.5 Ion-exchange media

Some ion-exchange resins occur naturally and have been used for hundreds of years.
They include clay peat, charred bone, and natural aluminosilicates. The recognition of ion
exchange as a process is generally attributed to H. S. Thompson and J. Thomas Way, who
were English agricultural chemists. Thompson observed in 1848 that soil treated with
either ammonium sulfate or ammonium carbonate adsorbed the ammonia and released
lime. He reported his results to Way and then conducted systematic studies, 1850-54. In
1935, B. A. Adams and E. L. Holmes observed that crushed phenolic phonograph records
were capable of ion exchange. This observation led to the development of synthetic organic
ion-exchange resins. With this development, the industrial use of ion exchange was rapidly
increased.
Resins are typically synthesized by copolymerization of styrene and divinylbenzene
(DVB). The styrene provides the backbone, and DVB is used as a crosslinker to stabilize
the structure. The resin is then reacted with an acid or base to produce the fixed charged
groups. Crosslinking varies radially within the resin. The extent is usually described by a
'nominal DVB content'. The degree of crosslinking is important because it determines the
internal pore structure (see transport Step 3 above). The greater the percentage of DVB,
the less the resin will swell when ions are exchanged, but the resin will have a tight pore
structure with low mass transfer rates. Commercial resins are 2 to 12% DVB.
Resin beads are synthesized as gel or macroporous materials. The macroporous resins
are polymerized in the presence of a third component that is insoluble in the polymer.
After this insoluble component is removed, large pores remain that allow the ions to have
improved access to the interior pore structure of the beads. Macroporous resins can be
useful for large ions like proteins, but they are more expensive, have lower capacity, and
are harder to regenerate than the gel resins. However, they are said to be more resistant to
thermal and osmotic shock as well as to oxidation and organic fouling than the gel-type
resins [4].
The following factors are important in the choice of an ion-exchange resin:
1 Exchange capacity (loading or productivity).
2 Fraction or percent removal of various ions from the liquid phase (selectivity).
3 Particle size and size distribution (flow throughput considerations).
4 Chemical and physical stability.
5 Regeneration requirements (chemicals, amounts required, loss in capacity).