Ion Exchange Resins
An ion exchange resin is a polymer with electrically charged sites at
which one ion may replace another. Natural soils contain solids with charged
sites that exchange ions, and certain minerals called zeolites are quite
good exchangers. Ion exchange also takes place in living materials because
cell walls, cell membranes, and other structures have charges. In natural
waters and in wastewaters, there are often undesirable ions and some of
them may be worth recovering. For example, cadmium ion is dangerous to
health but is not present usually at concentrations that would justify
recovery. On the other hand, silver ion in photographic wastes is not a
serious hazard, but its value is quite high. In either case, it makes sense
to substitute an ion such as sodium for the ion in the wastewater.
The Freundlich and Langmuir equations used for adsorption will also
describe ion exchange. However, the BET equation is not applicable because
multiple layers do not form; electrical charge must balance. The Donnan
membrane equilibrium discussed in the section about membranes is also relevant
to ion exchange because the charges on the resin backbone are localized
in an situation analogous to charged proteins than cannot pass through
Synthetic ion exchange resins are usually
cast as porous beads with considerable external and pore surface where
ions can attach. Whenever there is a great surface area, adsorption plays
a role. If a substance is adsorbed to an ion exchange resin, no ion is
liberated. Testing for ions in the effluent will distinguish between removal
by adsorption and removal by ion exchange. Of course, both mechanisms may
be significant in certain cases, and mass balances comparing moles removed
with moles of ions liberated will quantify the amounts of adsorption and
This next material is taken from a brochure about Dowex resins.
The amount of crosslinking depends on the proportions of different monomers
used in the polymerization step. Practical ranges are 4 % to 16 %. Resins
with very low crosslinking tend to be watery and change dimensions markedly
depending on which ions are bound. Properties that are interrelated with
(A) Moisture Content
A physical property of the ion exchange resins that changes with changes
in crosslinkage is the moisture content of the resin. For example sulfonic
acid groups attract water, and this water is tenaciously held inside each
resin particle. The quaternary ammonium groups of the anion resins behave
in a similar manner. The figure shows a plot of moisture content change
with changes in crosslinkage.
The total capacity of an ion exchange resin is defined as the total number
of chemical equivalents available for exchange per some unit weight or
unit volume of resin. The capacity may be expressed in terms of milliequivalents
per dry gram of resin or in terms of milliequivalents per dry gram of resin
or in terms of millequivalents per milliliter of wet resin.
The more highly crosslinked a resin, the more difficult it becomes to
introduce additional functional groups. Sulfonation is carried out after
the crosslinking has been completed and the sulfonic acid groups are introduced
inside the resin particle as well as over its surface. Likewise, the quaternary
ammonium groups are introduced after the polymerization has been completed
and they too are introduced both inside the particle as well as on its
surface. Fewer functional groups can be introduced inside the particles
when they are highly crosslinked and hence the total capacity on a dry
basis drops slightly.
This situation is reversed when a wet volume basis is used to measure
the capacity on a resin. Although fewer functional groups are introduced
into a highly crosslinked resin, these groups are spaced closer together
on a volume basis because the volume of water is reduced by the additional
crosslinking. (See above figure). Thus the capacity on a wet volume basis
increases as cross-linking increases. The next figure describes the changes
in capacity as crosslinking is changed.
(C) Equilibration Rate
Ion exchange reactions are reversible reactions with equilibrium conditions
being different for different ions. Crosslinkage has a definite influence
on the time required for an ion to reach equilibrium. An ion exchange resin
that is highly crosslinked is quite resistant to the diffusion of various
ions through it and hence, the time required to reach equilibrium is much
longer. Figure 3 illustrates the crosslinkage effect on the time required
for ethylene glycol to reach equilibrium.
In general, the larger the ion or molecule diffusing into an ion exchange
particle, or the more highly crosslinked the polymer, the longer will be
the time required to reach equilibrium conditions.
(D) Summary of Crosslinkage Effects
Copolymers of styrene containing low amounts of divinylbenzene (1-4%) are
characterized as follows:
1. High degree of permeability.
2. Contain a large amount of moisture.
3. Capacities are lower on a wet volume basis.
4. Equilibrium rates are high.
5. Physical stability is reduced.
6. Selectivity for various ions is decreased, but ability to accommodate
larger ions is increased. Copolymers of styrene containing high amounts
of divinylbenzene (12-16%) exhibit characteristics in the opposite direction.
The physical size of the resin particles is controlled during the polymerization
step. Screen are used to sieve resins to get a fairly uniform range of
sizes. Mesh sizes in the following table refer to U.S. Standard screens.
A higher mesh number means more and finer wires per unit area and thus
a smaller opening.
||Diameter of Particles
|20 - 50
|50 - 100
|100 - 200
|200 - 400
(a) Equilibration Rate
particle size of an ion exchange resin influences the time required to
establish equilibrium conditions. There are two types of diffusion that
must be considered in an ion exchange equilibrium. The first is called
film diffusion or the movement of ions from a surrounding solution to the
surface of an ion exchange particle. The second is called internal diffusion
and is the movement of ions from the surface to the interior of an ion
exchange particle. Film diffusion is usually the controlling reaction in
dilute solutions whereas internal diffusion is controlling in more concentrated
solutions. The particle size of an ion exchange resin affects both film
diffusion and internal diffusion. A fine mesh particle presents more
surface area for film diffusion and also contains less internal volume
through which an ion must diffuse. A decrease in particle size thus shortens
the time required for equilibration. This figure illustrates the shortening
of equilibration times for decreases in particle sizes.
(b) Flow Rate
exchange processes are usually carried out in columns with the resin resting
on a suitable support. Liquids may be processed either up-flow or down-flow
through such columns. The spherical particles of ion exchange resin resist
the flowing of a liquid through or around them. The smaller the particle
size, the greater will be this resistance against which a liquid must flow.
This resistance goes up very rapidly when particles smaller than 100 mesh
are employed. This figure illustrates the decrease in flow rates with decreasing
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