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 membranes.

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 ion exchange.

This next material is taken from a brochure about Dowex resins.

Crosslinkage

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 crosslinking are:

(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.

(B) Capacity

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.

Particle Size

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.
 
Mesh Range
Diameter of Particles
20 - 50 0.0331-0.0117 Inches 840-297  Micrometers
50 - 100 0.0117-0.0059 297-149
100 - 200 0.0059-0.0029 149-74
200 - 400 0.0029-0.0015 74-38
minus 400 < 0.0015 < 38 

              
(a) Equilibration Rate
The 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

Ion 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 particle sizes.
More properties

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