ion exchange.doc

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Fully Ionised Resin Systems

All ion exchange resins, whether cation or anion exchangers, strongly or weakly ionised, gelor macroporous, spherical or granular, can be viewed as solid solutions. The 2 phases in

ion exchange (i.e. the solution and the immobile solid) can be referred to simply asthe outside phase and the inside phase. Transfer of components takes place across theinterface between the phases, which is the surface of the bead or granule.

The inside phase of an ion exchange resin contains 4 necessary components:

A 3-dimensional polymeric network Ionic functional groups permanently attached to this network Counter-ions

A solvent

Under certain conditions there may be other components inside the resin such as a secondsolvent, co-ions and non-ionic solutes.

Polymeric network

Most ion exchange resins in commercial use are based on an organic polymer network,but inorganic polymers are also used. Regardless of the composition of the network,its primary function is to limit the solubility of the resin. The chemical nature of the

polymer network is a major factor in determining the physical and chemical stability of theresin.

The majority of strong-acid cation exchange resins and strong-base anion exchange resins arebased on the co-polymerisation of styrene and a cross-linking agent, divinylbenzene(DVB) to produce a 3-dimensional cross-linked structure. The structure is relatively welldefined and is fully ionised over the entire pH range.

The degree of cross-linking is governed by the ratio of DVB to styrene. These cross-linkedco-polymers swell in the presence of organic solvents, but have no ion exchange

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properties. To convert the co-polymers to water-swellable gels with ion exchangeproperties, ionic functional groups are added to the polymeric network.

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Functional Groups

Ionic sites, which give the polymer ion exchange properties, are added to the styrene-DVB polymer network by one or more chemical reactions.

Strong-acid cation exchange resins are prepared by sulfonating the benzene rings in thepolymer. The SO3

- groups are permanently fixed to the polymer network to give a negativelycharged matrix and exchangeable, mobile positive hydrogen ions.

The hydrogen ions can be exchanged on an equivalent basis with other cations such asNa+ , Ca2+ , K+ or Mg2+ , to maintain neutrality of the polymer. For example, 2 H+ ions areexchanged for 1 Ca2+ ion. The exchangeable ions are called counter-ions.

The liquid whose ions are being exchanged also contains other ions of unlike charge,such as Cl- for a solution of NaCl, where Na+ is exchanged. These other ions are called co-

ions.

Strong-base anion exchange resins require 2 reactions: chloromethylation and amination:

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The Cl- ions can be exchanged for other anions such as OH - , HCO3- , SO4

2- and NO3- .

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The Figure below showed a cation exchanger with fixed and mobile ions.

The ionised functional group converts the oil-swellable styrene-DVB copolymer to a water-swellable structure. A relatively low degree of functional substitution may be sufficient togive the copolymer some water swellability. The greater the degree of substitution, the moretendency there is for the bead to pull in water. The higher the cross-linking, the greater theresistance to uptake of water. At equilibrium, the amount of water taken up by the bead is afunction of the type and amount of ionic substitution, and the effective cross-linking of thepolymer structure. For most separation applications, the water content of the swollen beadvaries from 30 to 80%.


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The maximum ion exchange capacity of a strong-acid cation or strong-base anion exchangeris stoichiometric, based on the number of equivalents (eq) of mobile charge in theresin.

Thus, 1 mole H+ is one eq, whereas 1 mole Ca2+ is 2 eq. The exchanger capacity is usuallyquoted as eq/kg of dry resin or eq/L of wet resin, depending on whether wet or dry basisis used.

Fully ionised resins have a wet-volume capacity that varies with the water content, which isexpressed as equivalents per litre (eq/L). Volume capacity is a measurement ofthe maximum usefulness of the resin in a stoichiometric exchange. When all the availableexchange sites have been used, the ion-exchange process must be interrupted and theresin regenerated.

On the other hand, the dry-weight capacity is a measure of the extent of functional groupsubstitution in the resin. Fully ionised resins contains a fixed number of functional groupsper unit weight, and therefore have a constant dry-weight capacity. Most commercialresins have dry weight capacities of 5.0 ± 0.1 milliequivalents per dry gram (meq/g)

Commercial ion exchangers in the hydrogen, sodium and chloride form are available undertrade names of Amberlite, Duolite, Dowex, Ionac and Purolite. Typically they are in the formof spherical beads from about 40 m to 1.2 mm in diameter. When saturated with water, thebeads have typical moisture content from 40 - 65 wt%. When packed into a bed, they havebulk densities from 0.56 - 0.96 g/cm3 with fractional bed porosities of 0.35 - 0.40.

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Counter-ions

Fixed ionic sites in the resin structure must be balanced by a like number of ions of

the opposite charge to maintain electrical neutrality. These ions are called counter-ions.True ion exchange is the transfer of counter-ions between the outside and inside phases.Such an exchange is always on an equivalent basis.

Equivalency of exchange may be obscured if the exchange is accompanied by chemicalreaction. The most common case is exchange of an acid solution with the hydroxide form ofa resin. The anion of the acid enters the resin in exchange for a hydroxide ion which reacts with the hydrogen ion to form water.

Other reasons for an apparent lack of equivalency include the formation of precipitate or the

formation of a poorly ionised component which is physically adsorbed by the resin.





Solvent

Often water is the solvent in the resin. The amount of water in the inside phase is animportant factor in determining relative selectivity of the resin for different ions. The higherthe water content of the resin phase, the more the inside phase starts to resemble theoutside phase. As a general rule, as the water content increases, the difference inselectivity between ion species becomes less.

Any change in the solvent away from water changes the nature of the system.

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