Principle of ion exchange

Dealing with ion exchange applications in waste water treatment requires full knowledge about water composition to judge competing effects

Constitution and operating principles

Synthetic and industrially produced ion exchange resins consist of small, porous beads that are insoluble in water and organic solvents. The most widely used base-materials are polystyrene and polyacrylate. The diameter of the beads is in a range of 0.3 to 1.3 mm. The beads contain around 50% of water, which is dispersed in the gel-structured compartments of the material and in the pores.

Since water is dispersed homogenously through the bead, water soluble materials can move freely, in and out. To each of the monomer units of the polymer, so called “functional groups” are attached. These functional groups can interact with water soluble species, especially with ions. Ions are either positively (cations) or negatively (anions) charged. Since the functional groups are also charged, the interaction between ions and functional groups is exhibited via electrostatic forces. Positively charged functional groups (e.g. a quarternary amine) interact with anions and negatively charged functional group (e.g. a sulfonic-, phosphonic- or carboxylic acid group) will interact with cations.

The binding force between the functional group and the attached ion is relatively loose. The exchange can be reversed by another ion passing across the functional group. Then another exchange reaction can take place and so on. One exchange reaction can follow another.

The principle of selectivity

There is a huge variety of existing ions. Let us especially have a look at the different types of cations. We distinguish different types of alkali-cations (sodium, potassium, …), of earth- and rare-earth-alkali-cations (magnesium, calcium, strontium, barium, lanthanium, iridium, …), heavy metal ions (lead, cobalt, nickel, mercury, …), light metal cations (aluminium, beryllium, titanium, …), semi-metal-cations (germanium, gallium, indium, ..), noble metal cations (gold, silver, platinum, palladium, …).

By investigating the adsorption of different kinds of cations on ion exchange materials, it is found that different ions do interact differently with the functional groups of ion exchangers. Some are strongly bound, some are less strongly bound.

This results in the fact, that a weakly bound ion can preferably be displaced by a stronger binding ion. This effect is called the principle of “selectivity”. A more selective ion binds more strongly than a less selective ion.

The effect of selectivity can be used to remove distinct ions from water and to replace them with others. Applications are found not only in industry, but also in household, where the use of ion exchangers for water softening is well known. In water softening applications the hardness causing calcium-cations are bound and exchange the weaker binding sodium or hydrogen cations.

Especially for waste water applications so called “chelating resins” have been developed, which have a high selectivity for toxic heavy metal ions. These materials selectively adsorb the toxic components from the waste stream and leave less toxic components, such earth alkali and alkali metals untouched.

The high affinity of chelating resins towards heavy metal ions is accomplished via the formation of a so called “complex – bonding”. In this special type of bonding the functional group and the ion do not only interact via electrostatic forces. Additionally there is a so called “coordinative bonding” which supports and strengthens the interaction, making it tighter than a usual binding. In figure 7 there is an example of this type. The exchange of sodium ions by a copper ion is shown. The copper is bound via two electrostatic types of bonds and by the interaction of the sole electron pair of the nitrogen atom of the functional group.

The selectivity of ion exchange resins is expressed via the so-called selectivity series. Figure 8 shows the selectivity series for an IDA resin. As can be seen, the heavy metal ions bind more strongly than the alkali earth ions and these bind more strongly than alkali metals.

The principle of equilibrium and mass action

Selective ions displace less selective ions as mentioned previously. The exchange continues as long as there are enough selective ions available for the resin to exchange and/or until the functional groups are saturated with the selective ions.

In some cases this means that all of the functional groups have lost the less selective ion and have taken up a selective ion. It can also mean that the exchanger reaches a certain level of loading with a certain ratio of functional groups that remain un-exchanged and still carry the less selective ions. This effect is called the effect of equilibrium or in other words the effect of mass action.

This effect comes from the fact that the less selective binding ions still affect the degree of exchange. Due to the weaker interaction force however, its effect is not as great. Even though the attraction forces are weak, the effective force increases with the concentration of the less selective ions. For example, if the attraction force is 100 times weaker but the concentration is 100 times higher; the less selective ions can equal the higher selective species in the final reaction. This is called the impact of mass action. As a result, to understand the exchange rate in a certain exchange situation, both the binding force and the concentration levels must be considered.

Once water composition is understood, a better understanding of the efficiency of the process can be realized. If the ratio of concentrations of less selective to highly selective ions is low, high operating capacities resins can be expected. If the ratio of concentrations is high, lower operating capacities will be realized. Therefore the efficiency of a waste water treatment process always depends on the water composition which in most of the cases is so unique as the individual production process that is the source where it is deriving from.

Dr Stefan Neumann