Principles of Ion Exchange

Ion exchange is a process in which ions on the surface of a solid are exchanged for ions of a similar charge in a solution with which the solid is in contact.

Ion exchange can be used to remove undesirable ions from waste water. Cations (positive ions) are exchanged for hydrogen or sodium, and anions (negative ions) for hydroxide or chloride ions.

The cation exchange on a hydrogen cycle can be illustrated by the following reaction, using, in this example, the removal of calcium ions, which are one of the ions (Ca2+ and Mg2+) that cause hardness of water:

where R represents a cation exchange resin.

The anion exchange can be similarly illustrated by the following reactions:

When all the exchange sites have been replaced with calcium or sulfate ions, the resin must be regenerated. The cation exchanger can be regenerated by passing a concentrated solution of sodium chloride or a strong acid through the bed, while the anion exchanger, which in this case is of hydroxide form, must be treated by a solution of hydroxide ions, e.g., sodium hydroxide.

Ion exchange is known to occur with a number of natural solids, such as soil, humus, metallic minerals and clay.

Clay, and in some instances other natural materials, can be used for demineralization of drinking water. In the context of adsorption, the ability of aluminum oxide to make a surface ion exchange should be mentioned. The natural clay mineral, clinoptilolite, can be used for waste water treatment as it has a high selectivity for removal of ammonium ions; see also Section 5.8.

Synthetic ion exchange resins consist of a network of compounds of high molecular weight to which ionic functional groups are attached. The molecules are cross-linked in a three-dimensional matrix and the degree of the cross-linking determines the internal pore structure of the resin. Since ions must diffuse into and out of the resin, ions larger than a given size may be excluded from the interaction through a selection dependent upon the degree of cross-linking. However, the nature of the groups attached to the matrix also determines the ion selectivity and thereby the equilibrium constant for the ion exchange process. The cation exchangers contain functional groups such as sulfonic R-S03-H - carboxylic, R-COOH - phenolic, R-OH and phosphonic, R-P03H2 (R represents the matrix). It Is possible to distinguish between strongly acidic cation exchangers derived from a strong acid, such as H2S04, and weakly acidic ones derived from a weak acid, such as H2C03. It is also possible to determine a pK-value for the cation exchangers in the same way as for acids generally. Thus:

Anion exchange resins contain such functional groups as primary amine, R-NH2, secondary amine, R-R1NH, and tertiary amine R-R1-R2N groups and the quaternary ammonium group R-RiR2R3N+OH".

It can be seen that the anion exchanger can be divided into weakly basic and strongly basic ion exchangers derived from quaternary ammonium compounds.

It is also possible to introduce ionic groups onto natural material. This is done by using cellulose as a matrix, and due to the high porosity of this material it is possible to remove even high molecular weight ions such as proteins and polypeptides.

Preparation of cation exchange resin, using hydrocarbon molecules as a matrix, is carried out by polymerization of such organic molecules as styrene and methacrylic acid. The degree of cross-linking in styrene is determined by the amount of divinylbenzene added to the polymerization. This can be illustrated by the example shown Fig. 9.1.

Figure 9.1. Polymerization of styrene and vinylbenzene to form polystyrene with degree of cross-linking.

It is characteristic that the exchange occurs on a chemical equivalent basis. The capacity of the ion exchanger is therefore usually expressed as equivalents per liter of bed volume.

When the ion exchange process is used for reduction of hardness, the capacity can also be expressed as kg of calcium carbonate per m3 of bed volume. Since the exchange occurs on an equivalent basis, the capacity can be found based either on the number of ions removed or the number of ions released. Also, the quantity of regenerant required can be calculated from the capacity. However, neither the resin nor the regeneration process can be utilized with 100% efficiency.

Figure 9.2. Illustration of the preference of an ion exchange resin for a particular ion. The selectivity coefficient at 50% in solution can be found from the diagram to be 82/18 =4.6.

% in solution

Figure 9.2. Illustration of the preference of an ion exchange resin for a particular ion. The selectivity coefficient at 50% in solution can be found from the diagram to be 82/18 =4.6.

Figure 9.2 illustrates the preference of an ion exchange resin for a particular ion. The percentage In the resin is plotted against the percentage in solution.

The selectivity coefficient, Kab, is not actually constant, but is dependent upon experimental conditions. A selectivity coefficient of 50% in solution is often used = a-50%.

If we use concentration and not activity, it will involve, for monocharged ions: CB = CA

The selectivity of the resin for the exchange of ions is dependent upon the ionic charge and the ionic size. An ion exchange resin generally prefers counter ions of high valence. Thus, for a series of typical anions of interest in waste water treatment one would expect the following order of selectivity:

Similar for a series of cations:

But this is under circumstances where the internal pore structure of the resin does not exclude the ions mentioned from reaction. Organic ions are often too large to penetrate the matrix of an ion exchange, an effect which is, of course, more pronounced when the resins considered have a high degree of cross-linking. As most kinds of water and waste water contain several types of ions besides those which must be removed it is naturally a great advantage to have a resin with a high selectivity for the ions to be removed during the ion exchange process.

The resin utilization is defined as the ratio of the quantity of ions removed during the actual treatment to the total quantity of ions that could be removed at 100% efficiency; this is the theoretical capacity. The regeneration efficiency is the quantity of ions removed from the resins compared to the quantity of ions present in the volume of the regenerant used. Weak base resin has a significant potential for removing certain organic compounds from water, but the efficiency is highly dependent upon the pH.

It seems reasonable to hypothesize that an adsorption is taking place by the formation of a hydrogen bond between the free amino groups of the resin and hydroxyl- groups of the organic substance taken up. As pH decreases, so that the amino groups are converted to their acidic form, the adsorption capacity significantly decreases.

The exchange reaction between ions in solution and ions attached to the resin matrix is generally reversible. The exchange can be treated as a simple stoichiometric reaction. For cation exchange the equation is:

The ion exchange reaction is selective, so that the ions attached to the fixed resin matrix will have preference for one counter ion over another. Therefore the concentration of different counter ions in the resin will be different from the corresponding concentration ration in the solution.

According to the law of mass action, the equilibrium relationship for reaction (9.5) will give:

aA * aRBn where aB and aA are the activity of the ions B+ and An+ in the solution and correspondingly aRB and aRA are the activities of the resin in B- and A-form, respectively. Note that the activities are used, which means that the activity coefficients should be calculated as shown in Section 7.1.

As mentioned above the clay mineral, clinoptilolite, can take up ammonium ions with a high selectivity. This process is used for the removal of ammonium from municipal waste water in the U.S.A., where good quality clinoptilolite occurs. Clinoptilolite has less capacity than the synthetic ion exchanger, but its high selectivity for ammonium justifies its use for ammonium removal. The best quality clinoptilolite has a capacity of 1 eqv. or slightly more per liter. This means that 1 liter of ion exchange material can remove 14 g ammonium -N from waste water, provided all the capacity is occupied by ammonium ions. Municipal waste water contains approximately 28 g (2 eqv.) per m3, which means that 1 m3 of ion exchange material can treat 500 m3 waste water (which represents a capacity of 500 bed volumes). The practical capacity is, however, considerably less - 150-250 bed volumes - due to the presence of other ions that are taken up by the ion exchange material, although the selectivity is higher for ammonium that for the other ions present in the waste water. The concentration of sodium, potassium and calcium ions might be several eqv. per liter, compared with only 2 meqv. per liter of ammonium ions.

Clinoptilolite is less resistant to acids or bases than synthetic ion exchangers. A good elution is obtained by use of sodium hydroxide, but as the material is dissolved by sodium hydroxide a very diluted solution should be used for elution to minimize the loss of material. A mixture of sodium chloride and lime is also suggested as alternative elution solution.

The flow rate through the ion exchange column is generally smaller for clinoptilolite than for synthetic material resin -10 m/h as against 20-25 m/h.

The elution liquid can be recovered by air stripping, as mentioned in Section 7.6. The preconcentration on the ion exchanger makes this process attractive - the sludge problem is diminished and the cost of chemicals is reduced considerably. For further details about this method of recovery, see Jorgensen (1973 and 1975).

Another ion exchanger selective for nitrogen compounds is the above mentioned cellulose ion exchanger. It has a capacity of about 1 eqv./1, of which at least 50% is highly selective for proteins and other high molecular nitrogen organics. It makes the application of this ion exchanger attractive for industrial waste water with high concentrations of proteins and where recovery of the proteins is desirable.

A combination of chemical precipitation and ion exchange has developed as an alternative to the mechanical-biological-chemical treatment method. A flowchart of such a plant is shown in Fig. 9.3. After the chemical precipitation the waste water is treated on two ion exchangers (which, however, could be in one mixed bed column). The first ion exchanger is cellulose-based for removing proteins and reducing BOD5. The nitrogen concentration is here typically reduced from total N 30 mg/l to total N 15-20 mg/l due to the high selectivity of the cellulose-based ion exchanger for organic nitrogen compounds. The second column could be either clinoptilolite and/or activated alumina. A plant using this process has been in operation since 1973 in Sweden, giving results comparable with or even better than the generally applied 3 steps treatment (see Table 9.1).

Ion Exchange Whey

Figure fl.3. Flowchart of a combination of chemical precipitation and ion exchange. (A) a submersible pump, (B) the settling basin, (C) an intermediate vessel, where carbon dioxide Is added, (D) a carbon dioxide container (50 atm., 25 liters.), (E) a pump feeding the ion exchangers, (F) elution liquid, (G) a hand-pump, (H) a dosing pump.

The capital cost and operating costs are approximately the same as for a three-steps plant. However, the plant produces 2-4 times less sludge than the normal 3 step plant, giving a correspondingly lower sludge treatment cost.

0 0

Post a comment