Ionic exchange may be defined as a physical-chemical phenomenon in which a solu-tion changes ions with the surface of a porous solid, as depicted in Figure 5.13. The techniques used in ionic exchange are so similar to those used in adsorption that, for most engineering applications, it can be considered as a special case of adsorption.
In ionic exchange, the fluid phase is an aqueous electrolytic phase, and the solid is an electrolyte that is insoluble in the liquid, usually called ion exchange resin. The result of the operation is an exchange reaction between the electrolyte in solution and the resin.
Ionic exchange has several applications, such as water softening for steam generation, water deionization, purification of pharmaceutical products (such as vitamin B and antibi-otics), metallurgic processes, and the fractioning of mixtures through chromatography.
The development and scale-up of equipment which make use of ionic exchange are made in the same way as for adsorptive processes. The ionic exchange rate is dependent on the processes of mass transfer of ions from the solution to the sur-face of the solid particle, diffusion through the pores to the interior of the solid, ion exchange, and diffusion of the exchanged ions outside the pores, in a process similar to that described for adsorption.
5.8.1 ionic exchangers
Ionic exchangers are porous, insoluble solids, with ions and water in their structure.
There is a network of fixed ions and also moving ions (anions or cations) that are susceptible to exchange by ions of the same electrical charge present in electrolytic solutions. The solid acts in a selective way, by removing from those solutions the ions for which it has more affinity.
3.5
FIGuRE 5.12 Equilibrium condition for the process described in Example 5.1.
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According to their structure, ion exchangers are divided into mineral (either nat-ural or synthetic) and organic. According to their function, they can be cationic, anionic, or amphoteric, depending on the ions that can be exchaged with an external solution. They can be further divided into weak or strong exchangers, depending on the degree of exchange that can be achieved.
Cationic exchangers are those that have acid functional groups in their struc-ture, thus rendering them negatively charged; they are neutralized by cations that can be exchanged with cations present in solution. The main acid functional groups used to produce cationic exchangers are sulfonic acid and its derivatives (for strong exchangers) and carboxilic acids and their derivatives (for weak exchangers), and sodium (Na+) and hydrogen (H+) ions are the most frequently used exchange agents.
Anionic exchangers, in turn, have basic functional groups in their structure, ren-dering it positively charged; it is neutralized by anions that can be exchanged with anions present in solution. The main functional groups used are quaternary ammo-nium compounds and their derivatives (for the strong exchangers) and derived ter-tiary amines (for the weak exchangers), and chloride (Cl–) and hydroxide (OH–) ions are commonly used exchange agents.
FIGuRE 5.13 An anionic exchange process.
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Finally, amphoteric exchangers have both types of functional groups in their structure, thus acting as anionic and cationic exchange agents simultaneously.
5.8.2 the ionic exchange MechanisM
The mechanism describing ionic exchange is not yet fully understood, but there are some theories that explain satisfactorily the phenomena taking place. The electric double-layer theory is based on the principle that the contact between two phases with different chemical compositions containing charged molecules generates a differ-ence of potential where the separation of charges can be observed. There are several possible structures for the electric double layer. Adopting an anionic exchanger as an example, where the solid phase is positively charged and the ions to be exchanged are negative (anions), three structures may be described:
Helmholtz’s double layer
• : it considers that all the anions needed to neutral-ize the electric potential of the solid form a single layer around the solid, situated at a minimal distance d from its surface (Figure 5.14a).
Gouy’s double layer
• : its structure is completely diffuse, diminishing the charge intensity as a function of the distance to the solid’s surface (Figure 5.14b).
Stern’s double layer
• : this is an intermediate structure between the previous two: part of the anion layer is electrically dense and close to the solid’s sur-face, and the other part, farther from that sursur-face, is diffuse (Figure 5.14c).
The same mechanisms are valid when the solid is negatively charged and there are cations in solution.
It is worthwhile to note that electric layers are not as static or homogeneous as the above definitions may suggest; in fact, at different locations on the exchanger’s surface, different structures may be observed.
5.8.3 ion exchange equiliBriuM
An ion exchanger consists basically of a solid matrix with charged ionic groups (positive or negative) bound to it. Ions with opposed charges, called counterions, neutralize those charges, thus keeping the system in equilibrium. When placed in contact with an electrolytic solution, the ion exchanger allows the replacement of the ions in solution by the ions (of the same charge) in its structure; this induces a perturbation in the system until a new equilibrium is attained.
Ionic exchange occurs in a similar way as a stoichiometric reaction: with the decrease of the concentration of the counterions on the adsorbent’s surface, their concentration in the solution increases; simultaneously, the concentration of the ions in the original solution is decreased as their concentration increases at the surface of the adsorbent.
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FIGuRE 5.14 Three different types of electric double layer in ionic-exchange processes:
(a) Helmholtz, (b) Gouy, and (c) [following page] Stern.
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