Definition of terms and description of electromenbrane processes

Before discussing the various electromembrane processes and their applications in detail, the definition of some basic terms and the principle of these processes as well as the main components determining their performance shall be described briefly.

1.1.1 The ion-exchange membranes, and their structure and function

The key components in electromembrane processes are the ion-exchange membranes, which resemble ion-exchange resins in sheet form. There are two different types of ion-exchange membranes: (1) cation-exchange membranes, which contain negatively charged groups fixed to the polymer matrix, and (2) anion-exchange membranes, which contain positively charged groups fixed to the polymer matrix. In a cation-exchange membrane, the fixed negative charges are in electrical equilibrium with mobile cations in the interstices of the polymer as indicated in Fig. 1, which shows schematically the matrix of a cation-exchange membrane with fixed anions and mobile cations, the latter are referred to as counterions. The mobile anions are referred to as co-ions. They are more or less completely excluded from the polymer matrix because of their electrical charge, which is identical to that of the fixed ions. Due to the exclusion of the co-ions, a cation-exchange membrane permits predominantly the transfer ofcations.

© Counter-Ion A Co-Ion Q Fixed-Ion

Polymer Matrix

Figure 1 Schematic drawing illustrating the structure of a cation-exchange membrane.

Anion-exchange membranes carry positive fixed charges. Therefore, they are mainly permeable to anions. The properties of ion-exchange membranes are determined by different parameters such as the density of the polymer network, the hydrophobic or hydrophilic character of the matrix polymer, and the type and concentration of the fixed charges in the polymer. To which extent the co-ions are excluded from an ion-exchange membrane depends not only on membrane properties but also on the ion concentration of the feed solution.

The most desired properties of ion-exchange membranes are high permselectivity and low electrical resistance and good mechanical, chemical, and thermal stability. Many of today's available membranes meet most of these requirements. Especially, the cation-exchange membranes based on fluorocarbon polymers have quite satisfactory properties.

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