1.2 Electrodialysis
1.2.3 Ion-exchange membranes
Charged polymer membranes have been investigated for well over 100 years. Most natural and artificial membranes consist of polymers and the porous membranes investigated in the past inherently carry at least some ionizable groups (except in a few instances) and are therefore charged (Sélégny 1976). The basis of classic electrochemistry was founded around the turn of the 20th century with capacities like Ostwald, Nernst and Donnan being some of the most notable forerunners. Charged membranes have been investigated ever since, and while the results obtained in this field were mostly confusing during the 19th century, an increasing understanding of charged membranes and their basic functions has developed during the 20th century.
Commercially created ion-exchange membranes (IEM) are normally composed of crosslinked polymeric gels with fixed positive or negative charges. A fixed, immobile charge must be balanced by a dissolved mobile ion of opposite charge to preserve electroneutrality. Hence, to avoid precipitation or crystallization of the ionic groups inside an ion-exchange membrane, it is necessary to keep the membranes constantly wetted to preserve functionality. The polymers with high ion-exchange capacity have a strong tendency to swell, which is held in check by a high degree of crosslinking (Kesting 1985).
Two general different types of ion-exchange membranes exist. An ion exchange membrane with fixed positive charges allows primarily negative ions to enter and pass through the membranes internal matrix. This is then named an anion-exchange membrane (AEM). Typical fixed charges in anion exchange membranes are (Strathmann 1992):
+ +
+ +
+
+
− − = − −
− NH
3RNH
2R
3N R
2N R
3P R
2S
Similarly, a membrane with fixed negative charges allows the free passage of cations and is hence named a cation-exchange membrane (CEM). Fixed charges commonly used in cation exchange membranes are:
−
−
−
−
−
−
− − − − −
− SO
3COO PO
32HPO
2AsO
32SeO
3Normally the most strongly acidic or basic groups like the quaternary ammonium group (-R3N+) or the sulfonic group (-SO3-) are preferred in commercial membranes, since they are completely dissociated over nearly the entire pH-range.
The name "ion-exchange membrane" is perhaps a little misguiding, since ion exchange membranes work quite different from ion-exchange resins, normally employed in ion-exchange columns. In ion-exchange resins, ions of opposite charge are replaced by other ions of same general charge, while ions of the same charge as that of the resin are allowed free passage. An example on this is the demineralization of water, where calcium and magnesium ions are replaced by hydrogen or sodium ions, while anions like chloride and sulfate pass right through. In ion exchange membranes ions of opposite charge than the membranes fixed charges (counter-ions) can jump from one fixed group to the next and are hence mobile inside the membrane, while ions of same charge as the membrane (co-ions) are electrically repulsed by the fixed charges. The co-ions are still mobile inside the ion-exchange membrane, but because of the repulsion and the necessity of preserving electroneutrality, the concentration of co-ions is much lower compared to that of the counter-ions.
The principle is sketched on Figure 1.7 with a polymer carrying fixed sulfonic groups.
Figure 1.7 Migration of sodium ions through a cation exchange membrane.
The mobile ions are respectively sodium (counter-ions) and chloride (co-ions). The sketch shows how the sodium ions migrate through the polymer by "jumping" from one fixed sulfonic group till
excluding co-ions from ion exchange membranes is referred to as Donnan exclusion, which is elaborated on later in this chapter.
The Donnan exclusion and the selectivity of an ion-exchange membrane depend on different factors (Strathmann 1992). Both properties increase with:
• increasing concentration of fixed charges,
• increasing valence of the co-ions,
• decreasing valence of the counter-ions,
• decreasing electrolyte concentration in the mobile phase,
• decreasing affinity between the exchanger groups and the counter-ions.
Other important parameters for the ion-exchange membrane's properties include the density of the polymeric network, the hydrophobic and hydrophilic properties of the polymer matrix, the distribution of the charge density, and the morphology of the membrane.
To be most effective an ion-exchange membrane should possess the following properties:
• High permselectivity. The ion-exchange membrane should be highly permeable to counter-ions and impermeable to co-counter-ions.
• Low electrical resistance. The permeability for the counter-ions driven by an electrical potential gradient should be as high as possible.
• Good mechanical and form stability. The membrane should be mechanically strong to withstand handling during equipment assembling/disassembling as well as minor pressure changes during operation. Also low degree of swelling or shrinking in transition from dilute to concentrated electrolyte solutions is important to preserve form and avoid equipment leakage.
• High chemical stability. The membrane should be inert to the electrolyte solutions and their electrolysis products. This means stability in the operating pH-range and resistance to oxidizing agents.
Optimizing properties of ion-exchange membranes often result in a compromise, because the parameters involved most often work in opposite directions. Higher degree of cross-linking improves mechanical stability, but increases the electrical resistance as well. Higher concentration of fixed charges causes lower electrical resistance, but increases swelling which lowers mechanical stability. Hence, commercial ion-exchange membranes are usually manufactured with specific properties for different tasks. Membranes with increased chemical resistance, mechanical stability, or low diffusion rates can be obtained as needed for a given process. Monovalent-selective membranes, that are permeable to monovalent ions while excluding multivalent counter-ions as well as co-counter-ions, are also commercially available. These special grade membranes are used in the table salt production, when sodium chloride is concentrated from seawater with electrodialysis, thus only concentrating the sodium and chloride ions with a minimum of the metal, sulfate, calcium, or magnesium ions normally present in seawater.
A special version of an ion-exchange membrane is the bipolar membrane. The bipolar membrane is composed of two halves: an anion-exchange film and a cation-exchange film connected by a very thin (about 1 nm thick) neutral interface junction. A bipolar membrane is sketched on Figure 1.8.
When the membrane is placed in an electrical field as shown in the figure water molecules split into hydroxide ions and protons.
CEL AEL
TransitionLayer
H2O
H+
+
OH
C at h o d e
H2OH2O
A n o de
Figure 1.8 Schematics of bipolar membrane. Cations are rejected by the anion-exchange layer (AEL) and anions are rejected by the cation-exchange layer (CEL). The electrical potential drop across the transition layer catalyses dissociation of water that diffuses into the membrane.
The applications possible by this watersplitting process are many. By producing hydrogen and hydroxide ions while transferring electrical current and serving as an impermeable wall to both cations and anions, it is possible to regenerate acids and bases from mixed solutions.