Electrodialysis Membranes

Figure 8.1a shows a cut section of an electrodialysis filtering membrane. The filtering membranes are sheet-like barriers made out of high-capacity, highly cross-linked ion exchange resins that allow passage of ions but not of water. Two types are used: cation membranes, which allow only cations to pass, and anion membranes, which allow only anions to pass. The cut section in the figure is a cation membrane composed of an insoluble matrix with water in the pore spaces. Negative charges are fixed onto the insoluble matrix, and mobile cations reside in the pore spaces occupied by water. It is the residence of these mobile cations that gives the membrane the property of allowing cations to pass through it. These cations will go out of the structure if they are replaced by other cations that enter the structure. If the entering cations came from water external to the membrane, then, the cations are removed from the water, thus filtering them out. In anion membranes, the mechanics just described are reversed. The mobile ions in the pore spaces are the anions; the ions fixed to the insoluble matrix are the cations. The entering and replacing ions are anions from the water external to the membrane. In this case, the anions are filtered out from the water.

Figure 8.1b portrays the process of filtering out the ions in solution. Inside the tank, cation and anion membranes are installed alternate to each other. Two electrodes are put on each side of the tank. By impressing electricity on these electrodes, the positive anode attracts negative ions and the negative cathode attracts positive ions. This impression of electricity is the reason why the respective ions replace their like

— Mobile cation

Insoluble 'matrix

Fixed negative charge

— Mobile cation

Insoluble 'matrix

Fixed negative charge

Cathode " C

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