Electrodialysis is a process for the separation of electrolyte from a solvent, typically water. The process is widely used in the Desalination of water and process solutions. It uses a direct electrical current to transport ions through sheets of ion-exchanger membranes and is operated in a unit with at least three compartments, as shown in Figure 1. The terminal compartments house an anode and a cathode, between which a potential difference is applied to drive the ions through the electrolyte solutions and the membranes.
Two types of membranes are used: one which is preferentially permeable to the transport of anions (anion-selective), and one which is preferentially permeable to cations (cation-selective). Membranes are arranged alternately between the electrodes, forming individual compartments (or cells). The solution to be desalinated is held in one compartment and during current flow, anions move through the anion exchange membrane in the direction of the anode into adjacent compartment while cations move in the opposite direction into an adjacent compartment on the other side. Thus, overall, the solution becomes depleted in ions in one compartment and solutions in adjacent compartments become enriched in ions. In practice, solutions flow through the compartment to allow continuous operation and several hundred cell pairs (one concentrated and one diluted solution) are used.
The membranes generally used in electrodialysis are copolymers of styrene and divynylbenzene. Ion exchange characteristics are typically introduced by sulphonation in the case of cation-exchange, and by substitution with quaternary ammonium groups in the case of anion-selective materials. The degree of cross-linking of polymers determines the amount of water absorbed into the membrane after activation, creating channels through which ions can diffuse. The fixed charge groups—essentially electrostatically—repel ions of the same charge (e.g., anion in the case of sulphonate) and thus impart the appropriate ion-selective exchange characteristic.
Transport of ions through membranes is defined in terms of the transport number for a particular ion, which can be as high as 0.98 for some anion exchange membranes and is greater than 0.9 for cation-exchange membranes. A transport number less than 1 means that a proportion of the current is carried by ions of the opposite charge in the wrong direction. This behavior controls the current efficiency of the process, which ideally should be 100%, i.e., 1 F of electricity allows the transport of 1 mol of salt in each cell pair.