Electrically driven membrane processes 2 Electrodialysis

Ion-exchange membranes are frequently used in combination with an electrical potential difference (as in electrodialysis, for example see chapter VI, where electrical driven membrane processes are involved). Two forces now act on the ionic solutes a concentration difference and an electcicai potential difference. Under these circumstances transport of an ion can be described by a combination of these two processes. i,e. a Fickian diffusion and an ionic conductance. The resulting equation is known as the Nemst-Planck equadon -... 

As discussed by Pletcher 24, electrodialysis is an electrically driven membrane separation process. The main use of electrodialysis is in the production of drinking water by the desalination of sea-water or brackish water. Another large-scale application is in the production of sodium chloride for table salt, the principal method in Japan, with production exceeding 106 tonne per annum. 

Since the electrical resistance of the effiuent and parasitic currents are minimal at high level of impurities, specihc interest in electrically assisted membrane processes could increase due to more strict laws and legislation around effluents. The depletion of freshwater resources and the necessity to process brackish or seawater to produce potable water could promote the use of electrically assisted membrane processes in the future. Electrodialysis will have to compete with pressure-driven membrane processes such as reverse osmosis. The growing awareness of the unique cleaning ability of electrically ionized water (EIW) [47], a byproduct of electrodialysis, may be a factor to consider in the choice between ED and RO systems. NMR relaxation measurements were used to determine the water cluster size of electrically ionized water EIW. It is known that the water cluster size of EIW is signihcantly smaller than that of tap water. The smaller water cluster size is believed to enhance the penetration and extractive properties of EIW. Recently, EIW has been produced and used in several cleaning processes [47] in industry. 

Membrane processes are widely used in oil water separation. In general, a membrane is classified into two groups pressure-driven membrane and electrical membrane, known as electrodialysis. The most applicable process for oily wastewater removal is the former type. The pressure-driven membrane applications include microfiltration (MF), ultrafil-tration (UF), nanofiltration (NF), and reverse osmosis (RO). All of them are categorized by the molecular weight or particle size cut-off of the membrane as shown in Table 5. 

Most of the membrane processes are pressure driven. The notable exception to this is electrodialysis (ED) by which ions are separated under the influence of an electric field. In addition, the chapter on coupled transport covers processes which are driven under the influence of a concentration gradient. 

Ionic membranes are characterised by the presence of charged groups. Charge is, in addition to solubility, diffusivity, pore size and pore size distribution, another principle to achieve a separation. Charged membranes or ion-exchange membranes are not only employed in electrically driven processes such as electrodialysis and membrane electrolysis. There are a number of other processes that make use of the electrical aspects at the interface membrane-solution without the employment of an external electrical potential difference. Examples of these include reverse osmosis and nanofiltration (retention of ions), microfiltration and ultrafdtration (reduction of fouling phenomena), diffusion dialysis and Donnan dialysis (combination of Dorman exclusion and diffusion) and even in gas separation and pervaporation charged membranes can be applied... 

A third major class of physical separation is molecular separation, which is often based on membrane processes in which dissolved contaminants or solvents pass through a size-selective membrane under pressure. The products are a relatively pure solvent phase (usually water) and a concentrate rich in the solute impurities. Membrane processes including the special case of reverse osmosis to remove salts from water are discussed for the treatment of water in Chapter 5, Section 5.10. Electrodialysis, employing membranes alternately permeable to cations and to anions and driven by the passage of an electrical current (see Chapter 5, Section 5.10), is sometimes used to concentrate metal plating wastes and to reclaim dissolved metals. 

Membrane separations are driven by pressure, concentration, or electric field across the membrane and can be differentiated according to type of driving force, molecular size, or type of operation. Common membrane processes include microfiltration, ultrafiltration, nanofiltration/ reverse osmosis, gas separation, pervaporation, and dialysis/electrodialysis [3,4]. Some processes have been applied extensively for separation and purification of gas and liquid mixtures in industry. 

Ion-exchange processes can be driven by concentration gradients or by an electric field applied across the membrane (electrodialysis), the ions not merely being exchanged but actually passing across. 

Although fairly distinct in their capabilities, these processes have several features in common. The membrane materials used are generally polymeric in origin and are extensions, both in mode of preparation and in composition, of the cellulose acetate membranes pioneered by Loeb and Sourirajan. Another common feature shared by most of the processes bsted is the use of pressure as the driving force the exceptions are the different forms of dialysis. Thus, in hemodialysis, concentration replaces pressure as the driving force, while electrodialysis is driven by an electrical potential. 

---