Electrodialysis Donnan dialysis

Now the major application of dialysis is the artificial kidney and, as described in Chapter 12, more than 100 million of these devices are used annually. Apart from this one important application, dialysis has essentially been abandoned as a separation technique, because it relies on diffusion, which is inherently unselec-tive and slow, to achieve a separation. Thus, most potential dialysis separations are better handled by ultrafiltration or electrodialysis, in both of which an outside force and more selective membranes provide better, faster separations. The only three exceptions—Donnan dialysis, diffusion dialysis and piezodialysis—are described in the following sections. 

For radioactive effluent treatment, the relevant membrane processes are microfiltration, ulfrafiltration (UF), reverse osmosis, electrodialysis, diffusion, and Donnan dialysis and liquid membrane processes and they can be used either alone or in conjunction with any of the conventional processes. The actual process selected would depend on the physical, physicochemical, and radiochemical nature of the effluents. The basic factors which help in the design of an appropriate system are permeate quality, decontamination, and VRFs, disposal methods available for secondary wastes generated, and the permeate. 

Donnan dialysis combines the advantages of conventional dialysis (cheap driving force) with those of electrodialysis (charged membranes, selective transport across the membranes). It also makes use of the different mobility and different transport velocity of ions of the same or opposite charge. Donnan dialysis is now beginning to play a role in waste management. More details are available in our own reports on the utilization of sulphate and nitrate ions taken up from the post-nitrification spent liquors [16 ]. 

The desalination of brackish water by electrodialysis and the electrolytic production of chlorine and caustic soda are the two most popular processes using ion-exchange membranes. There are, however, many other processes such as diffusion dialysis, Donnan dialysis, electrodialytic water dissociation, etc. which are rapidly gaining commercial and technical relevance. Furthermore ion-exchange membranes are vital elements in many energy storage and conversion systems such as batteries and fuel cells. 

AMJ, located in Great Neck, NY, sells industrial equipment for electrodialysis and Donnan dialysis. Its special products division has tested successfully a prototype unit designed and built by AMJ for nickel recovery systems. At the time we were working in electrodialysis (1974), AMJ was engaged in further development work with Special Products under a secrecy agreement. 

The term dialysis covers separation methods that are based on the transport of molecules or ions through a semi-permeable membrane. A differentiation is made between various types of dialysis (passive dialysis, Donnan dialysis, and electrodialysis), according to the driving force and the type of separation membrane that is used. "... 

Active Donnan dialysis is employed most commonly with ion chromatography and is useful for clean-up of sample solutions at extreme pHs. Electrodialysis can be used with ion chromatography for the off-hne analysis of strongly alkaline samples containing trace amounts of common inorganic anions. ... 

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... 

Membrane processes that use ion-exchange membranes and electric potential difference as the driving force for ionic species transport are referred to as electromembrane processes (Strathmann, 2004). The following electro-membrane separation processes (Scheme 5.1) can be distinguished electrodialysis (ED), including variations such as electrodialysis reversal, electro-electrodialysis and bipolar membrane electrodialysis, electrodeionization (EDI), and Donnan dialysis (DD). 

Although the large scale industrial utilisation of ion-exchange membranes began only 20 years ago, their principle has been known for about 100 years [1]. Beginning with the work of Ostwald in 1890, who discovered the existence of a "membrane potential" at the boundary between a semipermeable membrane and the solution as a consequence of the difference in concentration. In 1911 Donnan [2] developed a mathematical equation describing the concentration equilibrium. The first use of electrodialysis in mass separation dates back to 1903, when Morse and Pierce [3] introduced electrodes into two solutions separated by a dialysis membrane and found that electrolytes could be removed more rapidly from a feed solution with the application of an electrical potential. 

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