Dialysis separations

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. 

Hammer, M. S., Borchardt, J. A., Dialysis Separation of Sewage Sludge... 

In 1861, Thomas Graham Bell in Glasgow, Scotland, carried out the first dialysis experiments (and coined the term dialysis ), separating crystalloids and cohoids in a solution. BeU predicted that this technique could have medical application, but this was not realized until nearly 100 years later in the work of Willem Kolff and then Belding Scribner, who made HD a feasible treatment in the early 1960s. Since then, HD and more recently PD have extended the lives of many people, sometimes for up to 20 or 30 years. 

Dialysis is not used very often in environmental applications, but a brief discussion is useful to compare it to other types of membrane processes. Dialysis separates solutes of different ionic or molecular size in a solution. The driving force is a difference in solute concentration across the membrane. The smaller ions and molecules will pass through the membrane, but the bigger particles cannot make it through the pore openings. 

The separation of substances by membranes is essential in industry and human life. Of the various separation membranes, the ion exchange membrane is one of the most advanced and is widely used in various industrial fields electrodialysis, diffusion dialysis, separator and solid polymer electrolyte in electrolysis, separator and solid polymer electrolyte of various batteries, sensing materials, medical use, a part of analytical chemistry, etc. 

Neutralization dialysis Separation of electrolyte and nonelectrolyte, desalination of water, etc. 

Dialysis separations are often used for removal of interferents in the sample matrix. The technique is based on differences in mobility of ionic or molecular constituents in a liquid phase during their transport across a semi-permeable membrane into a second liquid phase which need not be immiscible with the first. Mass transfer occurs between a donor phase and an acceptor phase separated by a membrane which selectively allows penetration of solutes by blocking the passage of macro-molecules or by differences in molecular diffusivities. The driving force of the mass transfer is the existence of a concentration gradient of the transferable solute between the two phases. 

The dialysis membrane is a critical factor contributing to the efficiency of the dialysis separation system. In addition to their specificity on allowing the passage of a certain species of analyte while obstructing interfering components, which is a common requirement for all dialysis systems, on-line systems demand better mechanical and kinetic properties. Dialysis membranes of FI systems are expected to allow the achievement of high transfer factors within very short contact times of usually less than 30 seconds, while the membranes should be able to withstand hundreds of analytical cycles without significant deterioration in their performance. 

A 14-crown-4 derivative has also been incorporated in a PVC membrane to construct a coated-wire lithium ion-selective electrode used for the determination of lithium in blood sera, after on-line dialysis separation in a FIA system [6]. An estimation of the overall selectivity of the method over the sodium content, including that of the dialysis membrane and the electrode, resulted in a selectivity coefficient k of 1/50. This selectivity is not sufficient to overcome interferences from relatively large fluctuations in sodium contents, and corrections based on a simultaneous evaluation of the sodium content are required under such circumstances. 

Sionkowski, G. and Wodzki R, Recovery and concentration of metal ions, I. Donnan dialysis, Separ. Sci. Technol., 30, 805-820, 1995. 

Flow techniques are particularly suitable for monitoring environmental parameters in waters. Moreover, flow methods offer unique possibilities for automatic sample pretreatment, involving inline dilution or filtration, adjustment of viscosity, ionic strength, or pH as well as removal of solid and colloidal matrix constituents via in-line dialysis separation techniques. 

Soil Dialysis separation Potentiometry (CI-ISE) 5.0-5000 mg 1 Slurry sampling/online removal of... 

The presence of colloidal sulfur in the same solution with suspended sulfur was shown by Debus on evaporating the hydrosol. When about two-thirds of the water is evaporated the solution is clear and yellow. Dialysis separates out sulfur just as in the case of the preparation of Raffo. An elaborate investigation of the fractional precipitation of sulfur by Sven Od6n has shown that the larger particles are more easily precipitated than the finer ultramicrons. 

Hichour, M., P rsin, E, Sandeaux, J. Gavach, C. (2000) Fluoride removal from waters by Donnan dialysis. Separation and Purification Technology, 18,1-11. 

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