Dialysis or Electrodialysis

This method has only been used for separating low-molecular weight salts and interfacially active substances from protective colloids and high polymer plastics and for fractionating pure plastics according to their molecular weight. 

Some examples of these four separation procedures are shown in Table 2.6. 

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To obtain pure sols, dialysis or electrodialysis, ion exchange, and peptization at high temperatures are used (Her, 1959). 

The product gels may be freed from impurity ions (usually present in large amounts) by dialysis or electrodialysis. However, even this procedure does not yield gels completely free from electrolytes. If completely pure products are desired, it is best to use the hydrolysis of alkoxides presented on p. 1652. 

The conventional techniques for the purification of low-molecular-weight compounds, such as distillation, sublimation, and crystallization, are not applicable to polymers. In some cases, it is possible to remove the impurities by cold or hot extraction of the finely powdered polymer with suitable solvents or by steam distillation. Separation of low-molecular-weight components from water-soluble polymers (e.g., poly(acrylic acid), poly(vinyl alcohol), poly(acryl amide)) can be accomplished by dialysis or electrodialysis. However, the most widely used method of purification is by reprecipitation in which the solution of polymer (concentration less than 5-10 wt.%) is dropped into a 4- to 10-fold excess of precipitant, with stirring. If necessary, this operation is repeated with other solvent/precipitant pairs until the impurities are no longer detectable. 

Carbonic anhydrase is assumed to be located at the surface of the membrane of the tubular cells [47, 48]. Carbonic anhydrase is a small zinc protein found in many animal tissues, but its concentration is highest in the kidney tubules cells, the erythrocyte, and some cells of the gastric mucosa. The substrates of the carbonic anhydrase reaction are carbon dioxide and water the product is carbonic acid. The enzyme has been purified from erythrocytes, and its molecular weight is about 30,000. The purified enzyme preparation contains 0.21% zinc, probably 1 atom of zinc per molecule of enzyme. The zinc is tightly bound to the enzyme molecule and cannot be removed by dialysis or electrodialysis. The presence of zinc in the molecule is essential to activity because when zinc is removed from the molecule by extended incubation with 1-10 phenanthroline, the enzyme s activity reflects the zinc content of the preparation. 

Membrane separation processes have been applied to many industrial production systems for the purpose of clarification, concentration, desalting, waste treatment, or product recovery. Broadly speaking, membrane filtration can be classified as microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and dialysis or electrodialysis. In this section, the discussion will only cover microfiltration and ultrafiltration, both of which are pressure-driven membrane processes. 

This class of membranes resemble those employed for dialysis or electrodialysis. However for the PM, transport selectivity is not based on the size of the permeating species or its charge, but on the specific solubility of complexes formed by the metd species and carrier in the membrane matrix. Effective separation of uranium from its mixture with iron and aluminum in dilute nitric acid was obtained with PVC sheets plasticized with TBP (57) and with dibutyl cresyl phosphate (DBCP) or dicresyl butyl phosphate (DCBP) (57,55) in a ratio of 1 3 (by weight). The U(VI) permeation... 

Dialysis or electrodialysis is a specialized diemical process to separate components using differences in diffusion rates through membranes. A difference in concentrations drives the flow of the components in question. This process is used in chemical purification systems. 

Process water applications include boiler water feed pretreatment before ion exchange or electrodialysis. RO is also used for ultrahigh-purity water production for use in laboratories, medical devices (kidney dialysis), microelectronic manufacturing (rinse fluids per ASTM D-19 D5127-90, 1990), and pharmaceutical manufacturing (purified water or water for injection as specified by USP). 

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. 

Ion-exchange membranes are currently used not only for more or less conventional separation processes like membrane electrolysis (mainly the chlor-alkali process), electrodialysis, dialysis or electro-ultrafiltration (cf. Table 2.1), but also in various... 

Membrane operation is a specific, but not exotic, operation. In fact it is a hybrid of classical heat and mass transfer processes (Figure 4.1). Direct contact mass transfer operations tend to reach equilibrium due to a difference of chemical potential between two phases that are put into contact. In the same way, temperature equilibrium is aimed at during heat transfer operations, for which driving force is a temperature gradient. In contrast, for membrane operations, by using the specific properties of separation of the thin layer material that constitutes the membrane, under the particular driving force that is applied, it is possible to deviate from the equilibrium that prevails at fluid-to-fluid interphase with classical direct contact mass exchange systems and to reorientate the mass transfer properties. In particular, this is the case with classical operations such as microfiltration (MF), ultrafiltration (UF), reverse osmosis (RO), gas separation (GS), pervaporation (PV), dialysis (DI) or electrodialysis (ED), for which a few characteristics are recalled in Table 4.1. 

Electrochemical cells (electrolysers, batteries and fuel cells) require separators, which allow a flow of specific ionic charges but prevent the transfer of chemical species which remain located either in the cathodic or in the anodic compartment. Among the various separators of electrochemical cells, the ion permeable organic membranes are also used for separation processes such as dialysis and electrodialysis. 

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. "... 

With reference to the very important difference between colloids and crystalloids, we may classify the electrical methods of separation as follows (1) separation of crystalloids from each other (2) separation of crystalloids from colloids (3) separation of colloids from each other. We shall discuss in detail processes 1 and 3. Process 2, most simply effected by dialysis or ultrafiltration, is considerably accelerated and made more effective by combination with electrical transport. This method was introduced by Schwerin and described by him and his collaborators in a number of patents (see ReitstStter, 1927). In the scientific literature, it was described for the first time by Morse and Pierce (1903). It was rediscovered by Dh6r6 and Gorgolewski (1910), who introduced the name electrodialysis dialyse ilectrique). The method was most extensively used by Pauli and his coworkers as a means of producing pure colloids this work was started in 1922. 

Other Uquid separation systems using membranes include dialysis and electrodialysis, where the driving force is now a concentration difference between the two sides of the membrane. The main use for dialysis is in blood processing, as a kidney replacement or booster, but both processes are also being used industrially, electrodialysis especially in desalination. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Non- or microporous, with fixed charge isotropic Dialysis Electrodialysis... 

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. 

Other methods that will not be discussed here, but also can be used to increase concentration (some with and some without concomitant evaporation) are reverse osmosis, ion exchange, dialysis, electrodialysis, osmotic distillation, and applications that involve fluidized beds, cooling towers, or evaporation of aerosols. 

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 ]. 

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