Donnan dialysis, membrane process

This three-aqueous phase system is based on a combination of liquid membrane (LM) and Donnan dialysis (DD) processes in the case of ion-exchange membranes or dialysis (D) processes in the case of neutral hydrophilic membranes. On the feed-side interface, the liquid membrane is loaded by the solute on the strip side, it is offloaded and recharged. Thus, strong acids, or bases, or salts with other cations and anions, used as respective pump solutes, provide a continuous process. 

Many related processes use charged membranes and/or EMF. Electrodialytic water dissociation (water splitting), diffusion dialysis, Donnan dialysis, and electrolysis are related processes. Electrowsis (chlorine-caustic) is a process of enormous importance much of which is processed through very special membranes. 

Donnan Dialysis Another nonelectrical process using ED membranes is used to exchange ions between two solutions. The common application is to use H to drive a cation from a dilute compartment to a concentrated one. A schematic is shown in Fig. 20-85. In the right compartment, the pH is 0, thus the H+ concentration is lO higher... 

Three other processes using ion exchange membranes (Donnan dialysis, diffusion dialysis and piezodialysis) are covered in Chapter 13. 

One dialysis process for which the membrane does have sufficient selectivity to achieve useful separations is Donnan dialysis. If salt solutions are separated by a membrane permeable only to ions of one charge, such as a cation exchange membrane containing fixed negatively charged groups, then distribution of two different cations M+ and N+ across the membrane can be expressed by the Donnan expression... 

In addition to the processes discussed so far there are two more electromembrane separation processes in which the driving force is not an externally applied electrical potential but a concentration gradient. The processes are referred to as diffusion dialysis and Donnan dialysis. Diffusion dialysis is utilizing anion- or cation-exchange membranes only to separate acids and bases from mixtures with salts. Donnan dialysis can be used to exchange ions between to solutions separated by an ion-exchange membrane. Both processes have so far gained only limited practical relevance [4] and will not be discussed in this chapter. 

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 is a membrane separation process that uses ion-selective membranes to prevent the flow of certain ions from one solution to another. A schematic of the process is presented in Figure 29.8. When a salt solution is separated from its corresponding acid by a cation-exchange membrane, the anions are excluded from the membrane, whUe the cations are redistributed across the membrane to attain Donnan equilibrium. By changing the salt solution periodically, it would be possible to shift the equilibrium favorably to effect simultaneous neutralization of acid on one side of the membrane (feed compartment) and acid recovery on the other side (receiver compartment). The driving force for ion migration is the chemical potential gradient for the cation across the membranes. 

During the last two decades, pressure-driven membrane processes namely reverse osmosis (RO), nanofiltration (NF), and ultrafiltration (UF) have found increased applications in water utilities and chemical industries. Unlike RO, NF, and UF, the Donnan membrane process (DMP) or Donnan dialysis is driven by an electrochemical potential gradient across an ion-exchange membrane. Theoretically, the DMP is not susceptible to fouling because particulate matter or large organic molecules do not concentrate on the membrane surface, as commonly observed with pressure-driven membrane processes. DMP has been used in the past in hydrometallurgical operations [19,20], for concentration of ionic contaminants [21,22] and for separation of... 

Donnan dialysis successfully involves mosaic membranes which are an important part of piezodialysis. Their basic advantage is the close juxtapoation of anion- and cation-exchange resins in the same membrane. It is a pity that research on piezodialysis, a desalination process with great promise and hopes, has been abandoned. However, one may expect work on it will be reassumed as soon as some difficulties originated from the deficiency of mosaic membranes, as well as some engineering problems, are overcome. 

There are a number of different membrane techniques which have been suggested as alternatives to the SPE and LLE techniques. It is necessary to distinguish between porous and nonporous membranes, as they have different characteristics and fields of application. In porous membrane techniques, the liquids on each side of the membrane are physically connected through the pores. These membranes are used in Donnan dialysis to separate low-molecular-mass analytes from high-molecular-mass matrix components, leading to an efficient cleanup, but no discrimination between different small molecules. No enrichment of the small molecules is possible instead, the mass transfer process is a simple concentration difference over the membrane. Nonporous membranes are used for extraction techniques. 

Ionomer membranes based on perfluorocarbon polymers became available In the late 196O s. These materials have excellent chemical resistance, thermal stability, mechanical strength and strong acid strength, A number of functionalities have been studied. Including carboxylate, sulfonate and sulfonamide, but only the first two are available as commercial materials. Ferfluorlnated lonomers have been evaluated as membranes In a variety of applications, such as water electrolysis, fuel cells, air driers, Donnan dialysis In waste metal recovery, and acid catalysts, but the primary interest in these materials is for the permselective membrane In electrochemical processes such as In the production of chlorine and caustic (58). 

Donnan dialysis The BAHLM systems with ion-exchange membranes, based on Donnan dialysis [18,19], will be considered below. Donnan dialysis is a continuously operating ion-exchange process. There are many theoretical models describing transport mechanisms and kinetics of DL) [18-26]. All transport kinetics models are based on Fick s or Nernst-Planck s equations for ion fluxes. In both cases, the authors introduce many assumptions and simplifications. 

There are several processes that are occurring in Donnan dialysis sample preparation. One process involves the normal transport ions of like charge across the membrane. At the start of dialysis there is a strong tendency for the any particular ion to diffuse from a high concentration zone on one side of the membrane to the low concentration zone on the other side. As this process occurs, corresponding transfer of the same charge ion travels back across the membrane in order to preserve electroneutrality. The counter ion does not travel across so there can exist different normality concentrations of ions on each side. The example of 90 % transport of a sample (Na+) Cl at 0.0010 N and a receiving electrolyte (Na" ) OH" at 0.050 N at 1 mL each is shown in Fig. 9.3. 

Donnan dialysis is an ion exchange process in which an ion exchange membrane is placed between two solutions, i.e., the feed (I) and receiver (II) solutions. The membrane is permeable to counter-ions and excludes co-ions. The ionic strength of the feed solution is relatively low compared with that of the receiver electrolyte solution. At equilibrium, if concentrations are used instead of activities, there is the following Donnan relationship,... 

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. 

The process can be carried out accordingly with anions through anion-exchange membranes. An example of anion Donnan dialysis is the sweetening of citrus juices. In this process hydroxyl ions furnished by a caustic solution replace the citrate ions in the juice. 

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