Donnan membrane process importance

For membrane processes involving liquids the mass transport mechanisms can be more involved. This is because the nature of liquid mixtures currently separated by membranes is also significantly more complex they include emulsions, suspensions of solid particles, proteins, and microorganisms, and multi-component solutions of polymers, salts, acids or bases. The interactions between the species present in such liquid mixtures and the membrane materials could include not only adsorption phenomena but also electric, electrostatic, polarization, and Donnan effects. When an aqueous solution/suspension phase is treated by a MF or UF process it is generally accepted, for example, that convection and particle sieving phenomena are coupled with one or more of the phenomena noted previously. In nanofiltration processes, which typically utilize microporous membranes, the interactions with the membrane surfaces are more prevalent, and the importance of electrostatic and other effects is more significant. The conventional models utilized until now to describe liquid phase filtration are based on irreversible thermodynamics good reviews about such models have been reported in the technical literature [1.1, 1.3, 1.4]. 

UF and RO models may all apply to some extent to NF. Charge, however, appears to play a more important role than for other pressure driven membrane processes. The Extended-Nemst Planck Equation (equation (3.28)) is a means of describing NF behaviour. The extended Nernst Planck equation, proposed by Deen et al. (1980), includes the Donnan expression, which describes the partitioning of solutes between solution and membrane. The model can be used to calculate an effective pore size (which does not necessarily mean that pores exist), and to determine thickness and effective charge of the membrane. This information can then be used to predict the separation of mixtures (Bowen and Mukhtar (1996)). No assumptions regarding membrane morphology ate required (Peeters (1997)). The terms represent transport due to diffusion, electric field gradient and convection respectively. Jsi is the flux of an ion i, Di,i> is the ion diffusivity in the membane, R the gas constant, F the Faraday constant, y the electrical potential and Ki,c the convective hindrance factor in the membrane. 

The Donnan equilibrium is important in biological systems where it governs the movement of water and electrolytes into and out of cells. Chemical processes within the living cell can control the permeability of the cell membranes to various species. 

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 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. 

The glass electrode is by far the most frequently used ion-selective electrode. It is applied in almost all laboratories for the determination of pH values. Depending on the composition of the glass, some other ions are also detectable. However, in most textbooks, the theoretical description of the potential-determining electrochemical processes does not take into account important recent findings. For instance, in the past, the potential difference at a glass electrode was described either as an effect of the adsorption of hydronium ions, as a diffusion potential within the glass membrane, as a Donnan potential or a poten-... 

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