Donnan membrane effect

Cumbal, L. and Sengupta, A.K. (2005) Arsenic removal using polymer-supported hydrated iron(III) oxide nanoparticles Role of Donnan membrane effect. Environmental Science and Technology, 39(17), 6508-15. 

H. K. Lonsdale and W. Pusch, Donnan-membrane effects in hyperfiltration of ternary systems. /. Chem. Soc., Faraday Trans. 1,71 (3) (1975) 501. 

This method has been widely and traditionally used for the study of the interaction of proteins in solution with smaller molecules and ions including surfactants. The principle rests on the fact that a membrane can be chosen such that, while the macromolecule is contained in the dialysis bag made from the membrane, the smaller molecule (and water) can move freely between the bag and the solution in the outer container. When equilibrium is established, analysis reveals the degree of binding of the smaller molecule by the macromolecule. [One complication is the Donnan membrane effect (11,12) but this can be overcome if the measurements are done in the presence of salt, e.g., 0.1 M NaCL]... 

The rejection of salts by nanofiltration membranes is more complicated and depends on both molecular size and Donnan exclusion effects caused by the acid groups attached to the polymer backbone. The phenomenon of Donnan exclusion is described in more detail in Chapter 10. In brief, charged groups tend to exclude ions of the same charge, particularly multivalent ions while being freely permeable to ions of the opposite charge, particularly multivalent ions. 

Many nanofiltration membranes follow these rules, but oftentimes the behavior is more complex. Nanofiltration membranes frequently combine both size and Donnan exclusion effects to minimize the rejection of all salts and solutes. These so-called low-pressure reverse osmosis membranes have very high rejections and high permeances of salt at low salt concentrations, but lose their selectivity at salt concentrations above 1000 or 2000 ppm salt in the feed water. The membranes are therefore used to remove low levels of salt from already relatively clean water. The membranes are usually operated at very low pressures of 50-200 psig. 

Ion implantation This study involves modifying the surface of nanofiltration membranes by ion implantation for increased salt rejection [55]. ions at two different intensities—lElO and 5E10 atoms/cm —were implanted on the surface of commercially available nanofiltration membranes to increase the negativity of the membrane surfaces. The objective was to increase the Donnan exclusion effect to improve salt rejection by the modified membranes. It was also noted that this modification did not significantly damage the semipermeable membrane surface. 

It seems therefore, that the established procedures involving high feed velocity across the membrane surface, additional turbulence promotion, etc., need to be applied and optimized. There is a need for a model for fouling in reverse osmosis which incorporates such factors as the added concentration polarization caused by the fouling layer, and Donnan exclusion effects due to charged foulants. Clearly there is scope for more detailed experimental work in this area. 

These perfluorinated membranes, similar to other ion-exchange materials, exhibit a Donnan exclusion effect. Figure 8 shows the concentration of co-ion of N-form Nafion with varying HCl... 

As illustrating the kind of evidence which can be brought forward, for the three membrane effects hitherto considered, viz (i) the unequal distribution of a simple electrolyte on the two sides of a membrane ordinarily quite permeable to the electrolyte, when an electrolytic colloid is present on one side of the membrane, (2) the influence of this unequal distribution on the apparent osmotic pressure of the colloijl, (3) the hydrolysis of the electrolytic colloid by the chemically inert membrane—a short account will be given of certain of the experimental results obtained by Donnan and Haras (Trans Chem Soc, 99, 1554 (1911)) in connection with the osmotic behaviour of Congo red (the sodium salt of diphenylbisazonaphthylamine sulphomc acid)... 

The Donnan ion effect is also reflected in the lower rejection of sodium chloride at the higher salinity level (0.5% vs. 0.1%). The chemistry of this membrane is shown as follows ... 

In reality, a state of equilibrium is reached where the ionic attraction of the counterions by the polyanion is just balanced by diffusion into the external solution, which is driven by the chemical potential gradient, which in turn arises from the difference in counterion concentrations between the two domains. The overall effect closely resembles Donnan membrane equilibria (4) (Figure 3). As a consequence of the difference in counterion concentrations, the osmotic pressure inside the polymer domain exceeds that of the external solution, and the expansion of the polyelectrolyte can be equated to the difference in osmotic pressures of the intramolecular and intermolecular solutions. 

With zinc in the spin-bath, the nature of the Donnan membrane established at the interface between the viscose filament and the spin-bath, and across which the diffusion processes and mass transfer must occur, is changed [170,187,188]. Although it has been proposed that the membrane retards acid diffusion into the filament, it is equally probable that the observed effect is caused by the structure suppressing diffusion of water from the filament. Tornell s study of viscose spinning has shown that zinc reduces deswelling of the filament [189]. 

NF is closely related to RO, and is sometimes called loose RO (Table 1.3). The basic principles of the RO process discussed above are appficable to NF except that the rejection of solutes depends both on molecular size and Donnan exclusion effects, which are due to the acid groups, e.g., carboxyfrc or sulphonic acid groups, on the polymer backbone. The equilibrium between the charged membrane and the bulk solution is characterised by an electric potential called the Donnan potential. Ions smaller than the pore size are rejected because of Donnan exclusion [6, 7, 17]. 

Selection of a membrane the membrane should have both the capability to retain the catalyst and to partially reject organic species, enabling control of the residence time in the reaction environment. In order to select a suitable membrane, rejection should be determined during operation of the photoreactor. Parameters such as transmembrane pressure, solution pH, molecular size of the pollutants and products/by-products of their degradation should be especially considered. In the case of NF an improvement of permeate quality could be obtained by taking advantage of the Donnan exclusion effect, provided that the membrane is selected properly. 

Cook et al. [2] also proposed a new mechanism whereby equilibrium of the bound zwitterions with a mobile phase containing a suitable electrolyte causes the establishment of a charged layer created by the terminal sulfonate groups of the zwitterion, which acts as a Donnan membrane. The magnitude and polarity of the charge on this membrane depends on the nature of the mobile-phase ions. The Donnan membrane exerts weak electrostatic repulsion or attraction effects on analyte anions. A second component of the retention mechanism is chaotropic interaction of the analyte anion with the quaternary ammonium functional group of... 

The above examples have made us understand the usefulness of Donnan membrane equilibrium in biology. However, there are cases where the same effect becomes a bane to the system. An example that can be cited is that of oedema in tissues. When the plasma albumin content falls below the normal value, salt and water retention takes place in the tissues. The movement of the concerned electrolytes In and out of the cell membranes occurs mainly due to tlie Donnan effect. Other developments like Increased secretion of aldosterone and vasopressin follow soon to result in a full-fledged oedema state. 

When the Donnan equilibrium is operative the entry of ions into the membrane is restricted. Consequently as the concentration of ions in the solution increases the resistance of the membrane remains constant until the concentration of ions in the solution reaches that of the fixed ions attached to the polymer network. At this point their effect will be swamped and the movement of ions will be controlled by the concentration gradient. 

Nanofiltration membranes are negatively charged and reject multivalent anions at a much higher level than monovalent anions, an effect described as Donnan exclusion. Nanofiltration membranes have MgS04 retention and water permeability claims. 

A semi-permeable membrane, which is unequally permeable to different components and thus may show a potential difference across the membrane. In case (1), a diffusion potential occurs only if there is a difference in mobility between cation and anion. In case (2), we have to deal with the biologically important Donnan equilibrium e.g., a cell membrane may be permeable to small inorganic ions but impermeable to ions derived from high-molecular-weight proteins, so that across the membrane an osmotic pressure occurs in addition to a Donnan potential. The values concerned can be approximately calculated from the equations derived by Donnan35. In case (3), an intermediate situation, there is a combined effect of diffusion and the Donnan potential, so that its calculation becomes uncertain. 

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