Donnan exchange

In the simplest case, = n = 1 in Eq. (10.25). For such conditions the reaction is called Don-nan exchange. Donnan exchange behavior has several implications ... 

Donnan exchange best applies to the sorption of major dissolved cations (> 1 O to 10 mol/kg) such as Na, Ca, and Mg + by minerals of practically constant surface charge, such as the smectite and vermiculite clays and zeolitic minerals. 

What is the power-exchange function and how is it related to ion exchange and Donnan exchange Give examples of the applicability of each of these approaches to competitive cation adsorption. How are activity coefficients dealt with in ion-exchange reactions ... 

In ion-exchange resins, diffusion is further complicated by electrical coupling effec ts. In a system with M counterions, diffusion rates are described by the Nernst-Planck equations (Helfferich, gen. refs.). Assuming complete Donnan exclusion, these equations canbe written... 

Membrane Efficiency The permselectivity of an ion-exchange membrane is the ratio of the transport of electric charge through the membrane by specific ions to the total transport of electrons. Membranes are not strictly semipermeable, for coions are not completely excluded, particularly at higher feed concentrations. For example, the Donnan eqmlibrium for a univalent salt in dilute solution is ... 

The exchange of ions and solvent between a swollen ionic network and the surrounding electrolyte is represented in Fig. 136, where the fixed ion is taken to be a cation. It is apparent that the equilibrium between the swollen ionic gel and its surroundings closely resembles Donnan membrane equilibria. 

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... 

FIG. 20-85 Schematic of Donnan dialysis using a cation-exchange membrane. 

Conversely, the fundamentals for the UDL he on the coextraction of counterions into the membrane therefore, the membrane is no longer permselective (Donnan failure) [9]. Ideally, when the ionophores are saturated by ions, the ion-ionophore complex functions as an ion-exchanger and the membrane shows an anion Nernstian response. The UDL can be estimated from the membrane composition, formation constant and coextraction coefficients obtained from the so-called sandwich membrane method [73]. 

Unlike the pellicular packings used for ion exchange, the packings used in ion exclusion are derived from totally sulphonated polymeric materials. Separation is dependent upon three different mechanisms Donnan exclusion, steric exclusion and adsorption/partitioning. 

Freed of other restrictions, a mobile ion may be expected to diffuse down any concentration gradient that exists between porous solid and liquid. In the particular case of ion exchange, there is an additional requirement that the resin and liquid phases should remain electrically neutral. Any tendency for molecules to move in such a way as to disturb this neutrality will generate a large electrostatic potential opposing further movement, known as the Donnan potential. 

The equilibrium (also known as the Donnan effect) established across a semipermeable membrane or the equivalent of such a membrane (such as a solid ion-exchanger) across which one or more charged substances, often a protein, cannot diffuse. Diffusible anions and cations are distributed on the two sides of the membrane, such that the sum of concentrations (in dilute solutions) of diffusible and nondiffusible anions on either side of the membrane equals the sum of concentrations of diffusible and nondiffusible cations. Thus, the diffusible ions will be asymmetrically distributed across the membrane and a Donnan potential develops. 

A terminological remark is due. An equilibrium between two media with different fixed charge density (e.g., an ion-exchanger in contact with an electrolyte solution) is occasionally termed the Donnan equilibrium. The corresponding potential drop between the bulks of the respective media is then termed the Donnan potential. By the same token, we speak of the local Donnan equilibrium and the local Donnan potential, referring, respectively, to the local equilibrium and the interface potential jump at the surface of discontinuity of the fixed charge density, considered in the framework of the LEN approximation. 

Concentration Method. The concentration procedure that was developed and evaluated was a RO-Donnan dialysis system (4). The initial objective during method development was to conduct membranescreening tests to evaluate the suitability of various RO and ion-exchange membranes. The four membranes considered for final evaluation on the basis of solute rejection, chlorine stability, and artifact production were the cellulose acetate and FT-30 (Film Tec) RO membranes, the Nafion cation-exchange membrane, and the ION AC MA 3475 anion-exchange membrane. 

For equilibrium conditions, Donnan ion exchange theory (6) may be used to describe quantitatively equilibria for trace quantities of chemical elements in the presence of high homologous salt concentrations. The radionuclides 90Sr and 137Cs constitute a very small fraction of the mass of the chemical constituents in rainfall thus the equilibrium distribution of these nuclides between solid and liquid phases may be estimated as... 

---